A Molecular Signal Integration Network Underpinning Arabidopsis Seed Germination.

Seed dormancy is an adaptive trait defining where and when plants are established. Diverse signals from the environment are used to decide when to initiate seed germination, a process driven by the expansion of cells within the embryo. How these signals are integrated and transduced into the biomechanical changes that drive embryo growth remains poorly understood. Using Arabidopsis seeds, we demonstrate that cell-wall-loosening EXPANSIN (EXPA) genes promote gibberellic acid (GA)-mediated germination, identifying EXPAs as downstream molecular targets of this developmental phase transition. Molecular interaction screening identified transcription factors (TFs) that bind to both EXPA promoter fragments and DELLA GA-response regulators. A subset of these TFs is targeted each by nitric oxide (NO) and the phytochrome-interacting TF PIL5. This molecular interaction network therefore directly links the perception of an external environmental signal (light) and internal hormonal signals (GA and NO) with downstream germination-driving EXPA gene expression. Experimental validation of this network established that many of these TFs mediate GA-regulated germination, including TCP14/15, RAP2.2/2.3/2.12, and ZML1. The reduced germination phenotype of the tcp14 tcp15 mutant seed was partially rescued through ectopic expression of their direct target EXPA9. The GA-mediated control of germination by TCP14/15 is regulated through EXPA-mediated control of cell wall loosening, providing a mechanistic explanation for this phenotype and a previously undescribed role for TCPs in the control of cell expansion. This network reveals the paths of signal integration that culminate in seed germination and provides a resource to uncover links between the genetic and biomechanical bases of plant growth.