Rice qGL3/OsPPKL1 Functions with the GSK3/SHAGGY-Like Kinase OsGSK3 to Modulate Brassinosteroid Signaling.

Brassinosteroids (BRs) are steroid hormones that play essential roles in plant growth and development. We previously cloned qGL3, a major quantitative trait locus regulating grain length in rice (Oryza sativa L.). The japonica variety N411 has extra-large grains compared with the indica variety 9311, and the recessive qgl3 allele from N411 contributes positively to grain length. qGL3 encodes a putative protein phosphatase with Kelch-like repeat domains (OsPPKL1), an ortholog of Arabidopsis thaliana bri1 SUPPRESSOR1 (BSU1). BSU1 positively regulates BR signaling, while overexpression of qGL3 induced BR loss-of-function phenotypes. Both qGL3N411 and qGL39311 physically interact with the rice GSK3/SHAGGY-like kinase OsGSK3, an ortholog of Arabidopsis BRASSINOSTEROID INSENSITIVE2 (BIN2). qGL39311 dephosphorylates OsGSK3, but qGL3N411 lacks this activity. Knocking-out of OsGSK3 enhances BR signaling and induces nuclear localization of Oryza sativa BRASSINAZOLE RESISTANT1 (OsBZR1). Unlike the dephosphorylation of BIN2 (which leads to protein degradation) in Arabidopsis, qGL3 dephosphorylates and stabilizes OsGSK3 in rice. These results demonstrate that qGL3 suppresses BR signaling by regulating the phosphorylation and stability of OsGSK3, which modulates OsBZR1 phosphorylation and subcellular distribution. Our study clarifies the role of qGL3 in the regulation of grain length and provides insight into BR signaling, including the differences between rice and Arabidopsis.