Role of OsTPP3 in the regulation of hypoxic germination in rice

Seed germination is a fundamental biological process in the life cycle of crops and highly regulated in response to environmental changes. Although rice has relatively high tolerance to hypoxic stress caused by flooding, its seed germination and early seedling establishment are extremely sensitive to hypoxic stress. Direct-seeding is becoming more and more popular in many rice-growing regions of the world, due to its lower labor demand and less production cost. However, low germination rate and weak seedlings often occur from the hypoxic stress caused by flooding of rainfall after sowing, or by waterlogging in low-lying areas due to lack of field flatness. Finding new gene resources tolerant to hypoxic stress during germination is of great significance for rice breeding and cultivation.

We have observed that low level of exogenous trehalose promoted rice hypoxic germination and increased coleoptile length while high level of trehalose inhibited hypoxic germination. Then we further found that OsTPP3, one of the trehalose-6- phosphate phosphatase (TPP) genes, plays a positive role in regulating hypoxic germination. Interestingly, over expression of OsTPP3 decreased aerobic germination rate, suggesting an optimal range of trehalose level during germination is needed to regulate the balance between the aerobic and hypoxic germination. Then MYB30, a transcription factor (TF) predicted to regulate OsTPP3 expression by TF prediction, was found to bind to the promoter region of OsTPP3 and activate its expression in our preliminary binding assay in rice protoplast and trans-activation assays. We also found that expression of OsTPP3 is significantly upregulated in OsMYB30-overexpressing seeds which also displayed a delayed seed germination phenotype, suggesting that OsMYB30-OsTPP3 module plays a key role in regulating seed germination, especially under hypoxic condition.

Based on our preliminary results, we plan to utilize plant molecular biology approaches and reverse genetic technology to investigate the molecular mechanism of OsMYB30- OsTPP3 module that regulates hypoxic tolerance during seed germination, and to screen for superior genotypes of OsMYB30 and OsTPP3 from our core rice germplasm collection for the creation of new rice germplasms of hypoxic tolerant rice during germination and early seedling establishment. The results of this study will provide new insight for the regulating mechanism of hypoxic germination and genetic resources for rice breeding and cultivation.