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Phosphate (PO43-, Pi) is the sole phosphorus (P) containing compound utilized for plant growth, leading to the widespread use of Pi containing fertilizers. An alternative form of P, phosphite (PO33-, Phi) has increasingly been used in many crop systems, including amenity turfgrass, not only as a nutrient source but also as a pesticide and biostimulant. There are, however, conflicting reports of the efficacy and benefits of Phi as a source of P. This study was conducted to determine the rate of uptake, translocation and fate of Phi when applied as a foliar treatment to amenity turfgrass, and to assess its role as a source of P nutrition, determining the effect Phi treatments have on turfgrass growth, P deficiency responses, tissue and soil P accumulations. Analysis of Phi treated turfgrass using High Performance Ion Chromatography determined that Phi is rapidly taken up and translocated, that sequential Phi treatments lead to cumulative increases in meristematic tissues, an increase in soil P levels and no in planta conversion to Pi. In P sufficient rootzones (> 35 ppm), foliar applied Phi increased biomass in shoot, crowns, and roots, but also led to a reduction in root to shoot ratios. In phosphorus deficient rootzones (< 5 ppm), foliar applied Phi led to growth reductions in leaf, crown and root, and repression of P deficiency responses.

© 2018 British Society for Plant Pathology The ascomycete fungus Microdochium nivale is a major pathogen of many species of the gramineae. Control measures rely heavily on chemical fungicides, making alternative means of disease reduction desirable. Phosphite (PO33−), has proven efficacy in reducing susceptibility of different species of gramineae to oomycetes, and has adverse effects on the invitro growth of numerous other pathogens. The effect of phosphorous acid (H3PO3), phosphoric acid (H3PO4), dihydrogen potassium phosphite (KH2PO3), dihydrogen potassium phosphate (KH2PO4) and potassium hydroxide (KOH) on the invitro mycelial growth and development of M. nivale was determined. Radial growth on amended potato dextrose agar (PDA) was used to calculate mean daily growth and percentage inhibition. PO33− had a significant inhibitory effect on mycelial growth, with EC50 values ranging between 35.9 and 40.99μgmL−1, whilst PO43− and KOH had no significant inhibitory effect. Microscopic examination of mycelia showed morphological deformities in hyphae growing on PO33− amended PDA, whilst hyphal growth was normal on PO43− and KOH amended PDA. Conidial germination of M. nivale was significantly reduced following immersion in solutions of 50, 100 and 250μgmL−1 of PO33−, while PO43− and KOH at the same concentrations induced no inhibitory affect. These results show that PO33− is a significant inhibitor of the growth of M. nivale and may have the potential to be used as a chemical control agent in the field.

The co-evolution of bacterial plant pathogens and their hosts is a complex and dynamic process. Host resistance imposes stress on invading pathogens that can lead to changes in the bacterial genome enabling the pathogen to escape host resistance. We have observed this phenomenon with the plant pathogen Pseudomonas syringae pv. phaseolicola where isolates that have lost the genomic island PPHGI-1 carrying the effector gene avrPphB from its chromosome are infective against previously resistant plant hosts. However, we have never observed island extinction from the pathogen population within a host suggesting the island is maintained. Here we present a mathematical model which predicts different possible fates for the island in the population; one outcome indicated that PPHGI-1 would be maintained at low frequency in the population long term if it confers a fitness benefit. We empirically tested this prediction and determined that PPHGI-1 frequency in the bacterial population drops to a low but consistently detectable level during host resistance. Once PPHGI-1-carrying cells encounter a susceptible host, they rapidly increase in the population in a negative frequency dependent manner. Importantly, our data show that mobile genetic elements can persist within the bacterial population and increase in frequency under favourable conditions.

Bacterial microcolonies with heterogeneous sizes are formed during colonization of Phaseolus vulgaris by Pseudomonas syringae. Heterogeneous expression of structural and regulatory components of the P. syringae type III secretion system (T3SS), essential for colonization of the host apoplast and disease development, is likewise detected within the plant apoplast. T3SS expression is bistable in the homogeneous environment of nutrient-limited T3SS-inducing medium, suggesting that subpopulation formation is not a response to different environmental cues. T3SS bistability is reversible, indicating a non-genetic origin, and the T3SSHIGH and T3SSLOW subpopulations show differences in virulence. T3SS bistability requires the transcriptional activator HrpL, the double negative regulatory loop established by HrpV and HrpG, and may be enhanced through a positive feedback loop involving HrpA, the main component of the T3SS pilus. To our knowledge, this is the first example of phenotypic heterogeneity in the expression of virulence determinants during colonization of a non-mammalian host. Ministerio de Ciencia e Innovación BIO2009-11516, AGL2010-22287-C02-2 Ministerio de Economía y Competitividad BIO2012-35641, BIO2015-64391- R

Pseudomonas syringae pv. phaseolicola causes halo blight of the common bean, Phaseolus vulgaris, worldwide and remains difficult to control. Races of the pathogen cause either disease symptoms or a resistant hypersensitive response on a series of differentially reacting bean cultivars. The molecular genetics of the interaction between P. syringae pv. phaseolicola and bean, and the evolution of bacterial virulence, have been investigated in depth and this research has led to important discoveries in the field of plant–microbe interactions. In this review, we discuss several of the areas of study that chart the rise of P. syringae pv. phaseolicola from a common pathogen of bean plants to a molecular plant‐pathogen supermodel bacterium. Taxonomy: Bacteria; Proteobacteria, gamma subdivision; order Pseudomonadales; family Pseudomonadaceae; genus Pseudomonas; species Pseudomonas syringae; Genomospecies 2; pathogenic variety phaseolicola. Microbiological properties: Gram‐negative, aerobic, motile, rod‐shaped, 1.5 µm long, 0.7–1.2 µm in diameter, at least one polar flagellum, optimal temperatures for growth of 25–30 °C, oxidase negative, arginine dihydrolase negative, levan positive and elicits the hypersensitive response on tobacco. Host range: Major bacterial disease of common bean (Phaseolus vulgaris) in temperate regions and above medium altitudes in the tropics. Natural infections have been recorded on several other legume species, including all members of the tribe Phaseoleae with the exception of Desmodium spp. and Pisum sativum. Disease symptoms: Water‐soaked lesions on leaves, pods, stems or petioles, that quickly develop greenish‐yellow haloes on leaves at temperatures of less than 23 °C. Infected seeds may be symptomless, or have wrinkled or buttery‐yellow patches on the seed coat. Seedling infection is recognized by general chlorosis, stunting and distortion of growth. Epidemiology: Seed borne and disseminated from exudation by water‐splash and wind occurring during rainfall. Bacteria invade through wounds and natural openings (notably stomata). Weedy and cultivated alternative hosts may also harbour the bacterium. Disease control: Some measure of control is achieved with copper formulations and streptomycin. Pathogen‐free seed and resistant cultivars are recommended. Useful websites: Pseudomonas–plant interaction http://www.pseudomonas‐syringae.org/; PseudoDB http://xbase.bham.ac.uk/pseudodb/; Plant Associated and Environmental Microbes Database (PAMDB) http://genome.ppws.vt.edu/cgi‐bin/MLST/home.pl; PseudoMLSA Database http://www.uib.es/microbiologiaBD/Welcome.html

The co-evolution of bacterial plant pathogens and their hosts is a complex and dynamic process. Plant resistance can impose stress on invading pathogens that can lead to, and select for, beneficial changes in the bacterial genome. The Pseudomonas syringae pv. phaseolicola (Pph) genomic island PPHGI-1 carries an effector gene, avrPphB (hopAR1), which triggers the hypersensitive reaction in bean plants carrying the R3 resistance gene. Interaction between avrPphB and R3 generates an antimicrobial environment within the plant, resulting in the excision of PPHGI-1 and its loss from the genome. The loss of PPHGI-1 leads to the generation of a Pph strain able to cause disease in the plant. In this study, we observed that lower bacterial densities inoculated into resistant bean (Phaseolus vulgaris) plants resulted in quicker PPHGI-1 loss from the population, and that loss of the island was strongly influenced by the type of plant resistance encountered by the bacteria. In addition, we found that a number of changes occurred in the bacterial genome during growth in the plant, whether or not PPHGI-1 was lost. We also present evidence that the circular PPHGI-1 episome is able to replicate autonomously when excised from the genome. These results shed more light onto the plasticity of the bacterial genome as it is influenced by in planta conditions.