Stress Tolerance Enhancement of Rice by Genetic Manipulation of a bHLH-Myc2 Transcription Factor

Rice yield is adversely affected by various abiotic and biotic stresses. Jasmonic acid (JA) signaling has been implicated in stress response of plants. The nuclear localized basic helix loop helix (bHLH) Myc2 transcription factor is known to be a master regulator of genes involved in the response of the JA-mediated signaling pathway during stress and plant development. Myc2 is also induced by wounding and mechanical damage, and is associated with resistance against herbivore insects. In order to understand the mode of action of Myc2 in stress response of rice, overexpresser (OE) and knock-down (KD) mutants for OsMyc2 were generated in rice. After 7 d of withholding water, OsMyc2 OE plants showed better stress tolerance with respect to their growth and development, and physiological traits such as relative water content, membrane stability, chlorophyll fluorescence, etc. in comparison with the wild type (WT) and KD plants. Similar results were obtained for response to salinity stress (150 mM NaCl in hydroponics) where OE seedlings showed less chlorosis and better shoot and root growth as compared to the WT and KD lines. Furthermore, non-choice feeding assay of the transgenic rice plants with a specialist herbivore Spodoptera frugiperda showed that the life cycle of the insect was affected when the larvae were fed with tissues of the OsMyc2 OE lines. Bioassay with blast fungus, Magnaporthe oryzae, did not show obvious difference with the number of lesions, but the size of lesions was smaller in OE lines relative to that in WT and KD lines. OsMyc2, in addition to its overexpression under various stresses, modulated the expression of genes in JA signaling and associated networks. These results suggested that the OsMyc2 transcription factor is involved in multiple stress responses and can be manipulated to enhance stress tolerance in rice.