Seed size controlling signal transduction pathway identified

Wu and Cai et al. reveal that the receptor-like kinase ERECTA regulates seed size in Arabidopsis.

Xuedan Wu and Wenqiang Tang

Institution College of Life Sciences, Hebei Normal University

Background: Background: Seed development begins with double fertilization. Double fertilization releases a yet-to-be identified signal that triggers cell proliferation followed by cell expansion of outer integument cells, which later develop into the seed coat. This rapidly increases the volume of developing seeds, making room for the developing embryo. Many genes that regulate seed size by controlling the development of outer integument cells have been identified. Among these genes, peptidase DA1 cleaves UBP15, reducing UBP15 protein abundance and inhibiting the proliferation of outer integument cells. However, the upstream signaling pathway that activates DA-UBP15 remains unclear.

Question: In an attempt to identify more seed size controlling genes, we established that the receptor-like kinase ERECTA plays an important role in regulating seed size in Arabidopsis. We were very interested to know how double fertilization activates ERECTA and what the downstream signaling pathway is that mediates the regulation of seed size by ERECTA.

Findings: We found that ERECTA promotes outer integument cell proliferation by regulating the DA1-UBP15 module via the MPKK4/5-MPK3/6 pathway in Arabidopsis. ERECTA activated MPK3/6 interact directly with and phosphorylate DA1, which destabilizes and inactivates DA1 protein, causing the accumulation of UBP15 protein to promote the proliferation of outer integument cells. Our study reveals a nearly completed ER-MKK4/5-MPK3/6-DA1-UBP15 signaling pathway that promotes the proliferation of outer integument cells in developing seeds. We also showed that in this signaling pathway, ERECTA does not need its cytoplasmic domains to regulate outer integument cell proliferation, indicating ERECTA might function as a co-receptor.

Next steps: Identifying the ligand and the ERECTA interacting receptor (possibly another receptor-like kinase) that bind ERECTA and regulate outer integument cell proliferation is challenging. However, doing so would help elucidate the mechanism by which double fertilization initiates outer integument cell proliferation to coordinate embryo and endosperm development during seed formation.

Xuedan Wu, Xingbo Cai, Baowen Zhang, Shuting Wu, Ruiju Wang, Na Li, Yunhai Li, Yu Sun, Wenqiang Tang. (2022). ERECTA regulates seed size independently of its intracellular domain via MAPK-DA1-UBP15 signaling.