The BEE2 gene integrates brassinosteroid signalling into a regulatory module controlling gibberellin-mediated germination

Abstract

Seeds need to integrate endogenous and environmental cues to ensure that germination and plant growth occur in the right season. This crucial developmental process is tightly regulated through the interplay between different hormonal signalling pathways. Although it is well established that gibberellins (GAs) promote germination and brassinosteroids (BRs) can partially rescue seed germination in the absence of GAs, the molecular basis underlying this hormone interaction is not fully understood. Here, we identified BRASSINOSTEROID ENHANCED EXPRESSION 2 ( BEE2 ) as a positive regulator of germination through interaction with a previously characterized GA-regulated network. In this network, endosperm expansion relies on the activation of an endosperm-specific α-expansin gene ( EXPA2 ) by NAC25/NAC1L TFs, an activity repressed by the RGL2 DELLA protein. We show that BEE2 is specifically expressed in the endosperm where it also regulates EXPA2 expression and that mutations in E-boxes present in the EXPA2 promoter compromise BEE2-mediated transactivation. We demonstrate that physical interaction allows RGL2 to repress EXPA2 activation by BEE2 and provide genetic evidence showing that the positive regulation exerted by BEE2 and NAC25 on EXPA2 has an additive effect on testa rupture and promote seed germination. Taken together, our results indicate that BEE2 seems to provide an additive function to the RGL2-NAC-EXPA2 regulatory module to control the germination process mainly when GAs levels are compromised. This mechanism adds an extra layer of complexity to the control of the germination process, helps explain BR-GA crosstalk in seeds and may provide seeds with an additional pathway to ensure germination under suboptimal conditions.