Abstract
Aluminum (Al) toxicity is widespread in acidic soils where the common bean (Phaseolus vulgaris), the most important legume for human consumption, is produced and it is a limiting factor for crop production and symbiotic nitrogen fixation. We characterized the nodule responses of common bean plants inoculated with Rhizobioum tropici CIAT899 and the root responses of nitrate-fertilized plants exposed to excess Al in low pH, for long or short periods. A 43–50% reduction in nitrogenase activity indicates that Al toxicity (Alt) highly affected nitrogen fixation in common bean. Bean roots and nodules showed characteristic symptoms for Alt. In mature nodules Al accumulation and lipoperoxidation were observed in the infected zone, while callose deposition and cell death occurred mainly in the nodule cortex. Regulatory mechanisms of plant responses to metal toxicity involve microRNAs (miRNAs) along other regulators. Using a miRNA-macroarray hybridization approach we identified 28 (14 up-regulated) Alt nodule-responsive miRNAs. We validated (quantitative reverse transcriptase-PCR) the expression of eight nodule responsive miRNAs in roots and in nodules exposed to high Al for long or short periods. The inverse correlation between the target and miRNA expression ratio (stress:control) was observed in every case. Generally, miRNAs showed a higher earlier response in roots than in nodules. Some of the common bean Alt-responsive miRNAs identified has also been reported as differentially expressed in other plant species subjected to similar stress condition. miRNA/target nodes analyzed in this work are known to be involved in relevant signaling pathways, thus we propose that the participation of miR164/NAC1 (NAM/ATAF/CUC transcription factor) and miR393/TIR1 (TRANSPORT INHIBITOR RESPONSE 1-like protein) in auxin and of miR170/SCL (SCARECROW-like protein transcription factor) in gibberellin signaling is relevant for common bean response/adaptation to Al stress. Our data provide a foundation for evaluating the individual roles of miRNAs in the response of common bean nodules to Alt.
Highlights
Legumes are second only to the Graminiae in their importance to humans
Response of Common Bean Plants to Al Toxicity The objective of this work was to characterize the response of Symbiotic nitrogen fixation (SNF) common bean plants to acidic/Al-toxicity stress, aiming to describe the symptoms present as well as the miRNAs differentially expressed in active nodules from plants exposed to Al toxicity (Alt), something that is yet poorly documented for this or other legumes
Plantlets adapted to grow in hydroponic conditions were inoculated with R. tropici CIAT 899 that is acid tolerant (Graham et al, 1994), when functional nodules were formed stress was imposed by changing the nutrient solution to one containing 70 μM AlCl3
Summary
Legumes are second only to the Graminiae in their importance to humans. Grain legumes provide more than one–third of humankind nutritional nitrogen requirements. A hallmark trait of legumes is their ability to establish mutualistic symbioses with nitrogen-fixing bacteria collectively known as rhizobia. Symbiotic nitrogen fixation (SNF) by differentiated bacteroids takes place in specialized rhizobia-induced root nodules. This process involves a tight association between the two symbionts. SNF reduces the cost of legume cultivation and makes them valuable source of soil nitrogen to other crops (Graham and Vance, 2003)
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