Arabidopsis SIGMA FACTOR BINDING PROTEINs function antagonistically to WRKY75 in abscisic acid-mediated leaf senescence and seed germination.

The plant specific VQ gene family participates in diverse physiological processes but little information is available on their role in leaf senescence. Here, we show that the VQ motif-containing proteins, Arabidopsis SIGMA FACTOR BINDING PROTEIN1 (SIB1) and SIB2 are negative regulators of ABA-mediated leaf senescence. Loss of SIB1 and SIB2 functions resulted in increased sensitivity in ABA-induced leaf senescence. In contrast, overexpression of SIB1 significantly delayed this process. Moreover, biochemical studies revealed that SIBs interact with the WRKY75 transcription factor. Loss of WRKY75 functions decreased sensitivity to ABA-induced leaf senescence, while overexpression of WRKY75 significantly accelerated this process. Chromatin immunoprecipitation assays revealed that WRKY75 directly binds to the promoters of GOLDEN 2-LIKE1 (GLK1) and GLK2, repressing expression. SIBs repress the transcriptional functions of WRKY75 and negatively regulate ABA-induced leaf senescence in a WRKY75-dependent manner. In contrast, WRKY75 positively regulates ABA-mediated leaf senescence in a GLK-dependent manner. In addition, SIBs function antagonistically to WRKY75 in ABA-mediated seed germination. These results demonstrate that SIBs can form a complex with WRKY75 to antagonistically regulate ABA-mediated leaf senescence and seed germination.