The RNA binding protein OsCP31A modulates the expression of NDH complex genes involved in abiotic stress tolerance mechanism

Abstract

Drought is a serious threat to crops causing over $ 10 billion worth of damage annually around the world. To overcome this problem, biotechnological approaches have been made to change the root architecture or control the guard cell closure of plants. Another solution for drought tolerance modulation is via the homeostasis of chloroplast, which can be achieved by using chloroplast genes. The regulation of chloroplast genes including RNA processing is controlled by the nuclear encoded RNA binding protein, chloroplast ribonucleoprotein (cpRNP). Thus, we tried to isolate a cpRNP related to abiotic stresses in rice. Based on the phylogenetic tree between 10 Arabidopsis and 8 rice cpRNPs, we isolated Os09g0565200 designated, OsCP31A. We constructed overexpressors and knock down plants of CP31A for functional analyses. OsCP31A was mainly expressed in green tissues throughout development and was localized in the stroma of chloroplasts. Like cpRNPs in other crops, OsCP31A also had broad binding affinity towards chloroplast RNAs. Using overexpression (OsCP31AOX), non-transgenic (NT) and knock-down (OsCP31ARNAi) plants, we treated drought stresses and discovered drought tolerance in OsCP31AOX. To understand the OsCP31A-mediated drought tolerance mechanism, we examined expression levels of chloroplast genes in OsCP31OX and OsCP31RNAi. From this experiment, we finally identified NDH complex RNAs that were relatively stable in OsCP31AOX and less stable in OsCP31A RNAi. The NDH complex, generally known as a protein complex forms a supercomplex with PSI and transports electrons under abiotic stress conditions. The NDH complex-controlling electron transport system is called a cyclic electron transport (CET). We assumed that active NDH complex-controlling CET in OsCP31AOX may enhance drought tolerance under drought stress. To determine the active NDH-dependent CET in OsCP31AOX, we observed the chlorophyll fluorescence in OsCP31AOX and OsCP31ARNAi. When actinic light was turned off, CET was more responsive in OsCP31AOX compared to that of NT. However, the responsiveness was reduced in OsCP31ARNAi. Finally, we analyzed ATP content produced by the NDH-dependent CET. Interestingly, ATP content of OsCP31ARNAi was comparably low under normal growth conditions when compared to that of OsCP31AOX and NT plants. Under drought conditions, the ATP content was highest in OsCP31AOX followed by NT and OsCP31ARNAi. Taken together, we propose overexpression of OsCP31A is sufficient for improving drought tolerance by regulating the expression of NDH complex genes.