Heterologous expression of rice 9-cis-epoxycarotenoid dioxygenase 4 (OsNCED4) in Arabidopsis confers sugar oversensitivity and drought tolerance

San-Gwang Hwang, Chia-Yun Lee, Ching-Shan Tseng

The 9-cis-epoxycarotenoid dioxygenases OsNCED4 was cloned from rice in conjunction with OsNCED 1-3 and 5, of which 3 has been shown to function in ABA biosynthesis and alteration of leaf morphology. In higher plants, NCEDs have been shown to be key enzymes controlling ABA biosynthesis and belong to a differentially expressed gene family. Aside from OsNCED3, it remains largely unknown if other OsNCED genes are involved in ABA biosynthesis in rice. Thus, transgenic Arabidopsis plants overexpressing OsNCED4 were generated in the 129B08/nced3 mutant background to explore OsNCED4 function in ABA biosynthesis. Heterologous expression of OsNCED4 in Arabidopsis increased ABA levels and altered plant size and leaf shape, delayed seed germination, caused sugar oversensitivity in post-germination growth, and enhanced tolerance to drought. The native OsNCED3 and OsNCED4 promoters were expressed in an overlapping pattern in rice seeds and young seedlings, suggesting possible functional redundancy between OsNCED3 and OsNCED4. At the one-leaf stage, similar regulation of OsNCED3 and OsNCED4 gene expression in roots or leaves in response to moderate salt stress (150 mM NaCl) was observed. Like OsNCED3, OsNCED4 is functionally active in ABA biosynthesis in rice. OsNCED3 and OsNCED4 might play redundant roles in controlling ABA biosynthesis in rice, as suggested by GUS staining assay, but this should be further analyzed through complementation of rice NCED knockout mutants.