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With the objective to examine maize plant response to whole rhizosphere microbial communities in terms of growth and nutrition, we performed a greenhouse pot experiment with full mineral fertilization except for N and P, which were applied in a fully factorial set-up (Without, with N, with P and with N+P). Rhizosphere microbial communities were measured in terms of mycorrhiza formation and biomarker fatty acids. Whole rhizosphere microbial communities promoted plant growth with single N fertilization (P limitation), whereas negative plant growth responses were observed without fertilization (N and P limitation), single P fertilization (N limitation) and dual N and P fertilization (no nutrient limitation). Whole rhizosphere microbial communities increased and decreased shoot N content without and with N fertilization, respectively. On the other hand whole rhizosphere microbial communities increased shoot P content both with and without P fertilization, though more without P. The observed plant growth and shoot nutrient response to whole rhizosphere microbial communities seems to be linked to the root association with arbuscular mycorrhizal fungi (AMF) and abundance of rhizosphere microorganisms. In conclusion, our results show that maize plant response to whole rhizosphere microbial communities in terms of growth and nutrition is strongly associated with N and P fertilization balance.

Bioremediation is an environmentally sound and cost-effective strategy to restore soils contaminated with petroleum oil. In this study, native bio-surfactant-producing bacteria able to use polycyclic aromatic hydrocarbons (PAHs: anthracene [ANT] or phenanthrene [PHE]) as their sole carbon source were isolated from soil contaminated with weathered petroleum oil (WPO) collected near “La Venta” gas processing complex, in Tabasco, Mexico. Bacteria able to grow in 200 ppm of ANT or PHE and having high bio-surfactant activity were selected. Included were bacteria identified as Sphingobium sp., Bosea sp., Pseudomonas sp., Rhodococcus sp. and Phenylobacterium sp. Different consortia of these bacteria ± bio-stimulation with glucose, ammonium nitrate, or both, were examined for their capacity to degrade either WPO or fresh crude petroleum oil (FCPO) in soil microcosms. No efficient short-term (24 days) degradation of WPO using bio-augmentation and bio-stimulation strategies was possible using two bacterial consortia conformed by combinations of these bacterial isolates. Best results were obtained with a consortium composed of Pseudomonas cloritidismutans and Rhodococcus qingshengii, identified by 16S rRNA sequencing, which led to an 86% reduction of FCPO contaminants in soil microcosms, 90 days after treatment. However, the bioremediation efficiency of this consortium was significantly reduced by moderate salinity levels in the soil. Although promising, these results highlight the adverse effect that weathering and/or salinity may have on the remediation of petroleum oil-contaminated soils.

Twenty isolates of Trichoderma were obtained from orchards located in three main mango-producing States in Mexico: Chiapas, Oaxaca, and Michoacan, which represent different agronomical management practices and levels of soil fertility. Phylogenetic analysis showed that Trichoderma isolates belong to the following taxa: Hypocrea lixii (10 isolates), Hypocrea jecorina (four isolates), Trichoderma asperellum (three isolates), Trichoderma spirale (two isolates), and Trichoderma brevicompactum (one isolate). The genus Hypocrea is the teleomorph (sexual) stage of the genus Trichoderma, anamorph stage. Seventeen Trichoderma isolates showed at least 67% growth inhibition against the phytopathogenic fungus Colletotrichum gloeosporioides ATCC MYA 456 and three Trichoderma isolates showed complete overgrowth of this pathogen. One member of this group, identified as T. asperellum T8a, was able to control C. gloeosporioides ATCC MYA 456 in vitro and in vivo, as well as five C. gloeosporioides isolates obtained from mango orchards from the State of Oaxaca. Assay of the lytic enzymes involved suggest that cellulases of T. asperellum T8a play a role in biological control against C. gloeosporioides ATCC MYA 456 more than chitinase or glucanase. Thus, native T. asperellum T8a associated with mango trees can be used to enhance mango production, controlling anthracnose through cellulase activity.

Nitrogenase activity and trehalose accumulation were measured in nodulated and non-nodulated common beans ( Phaseolus vulgaris ) that were exposed to drought. Plants were infected with the Fix + Rhizobium sp NGR234, or a Fix − derivative (NGRΩ fixF ), or high trehalose-producing, native rhizobia. Trehalose content increased significantly while acetylene reduction activity (ARA) decreased in the nodules of plants exposed to drought. Nevertheless, ARA decreased at a slower rate in nodules with high trehalose levels. Under water stress, nodules infected with NGRΩ fixF tended to accumulate more trehalose than nodules infected with wild-type NGR234 (9±0.1 vs 8±0.1 mg g −1 dw, respectively). Highest trehalose accumulations were registered in nodules of plants infected with native rhizobia (average 16 mg g −1 dw), and these plants also exhibited the highest relative water content (65%), while in plants infected with the NGRΩ fixF RWC was significantly lower (56%). Our results suggest that nodule trehalose may protect bacterial nitrogenase activity under drought conditions, and that both trehalose and biological nitrogen fixation (BNF) contribute to drought tolerance.

Pachyrhizus ahipa and Pachyrhizus erosus are tuber legume crops with high yield potential, high nutritional value, low N fertiliser and low pesticide requirement and have a potential use in world-wide cultivation. One of the main advantages of these two crop species are their ability to produce high biomass without N fertiliser application. In some climatic regions P. erosus is reproductively pruned in order to obtain economic yields, but little is known about how the pruning influences the capacity of these tuber legumes to fix nitrogen. Two experiments were carried out to investigate the effect of flower pruning on the yield of tuberous roots, the capacity of N 2 fixation and net N balance. In the first experiment two landraces of P. ahipa and three cultivars of P. erosus were grown with or without flower pruning. The difference method was used to estimate N 2 fixation using maize and sorghum as reference crops. In the second experiment 15 N isotopic dilution methodology was used to determine N 2 fixation in the same cultivars as in Experiment 1, using the same reference crops, but tuber legumes were only grown with pruning of flowers. In the first experiment the amounts of nitrogen fixed ranged from 58 to 80 kg N ha −1 for P. ahipa and from 163 to 216 kg N ha −1 for P. erosus , and this variable was not significantly affected by flower pruning treatments, even-though treatments with flower pruning were harvested earlier. With pruning of flowers the amount of N in the residue ranged from 55 to 151 kg N ha −1 and their N concentration ranged from 32 to 35 g kg −1 . In the second experiment the % N derived from the atmosphere (Ndfa) was estimated to range from 55 to 69% for P. ahipa and from 68 to 77% for P. erosus , while the amount of N fixed ranged from 74 to 95 kg N ha −1 and from 172 to 190 kg N ha −1 , respectively, for these species. Positive net N balances in the system ranged from 12 to 18 kg N ha −1 for P. ahipa and from 55 to 81 kg N ha −1 for P. erosus .

Seasonal variations of trehalose contents in roots and root-nodules of five legumes in a tropical deciduous forest in Jalisco, Mexico, were determined. The tree species were: Lonchocarpus eriocarinalis and Erythrina lanata (sub-family Papilionoideae) and Piptadenia constricta , Albizia occidentalis and Lysiloma microphylum (sub-family Mimosoideae). Trehalose accumulation in nodules and roots varied seasonally and among species. For example, the Papilionoid-species retained nodules longer than those in the Mimosoideae (5 and 4 months, respectively), and accumulated the highest amounts of trehalose (average values of 178 vs. 2.88 mg g −1 nodule (d. wt), respectively). Generally, maximum trehalose contents in nodules and roots were observed in November, at the beginning of the dry season.