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

Abstract

Brassinosteroids (BRs) are steroid hormones that play essential roles in plant growth and development. We previously cloned <i>qGL3</i>, a major quantitative trait locus regulating grain length in rice (<i>Oryza sativa</i>). The <i>O. sativa japonica</i> var N411 has extra-large grains compared with the <i>O. sativa indica</i> var 9311, and the recessive <i>qgl3</i> allele from N411 contributes positively to grain length. <i>qGL3</i> encodes a putative protein phosphatase with Kelch-like repeat domains, an ortholog of Arabidopsis (<i>Arabidopsis thaliana</i>) <i>brassinosteroid-insensitive1</i> SUPPRESSOR1 (BSU1). BSU1 positively regulates BR signaling, while overexpression of <i>qGL3</i> induced BR loss-of-function phenotypes. Both qGL3^N411 and qGL3⁹³¹¹ physically interact with the rice glycogen synthase kinase 3 (GSK3)/SHAGGY-like kinase 3 (OsGSK3), an ortholog of Arabidopsis BR INSENSITIVE2 (BIN2). qGL3⁹³¹¹ dephosphorylates OsGSK3, but qGL3^N411 lacks this activity. Knocking out <i>OsGSK3</i> enhances BR signaling and induces nuclear localization of <i>O. sativa</i> 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.