ERECTA regulates seed size independently of its intracellular domain via MAPK-DA1-UBP15 signaling

ERECTA regulate seed size by regulating the proliferation of outer integument cells via an MKK4/5-MPK3/6-DA1-UBP15 signaling pathway in Arabidopsis, and is independent of its intracellular domain. Seed size is determined by the coordinated growth of the embryo, endosperm, and integument. Growth of the integument is initiated by signal molecules released from the developing endosperm or embryo. Although recent studies have identified many components that regulate seed size by controlling integument growth, the upstream signals and the signal transduction pathway that activate these components after double fertilization are unclear. Here, we report that the receptor-like kinase ERECTA (ER) controls seed size by regulating outer integument cell proliferation in Arabidopsis thaliana. Seeds from er mutants were smaller, while those from ER-overexpressing plants were larger, than those of control plants. Different from its role in regulating the development of other organs, ER regulates seed size via a novel mechanism that is independent of its intracellular domain. Our genetic and biochemical data show that a MITOGEN-ACTIVATED PROTEIN KINASE (MAPK) signaling pathway comprising MAPK-KINASE 4/5, MAPK 3/6 (MPK3/6), DA1, and UBIQUITIN SPECIFIC PROTEASE 15 (UBP15) functions downstream of ER and modulates seed size. MPK3/6 phosphorylation inactivates and destabilizes DA1 to increase the abundance of UBP15, promoting outer integument cell proliferation and increasing seed size. Our study illustrates a nearly completed ER-mediated signaling pathway that regulates seed size and will help uncover the mechanism that coordinates embryo, endosperm, and integument growth after double fertilization.

Automated summary

ERECTA regulates seed size by regulating the proliferation of outer integument cells through an MKK4/5-MPK3/6-DA1-UBP15 signaling pathway in Arabidopsis. This study uncovers a novel mechanism of ER in seed size regulation, which is independent of its intracellular domain. The identified MAPK signaling pathway downstream of ER, involving MPK3/6, DA1, and UBP15, plays a crucial role in modulating seed size by promoting outer integument cell proliferation.