ABSTRACT
Micro-RNAs from legume plants are emerging as relevant regulators of the rhizobia nitrogen-fixing symbiosis. In this work we functionally characterized the role of the node conformed by micro-RNA319 (miR319) - TEOSINTE BRANCHED/CYCLOIDEA/PCF (TCP) transcription factor in the common bean (Phaseolus vulgaris) - Rhizobium tropici symbiosis. The miR319d, one of nine miR319 isoforms from common bean, was highly expressed in root and nodules from inoculated plants as compared to roots from fertilized plants. The miR319d targets TCP10 (Phvul.005G067950), identified by degradome analysis, whose expression showed a negative correlation with miR319d expression. The phenotypic analysis of R. tropici-inoculated composite plants with transgenic roots/nodules overexpressing or silencing the function of miR319d demonstrated the relevant role of the miR319d/TCP10 node in the common bean rhizobia symbiosis. Increased miR319d resulted in reduced root length/width ratio, increased rhizobial infection evidenced by more deformed root hairs and infection threads, and decreased nodule formation and nitrogenase activity per plant. In addition, these plants with lower TCP10 levels showed decreased expression level of the jasmonic acid (JA) biosynthetic gene: LOX2. The transcription of LOX2 by TCPs has been demonstrated for Arabidopsis and in several plants LOX2 level and JA content have been associate with TCP levels. On this basis, we propose that in roots/nodules of inoculated common bean plants TCP10 could be the transcriptional regulator of LOX2 and the miR319d/TCP10 node could affect nodulation through JA signaling. However, given the complexity of nodulation, the participation of other signaling pathways in the phenotypes observed cannot be ruled out.
ABSTRACT
Plant hormones have become appropriate candidates for driving functional plant mycorrhization programs, including the processes that regulate the formation of arbuscules in arbuscular mycorrhizal (AM) symbiosis. Here, we examine the role played by ABA/GA interactions regulating the formation of AM in tomato. We report differences in ABA and GA metabolism between control and mycorrhizal roots. Active synthesis and catabolism of ABA occur in AM roots. GAs level increases as a consequence of a symbiosis-induced mechanism that requires functional arbuscules which in turn is dependent on a functional ABA pathway. A negative interaction in their metabolism has been demonstrated. ABA attenuates GA-biosynthetic and increases GA-catabolic gene expression leading to a reduction in bioactive GAs. Vice versa, GA activated ABA catabolism mainly in mycorrhizal roots. The negative impact of GA3 on arbuscule abundance in wild-type plants is partially offset by treatment with ABA and the application of a GA biosynthesis inhibitor rescued the arbuscule abundance in the ABA-deficient sitiens mutant. These findings, coupled with the evidence that ABA application leads to reduce bioactive GA1, support the hypothesis that ABA could act modifying bioactive GA level to regulate AM. Taken together, our results suggest that these hormones perform essential functions and antagonize each other by oppositely regulating AM formation in tomato roots.
