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Sugar beet plants in the experimental plots of Bangladesh Agricultural University showed wilting symptom along with white cottony mycelia and brown sclerotia on crowns and surrounding areas on the soil surface. We estimated 9.74% of plants in 144 sugar beet plots were infected with Sclerotium rolfsii that showed root rot symptom. Isolation of the fungus from infected tissues, and obtaining pure culture was carried out following standard protocol on potato dextrose agar (PDA) medium. Pure culture on PDA produced fluppy cottony mycelia with clamp connections and brown sclerotia ranging from 0.5 to 1.5 mm, typical of S. rolfsii within 3 weeks of incubation at ambient laboratory temperature (22 ± 2 °C). We further confirmed the identity of two isolates BTSB2 and BTSB6 by performing phylogenetic analysis of the internal transcribed spacer sequences along with morphological features. We also proved Koch’s postulate to confirm that isolate of S. rolfsii is the causal agent of sugar beet root rot, not a secondary fungal contaminant. Isolate BTSB2 grew well on PDA and OMA with an average radial growth rate of 2.3 and 2.1 mm/day, respectively. We found mannitol and glucose as the best carbon sources to support radial growth, whereas sucrose and starch supported maximum sclerotia production on synthetic medium. Highest radial growth was favored by media pH 5.0–7.0 at temperature 25 °C. This study characterized S. rolfsii as a causal agent of root rot of sugar beet in Bangladesh by detailed morphological and molecular investigations. Optimization of factors that support quick growth of the pathogen on synthetic media will help in easy production of inocula required for large-scale future inoculation studies.

Soil-plant–microbes relations within the plant rhizosphere are the determinants of plant and soil health, which is important for soil ecological environment for plant-microbe interactions. Plant growth-promoting rhizobacteria (PGPR) are considered to encourage plant growth and development directly or indirectly in soil. PGPR can demonstrate a diversity of characteristics responsible .for influencing plant growth and development. During this study, Twenty four different bacterial isolates were isolated, and detailed morphological, biochemical, and physiological characterizations of those isolates were accomplished. This experiment was performed with the 24 bacterial isolates to see their gram stain test, KOH test, catalase activity, cellulose degradation capability, in dole acetic acid (IAA) production, and phosphate solubilization activities, and also tested for growth within the different arsenic and salt stress conditions and 37°C temperature. Results revealed that among the rhizobacterial isolates, fifteen bacterial isolates were negative and nine was positive in gram reaction, while some were showed high IAA production ability, phosphate solubility capability, and cellulose degradation capacity within the culture media. The isolates were isolated from paddy soils and a few were characterized by a yellow color, flat elevation, and gram-positive, while some were characterized because of the yellowish color with round colony shape, raised elevation, gram-negative, and every one the isolates were positive in catalase production capacity and phosphate solubilization activity which is able to increase the available phosphorus within the soil for plants and also produced indole acetic acid that may use as a hormone to be used in growth enhancement of plants. Hence, these isolates need to be tested further for their effect on arsenic dynamics at the plant rhizosphere, selection of suitable plant species for the bacterial association, bacterial effect on arsenic uptake by plants, and potentials for field applications for sustainable agriculture.

Molecular mapping and application of quantitative trait loci (QTL) associated with a higher level of grain Zinc is a viable option to enhance zinc content in rice through breeding. An F2 population derived from a cross between a high yielding variety, BRRI dhan28, and a locally adapted Zn enriched cultivar, Kalobokri was used to map QTLs associated with higher levels of Zn in rice grain. The F2:3 progenies varied significantly (P ＜ 0.0001) in Zinc contents with a mean value remarkably higher than those in the superior parent. Through marker by trait analysis using IciMapping, we detected a large-effect QTL, qGZn3 on chromosome 3 between RM5419 and RM1164 spanning 1.83 Mb interval at the 0.05 level of significance with a threshold LOD of 10.61. This QTL showed a21.1% (R2 value) contribution to the total phenotypic variation for zinc content in the unpolished rice grains with 4.68 μg/g additive effect of Kalobokri alleles. We also detected 11 metal homeostasis related genes within the interval of qGZn3. In-silico analysis showed that four expressed sequence tags of one candidate gene (LOC_Os03g22810) encoding Cu/Zn superoxide dismutase, a metal-binding protein, are highly active in the endosperm as well as in the embryonic tissue of rice grain. Taken together, our results suggest that qGZn3 is a major QTL associated with high grain Zn content in the F2 progenies of rice. Our findings offer valuable genetic resources to facilitate breeding for high yielding and Zinc-enriched rice variety.

Wheat blast is a fungal disease of wheat which is caused by a host-specialized ascomycete Magnaporthe oryzae Triticum (MoT) pathotype (Anamorph Pyricularia oryzae Triticum). It was first discovered in Paraná state of Brazil in 1985 and then gradually spread in wheat growing areas in several countries in Brazil, Bolivia, Paraguay and Argentina. It is a fearsome disease of wheat which causes yield loss up to 100%. It is now a threat to wheat production in about 3 million hectares of wheat growing areas in South America. Application of fungicides to control this disease is unreliable and breeding resistant varieties is also not durable. At conducive environmental conditions (18-30°C and > 80% RH) during ear emergence or grain filling, this disease can become an epidemic and devastate wheat crop within a week. In 2016, MoT emerged as an epidemic in eight districts of Bangladesh and has so far destroyed more than 15,000 hectares of wheat. The wheat blast pathogen in Bangladesh is determined as a South American lineage of M. oryzae. Scientists fear that MoT may spread to neighbouring South Asian wheat-growing countries and thus pose a serious threat to food and nutritional security of South America.

A lot of variations in vegetable amaranth germplasm have been observed in Bangladesh. It has been used as a cheap source of antioxidants, nutrients, protein, and dietary fiber. But no efforts had not been taken to know the status of antioxidant content, polyphenol, flavonoid, antioxidant vitamins and minerals, dietary fiber, nutritional and agronomic traits. In this study, Forty-three vegetable amaranth genotypes were evaluated to determine the status of total antioxidant content, polyphenol, flavonoid, antioxidant vitamins and minerals, dietary fiber, nutritional and agronomic traits and the magnitude of genetic diversity based on the contribution of those traits for meaningful grouping and proper utilization in future breeding program. The experiment was carried out in an open experimental field at Bangabandhu Sheikh Mujibur Rahman Agricultural University, Bangladesh in a randomized complete block design with three replications. Multivariate (Principal component and cluster) analysis was done us...

The opportunistic fish pathogen, Enterococcus faecalis has been reported to cause mass mortality in several fish species in different countries. The objectives of this study were to (i) identify E. faecalis from the diseased fishes through molecular techniques; (ii) assess the antibiotic susceptibility profile of E. faecalis isolates; and (iii) control disease in tilapia fish by treatment with medicinal plant extracts. A total of 48 isolates were phenotypically identified as Enterococcus species from tilapia, stinging catfish and walking catfish cultivated in several fish farms in Gazipur. Ten randomly selected isolates were identified as E.faecalis by 16S rRNA gene sequencing. Artificial infection revealed that most of the isolates caused moderate to high mortality in fishes with characteristic disease symptoms. These isolates exhibited resistance to multiple antibiotics in vitro. Bioassay revealed that organic extracts of Tamarindus indica and Emblica officinalis leaves, Allium sativum bulb, and Syzygium aromaticum bud inhibited the growth of E. faecalis. Methanol extracts of A. sativum and methanol and acetone extracts of S. aromaticum significantly reduced the mortality of fish artificially infected with E. faecalis as both preventive and therapeutic agents. This is the first report on molecular identification, and herbal control of fish pathogenic E. faecalis in Bangladesh.

Ansary M.W.R., Haque E., West H.M., Rahman M.M., Akanda A.M., Wang Y., Islam M. T. (2016): Medicinal plant extracts and protein kinase C inhibitor suppress zoosporogenesis and impair motility of Phytophthora capsici zoospores. Plant Protect. Sci., 52: 113–122. The effects of water and acetone extracts from 100 medicinal plants growing in Bangladesh, along with a selective inhibitor of protein kinase C (PKC), chelerythrine chloride, were tested on zoosporogenesis (release of zoospores from the sporangia) and motility of Phytophthora capsici zoospores. Among 10 active crude acetone extracts, those from Psidium guajava and Nigella sativa (100 μg/ml) suppressed zoosporogenesis relative to the control (100% zoospore release) to 60 and 40% released, respectively and inhibited motility of 100% of the zoospores within 60 min of treat ment. Chelerythrine chloride also suppressed zoosporogenesis (30% released) at 0.1 μg/ml and inhibited motility of 100% zoospores at 0.2 μg/ml within 60 minutes. Among water extracts of 100 medicinal plants, 56 impaired motility of zoospores in a dose- and time-dependent manner. Diluted (20-fold) water extracts of 10 plants including Ocinum gratissimum, Terminalia bohera , and Duranta plumeri inhibited motility and subsequently caused lysis of zoospores.

Dragon fruit (Hylocereus polyrhizus) is a high value newly introduced fruit crop in Bangladesh. It has drawn considerable public attention due to its appealing flesh color, sweet taste and fruit qualities. Recently, basal rot of dragon fruit plants was observed in several farmer's fields, nurseries and in the research field of Bangabandhu Sheikh Mujibur Rahman Agricultural University (BSMRAU) where about 10-15% of plants were infected in each location. Initially, the symptoms appeared in the basal part near the soil as brown lesions which gradually extended to the upper stem and finally becoming soft and watery (Figure 1a). Infected plants were collected from Kapasia of Gazipur district (Latitude 24.266 and Longitude 90.633) to isolate the causal organism. Isolations were carried out following the procedure reported by Briste et al. (2019). Briefly, infected plant parts were surface sterilized in 2% NaOCl for 1 min followed by 70% ethanol for 5 min and rinsed 3 times with sterile double distilled water. A large piece of a surface sterilized plant was cut into small pieces (2 mm × 2 mm) from the margin of the necrotic lesion and placed on half strength potato dextrose agar (PDA) and incubated for 7 days at 25 °C. The BTFD1 and BTFD4 isolates were purified from single spores resulting in white colonies with a growth rate of 1cm/day on PDA (Figure 1b). Colonies produced single celled microconidia from unbranched, short monophialidic conidiophores and septate macroconidia as well as chlamydospores in PDA which is consistent with Fusarium oxysporum (Figure 1c). To confirm the identity of the isolates, the internal transcribed spacer (ITS1, 5.8S rRNA and ITS2) and translation elongation factor-1alpha (EF-1α) were amplified using primers ITS-1/ ITS-4 and EF1-728F/ EF1-986R, respectively (Surovy et al. 2018). The ITS sequences of the isolates BTFD1 and BTFD4 (GenBank accession # MN727096 and MN727095, respectively) showed 100% similarity with the sequence from F. oxysporum strain JJF2 (MN626452). Sequence identity for EF-1α (GenBank accession # MN752123 and MN752124, respectively) was 100% with the sequence from F. oxysporum strain CAV041_EO (MK783088). The isolates (BTFD1 and BTFD4) were identified as F. oxysporum based on the aligned sequences of ITS and EF-1α, molecular phylogenetic analyses by maximum likelihood tree (Figure 2a) and maximum parsimony tree methods (Figure 2b). The isolates were stored at 4°C on dried filter paper as well as in an ultra-low temperature freezer (-80°C) at IBGE, BSMRAU, Bangladesh and are available on request. To ensure pathogenicity, isolate BTFD1 was grown on PDA, incubated at 25°C for 7 days and 250 ml conidial suspension (with 1 × 105 conidia/ml) was prepared. Twelve,three-month-old healthy dragon fruit plants were inoculated. Pathogenicity tests were carried out in two sets using three replications in each set. In one set, only the basal part of the plants was dipped into the conidial suspension and in another set the whole plant was dipped into the conidial suspension for two hours. Sterile distilled water was also used in another set of plants as a control. The inoculated plants were placed on wet tissue in a plastic box (31cm × 24cm × 8cm) covered and incubated at 25°C. After 10 days, all inoculated plants in both sets developed rot symptoms similar to those observed in the field, while the control plants remained healthy (Figure 1d). The pathogen was successfully re-isolated from the inoculated symptomatic parts on half strength PDA medium and had morphology as characterized before, thus fulfilling Koch's postulates. This disease has been reported in Argentina and Malaysia (Wright et al. 2007; Hafifi et al. 2019). To the bet of our knowledge, this is the first report of Fusarium basal rot of dragon fruit in Bangladesh caused by F. oxysporum.

Phytophthora capsici is the most destructive peronosporomycete pathogen that causes damping-off and blight on many vegetable crops including pepper and cucurbits worldwide resulting in serious economic losses. This study was conducted to isolate and characterize novel beneficial bacteria from cropping field soils and evaluate their effects on several asexual life stages of P. capsici in vitro, and if these bacteria could suppress damping-off disease in vivo. Out of 30 bacterial isolates from this study, B-1, B-10 and B-17 significantly inhibited hyphal growth through induction of excessive branching, swelling, and cellular disintegration of P. capsici. These isolates were identified as Pseudomonas spp. through 16S rRNA gene sequencing. Live cells or culture supernatants of these antagonists also significantly suppressed formation of sporangia, release of zoospores and mycelial growth in liquid culture as revealed by dry weight of the pathogen. Interestingly, zoospores of P. capsici became immotile within a very short period of time after exposure to the 10-fold diluted culture supernatants of B-1, B-10 and B-17. However, at higher concentration of culture supernatants (5-fold diluted), part of the immotile zoospores lysed within 30–60 min. Cucumber plants grown from Pseudomonas treated seeds showed enhanced resistance to the damping-off disease caused by P. capsici compared with plants grown from non-treated seeds. Results from this study demonstrated that Pseudomonas spp. from native agro-ecological niche have great potential for controlling cucumber damping-off disease and may be used in integrated disease control to reduce reliance on synthetic chemicals.

This investigation was aimed to evaluate 22 promising vegetable amaranth genotypes for antioxidant, leaf quality, and agronomic traits for two test seasons. The experiment was conducted to study the degree of genetic parameters, associations among different traits, and their contribution towards foliage yield. The analysis of variance for twelve traits was found highly significant. High mean, high range of variability, and high genotypic variance were observed for all the traits except content of Ca, protein, and carotenoid. Close differences between genotypic and phenotypic variances and genotypic and phenotypic coefficients of variation were observed for all the traits. High to moderate genotypic coefficients of variation and heritability coupled with high to moderate genetic advance in percent of mean was observed for all the traits. Considering all genetic parameters, selection based on Fe, Zn, Mn, ascorbic acid content, fiber content, plant height, leaves plant -1 , diameter of stem base, and foliage yield would be effective for the improvement of vegetable amaranth. Foliage yield had a significant positive genotypic correlation with three antioxidant traits (ascorbic acid, Fe, and Mn), two leaf quality traits (protein and fiber), and three agronomic traits (plant height, leaves plant -1 , and stem base diameter). Fiber content, leaves plant -1 , plant height, and stem base diameter had high positive direct effects and Fe, Mn, and carotenoid exhibited moderate positive direct effects on foliage yield. Based on genetic interrelationships and path coefficient values, direct selection for antioxidants (Fe and Mn), fiber content, leaves plant -1 , plant height, and stem base diameter would significantly improve the foliage yield of vegetable amaranth. Selection based on protein content concomitantly required considering the Fe and Ca contents of the genotypes.

Antifungal resistance and toxicity problems of conventional fungicides highlighted the requirement of search for new safe antifungal agents. To comply with the requirement, we discovered four new non-cytotoxic lipopeptides, gageopeptides A-D, 1-4, from a marine-derived bacterium Bacillus subtilis. The structures and stereochemistry of gageopeptides were determined by NMR data analysis and chemical means. Gageopeptides exhibited significant antifungal activities against pathogenic fungi Rhizoctonia solani, Botrytis cinerea, and Colletotrichum acutatum with minimum inhibitory concentration (MIC) values of 0.02-0.06 μM. In addition, these lipopeptides showed significant motility inhibition and lytic activities against zoospores of the late blight pathogen Phytophthora capsici. These compounds also showed potent antimicrobial activity against Gram positive and Gram negative bacteria with MIC values of 0.04-0.08 μM. However, gageopeptides A-D did not exhibit any cytotoxicity (GI5025 μM) against cancer cell lines in sulforhodamine B (SRB), 3-[4,5-dimethylthiazol-2-yl]-2,5-diphenyltetrazolium bromide (MTT), and WST-1 ((4-[3-4-iodophenyl]-2-(4-nitrophenyl)-2H-5-tetrazolio)-1,3-benzene disulfonate)) assays, demonstrating that these compounds could be promising candidates for the development of non-cytotoxic antifungal agents.

Phosphorus (P) fixation limits availability of P to plants in tropical soil, which is a major constraint for crop production. The aim of our research was to isolate, screen and characterize phosphate solubilizing bacteria (PSB) from wheat and evaluate their efficacy in P nutrition in wheat. Upon screening, 9 isolates showing varying level of phosphate solubilizing activity in both agar plate and broth assays using Pikovskaya’s medium were obtained. The pH of the culture media was decreased with increased bacterial growth suggesting that they might secrete organic acids to solubilize insoluble phosphorus. In vitro wheat seedling bioassay with two superior PSB isolates (PSB1 and PSB8) and varying sources of P revealed that both isolates significantly enhanced seedling growth (shoot and root length, shoot and root dry weight) and nutrient contents (%N, %P and %K) in plant tissue compared to control (no PSB). The performance of PSB8 was superior to PSB1 in respect of all the parameters studied. The PSB8 was tentatively identified as Pseudomonas sp. through 16S rRNA gene sequencing. Our results suggest that Pseudomonas sp. PSB8 isolated from wheat might be useful for improving P nutrition in wheat in soils with low available P.

Strawberry (Fragaria × ananassa) is a newly introduced small fruit crop in Bangladesh, which has gained considerable interest from both growers and consumers. Increasing incidence of wilt and crown rot diseases is a challenge for strawberry growers. In March 2016, wilted foliage and dead strawberry plants of cv. Strawberry Festival were found in a strawberry nursery in Rajshahi district of Bangladesh affecting 10% of the plants. Symptoms included wilting of foliage, drying and withering of older leaves, stunted growth, and reduced fruit production, leading to eventual plant collapse. A lengthwise cut through the crown of diseased plants showed distinct brown to reddish discoloration similar to that described by Koike et al. (2009). The crowns were surface sterilized in 2% NaOCl for 1 min followed by 70% ethanol for 5 min. Small pieces (2 × 2 mm) of the internal discolored tissue were placed on half-strength potato dextrose agar (PDA) medium and incubated for 7 days at 25°C. Single-spore isolates (BRSP-02 and BRSP-12) were obtained and plated to PDA, cornmeal agar, and V8 juice agar. Isolates had a growth rate 0.8 cm/day on PDA medium and produced sporodochia with septate macroconidia. Colonies also produced single-celled microconidia from unbranched, short, monophialidic conidiophores on PDA consistent with Fusarium oxysporum (Nelson et al. 1983). The internal transcribed spacer (ITS1, 5.8S rRNA, and ITS2), actin (ACT), and translation elongation factor 1-alpha (EF-1α) of the isolates were amplified using primers ITS-1/ITS-4, ACT-512F/ACT-783R, and EF1-728F/EF1-986R, respectively (Carbone and Khon 1999; White et al. 1990). ITS sequences of the isolates BRSP-02 and BRSP-12 (GenBank accession nos. LC383470 and LC383471, respectively) showed 99% similarity with the sequence from F. oxysporum strain SHBV2 (KY090783). Sequence identity for EF-1α (GenBank accession nos. LC383472 and LC383473, respectively) was 98 to 99% with the sequence from F. oxysporum isolate BRIP59170 (KX434915). The ACT sequences (GenBank accession nos. LC383784 and LC383785, respectively) showed 97 to 98% similarity with the sequence from F. oxysporum isolate C-11 (KY798318). Based on the aligned sequences of ITS and EF-1α, molecular phylogenetic analysis by the maximum likelihood method and maximum parsimony method, the isolates (BRSP-02 and BRSP-12) were named as F. oxysporum. The isolates are archived in the Department of Biotechnology at Bangabandhu Sheikh Mujibur Rahman Agricultural University in Bangladesh on dried filter paper at 4°C and are available on request. To confirm the pathogenicity of the fungus, five 3-month-old tissue-cultured ‘Strawberry Festival’ plants were inoculated by dipping the roots into either 400 ml of conidial suspension (with 5 × 10⁶ conidia/ml) of the isolate BRSP-12 or into sterile distilled water (control) for 15 min. The inoculated plants were transplanted into plastic pots with artificial peat-based soil mix and maintained in the greenhouse with a high daytime temperature of 25°C and low nighttime temperature of 20°C. After 3 weeks, all plants inoculated with isolate BRSP-12 were stunted and developed wilt symptoms similar to those observed in the field, whereas the control plants remained healthy. Plants were then dug out, and crowns were cut longitudinally to check for internal discoloration. Light orange discoloration of crown tissues was observed in inoculated plants, but all the control plants remained symptomless. The fungus was successfully reisolated on half-strength PDA medium from 100% of crowns and had morphology as characterized before, fulfilling Koch’s postulates. To the best of our knowledge, this is the first report of Fusarium wilt of strawberry in Bangladesh. This disease has been reported from a number of other countries including Argentina, Australia, China, South Korea, Spain, the United States, and Japan (Koike et al. 2009; Williamson et al. 2012).

Plant growth promoting rhizobacteria (PGPR) are the rhizosphere bacteria that may be utilized to augment plant growth and suppress plant diseases. The objectives of this study were to identify and characterize PGPR indigenous to cucumber rhizosphere in Bangladesh, and to evaluate their ability to suppress Phytophthora crown rot in cucumber. A total of sixty six isolates were isolated, out of which ten (PPB1, PPB2, PPB3, PPB4, PPB5, PPB8, PPB9, PPB10, PPB11 and PPB12) were selected based on their in vitro plant growth promoting attributes and antagonism of phytopathogens. Phylogenetic analysis of 16S rRNA sequences identified these isolates as new strains of Pseudomonas stutzeri, Bacillus subtilis, Stenotrophomonas maltophilia and B. amyloliquefaciens. The selected isolates produced high levels (26.78 to 51.28 μg mL−1) of indole-3-acetic acid, while significant acetylene reduction activities (1.79 to 4.9 µmole C2H4 mg-1 protein h-1) were observed in eight isolates. Cucumber plants grown from seeds that were treated with these PGPR strains displayed significantly higher levels of germination, seedling vigor, growth, and N content in root and shoot tissue compared to non-treated control plants. All selected isolates were able to successfully colonize the cucumber roots. Moreover, treating cucumber seeds with these isolates significantly suppressed Phytophthora crown rot caused by Phytophthora capsici, and characteristic morphological alterations in Ph. capsici hyphae that grew towards PGPR colonies were observed. Since these PGPR inoculants exhibited multiple traits beneficial to the host plants, they may be applied in the development of new, safe, and effective seed treatments as an alternative to chemical fungicides.

The aim of this study was to investigate the effects of salinity on mineral nutrient distribution along roots and shoots of seven selected rice (Oryza sativa L.) genotypes, namely Pokkali, PVSB9, PVSB19, PNR381, PNR519, NS15 and Iratom24 differing in salt tolerance. The rice plants were grown in pots and subjected to varying levels of salinity stress (0, 3, 6, 9, 12 and 15 dS m−1). Mineral nutrients distribution in shoots and roots was measured after harvesting the plants at 95 days after transplanting. The responses of salt stress on mineral nutrient uptake and distribution along plant organs significantly differed among the rice genotypes. The contents of Na+ and Cl− in the roots and shoots of resistant genotypes (PVSB9, PNR381 and Pokkali) were significantly lower than the susceptible rice genotype (NS15). The concentrations of Na+ and Na+/K+ in shoots of sensitive rice genotype (NS15) sharply increased with increasing salinity above 6 dS m−1 than those of tolerant genotypes. The highest concentration ...

The biocontrol bacterium Lysobacter sp. SB-K88 suppresses damping-off disease in sugar beet and spinach caused by Aphanomyces cochlioides and Pythium sp. through characteristic plant colonization and antibiosis against the pathogens. This study aimed to unravel further details on mode of antagonism of SB-K88 against a damping-off pathogen A. cochlioides AC-5. The SB-K88 substantially inhibited growth and decomposed AC-5 mycelia and suppressed the release of zoospores from the hyphae. The excised root tips of sugar beet seedlings from seeds previously inoculated with SB-K88 were less attractive to AC-5 zoospores. Although aerial growth was not affected, however, root hairs of SB-K88 inoculated sugar beet seedlings were remarkably shorter and thicker than those of uninoculated control. When exposed to zoospores, the SB-K88 inhibited motility of zoospores and/or caused lysis, and then aggregated around the dead cystospores or lysed residues within 3–6 h likely to be micro-predatory behavior to a eukaryotic organism. Confocal laser scanning microscopic analysis revealed that number of lipid bodies and activities of mitochondria were markedly increased in the affected hyphae compared with control hyphae as visualized by established vital stains. Taken together, these results suggest that Lysobacter sp. SB-K88 suppresses damping-off diseases through exerting multifaceted antagonistic effects against the peronosporomycetes.

We previously demonstrated that xanthobaccin A from the rhizoplane bacterium Lysobacter sp. strain SB-K88 suppresses damping-off disease caused by Pythium sp. in sugar beet. In this study we focused on modes of Lysobacter sp. strain SB-K88 root colonization and antibiosis of the bacterium against Aphanomyces cochlioides , a pathogen of damping-off disease. Scanning electron microscopic analysis of 2-week-old sugar beet seedlings from seeds previously inoculated with SB-K88 revealed dense colonization on the root surfaces and a characteristic perpendicular pattern of Lysobacter colonization possibly generated via development of polar, brush-like fimbriae. In colonized regions a semitransparent film apparently enveloping the root and microcolonies were observed on the root surface. This Lysobacter strain also efficiently colonized the roots of several plants, including spinach, tomato, Arabidopsis thaliana , and Amaranthus gangeticus . Plants grown from both sugar beet and spinach seeds that were previously treated with Lysobacter sp. strain SB-K88 displayed significant resistance to the damping-off disease triggered by A. cochlioides . Interestingly, zoospores of A. cochlioides became immotile within 1 min after exposure to a SB-K88 cell suspension, a cell-free supernatant of SB-K88, or pure xanthobaccin A (MIC, 0.01 μg/ml). In all cases, lysis followed within 30 min in the presence of the inhibiting factor(s). Our data indicate that Lysobacter sp. strain SB-K88 has a direct inhibitory effect on A. cochlioides , suppressing damping-off disease. Furthermore, this inhibitory effect of Lysobacter sp. strain SB-K88 is likely due to a combination of antibiosis and characteristic biofilm formation at the rhizoplane of the host plant.

In a survey of plant secondary metabolites regulating the behavior of phytopathogenic Aphanomyces cochlioides zoospores, we found that leaf extracts of Amaranthus gangeticus and cotyledon extracts of pea (Pisum sativum) remarkably halted the motility of zoospores. Bioassay-directed fractionation of A. gangeticus and pea constituents revealed that the halting activity was dependent on a single chemical factor (halting factor). The active principle was identified as nicotinamide (1) by comparing its biological activity and spectroscopic properties with those of the authentic compound. Nicotinamide (1) showed potent halting activity toward the zoospores of A. cochlioides and A. euteiches, but it exhibited very less activity against other Oomycetes, Pythium aphanidermatum and Phytophthora infestans zoospores. Interestingly, the zoospores halted by nicotinamide (1) encysted within 10-15 min and then the resulting cystospores regenerated zoospores instead of germination. Nicotinamide (1) and related compounds were subjected to the halting activity bioassay to elucidate the structure- activity relationships. These bioassays revealed that part structures of (A) the aromatic ring containing at least one nitrogen atom, (B) carbonyl-like group adjacent to the aromatic ring and (C) hydrogen atoms on the amide group are responsible for the strong activity. So far, this is the first report of halting activity of nicotinamide (1) against fungal zoospores

Peronosporomycetes are devastating pathogens to numerous crop, ornamental, and native plants. They are phylogenetically distinct from those of fungi and hence most of the fungicides are ineffective against them. A large body of literature reveals that several bacterial genera such as Pseudomonas, Bacillus, Burkholderia, Lysobacter, Enterobacter, etc. exert antagonistic activities against the peronosporomycete phytopathogens in both in vitro and in vivo conditions. These bacterial strains originated from diverse habitats and some of them showed high promise for biocontrol of plant diseases caused by these notorious pathogens. Mechanisms of biocontrol by these bacterial antagonists include (1) antibiosis, including biosurfactant activity, (2) secretion of lytic enzymes, (3) competition for nutrients (C and Fe), (4) high plant and hyphal colonization, (5) hyperparasitism, and (6) development of induced systemic resistance in the host plants. This chapter comprehensively reviews advances of research on biocontrol of peronosporomycete phytopathogens by bacterial antagonists including the mode of actions of the antagonistic principles against the pathogens. Recent advances on genome sequencing of several peronosporomycetes and biocontrol agents will provide basis for better understanding of bacteria–plant–pathogen interactions and development of improved strains that will potentially function as effective biocontrol agents against the notorious peronosporomycete phytopathogens for low input sustainable agriculture.

Low level of soluble soil phosphorus (P) is a serious constraint in crop production in tropical and subtropical soils. Many plant-associated bacteria or plant probiotics can solubilize P from either organic- or inorganic-bound phosphates thereby facilitating plant growth. Understanding the bacterial contribution to plant P nutrition and opportunities for manipulating specific bacterial strain to enhance P availability in soil has, therefore, been of considerable interest over many decades. This interest is accentuated by rising costs of P fertilizer and because of low efficiency of P use by plants from soil and fertilizer sources. Bacteria from diverse taxonomic genera such as Pseudomonas, Bacillus, Klebsiella, Streptomyces, Burkholderia, Pantoea, Enterobacter, etc., can solubilize soil insoluble P and increase growth and yield of crops. The widely recognized mechanisms of phosphate solubilization mediated by these plant-associated bacteria are production of organic acids (such as gluconic, citric, and oxalic) and/or secretion of hydrolytic enzymes (such as phytases, phosphatase, etc.). Despite their potential as low-input practical agents for plant P nutrition, application of phosphate solubilizing bacteria (PSB) has been hampered by their inconsistent performance in the field. Hence, the full potentials of PSB have not yet been achieved for P nutrition in major crop production. Therefore, better understanding on how they interact with roots and other organisms in the rhizosphere is needed. This chapter reviews advances on PSB research and their potential for phosphorus nutrition in crop plants, with special reference to rice. Mode of action of phosphate solubilization by bacteria and their uses as biofertilizer for eco-friendly low-input sustainable crop production are also discussed.

The Lysobacter spp. are ubiquitous inhabitants in the diverse environment that have some unique features including gliding motility and high genomic G + C ratio (65–72%). They produce a wide variety of antimicrobial compounds (lytic enzyme and antibiotics) and thus have high promise to biocontrol of plant diseases caused by fungi, peronosporomyctes, nematodes, and bacteria. Among 21 identified species, biocontrol activity of Lysobacter enzymogenes strains C3 and 3.1T8 and Lysobacter sp. SB-K88 has extensively been investigated. Mechanisms of biocontrol of phytopathogens by Lysobacter spp. include (1) secretion of lytic enzymes and/or antibiotics including tetramic acid-containing macrocyclic lactams, (2) high plant colonization through characteristic perpendicular attachment, (3) development of induced systemic resistance in host plants, (4) hyperparasitism involving types III, IV, and VI secretion systems, and (5) exhibition of micropredatory behavior to the pathogens. Lysobacter spp. were also found as functional bacteria in the disease-suppressive soils. This chapter reviews the advances of research on biocontrol activity of plant diseases by bacterial antagonists Lysobacter spp., with special reference to Lysobacter sp. SB-K88. Mode of antagonism and potential use of Lysobacter spp. as biological control agents are also discussed.