Role of <i>BraRGL1</i> in regulation of <i>Brassica rapa</i> bolting and flowering

Abstract

Gibberellin (GA) plays a major role in controlling <i>Brassica rapa</i> stalk development. As an essential negative regulator of GA signal transduction, DELLA proteins may exert significant effects on stalk development. However, the regulatory mechanisms underlying this regulation remain unclear. In this study, we report highly efficient and inheritable mutagenesis using the CRISPR/Cas9 gene editing system in <i>BraPDS</i> (phytoene desaturase) and <i>BraRGL1</i> (key DELLA protein) genes. We observed a loss-of-function mutation in <i>BraRGL1</i> due to two amino acids in GRAS domain. The flower bud differentiation and bolting time of <i>BraRGL1</i> mutants were significantly advanced. The expression of GA-regulatory protein (<i>BraGASA6</i>), flowering related genes (<i>BraSOC1</i>, <i>BraLFY</i>), expansion protein (<i>BraEXPA11</i>) and xyloglucan endotransferase (<i>BraXTH3</i>) genes was also significantly upregulated in these mutants. <i>BraRGL1</i>-overexpressing plants displayed the contrasting phenotypes. <i>BraRGL1</i> mutants were more sensitive to GA signaling. BraRGL1 interacted with BraSOC1, and the interaction intensity decreased after GA₃ treatment. In addition, BraRGL1 inhibited the transcription-activation ability of <i>BraSOC1</i> for <i>BraXTH3</i> and <i>BraLFY</i> genes, but the presence of GA₃ enhanced the activation ability of <i>BraSOC1</i>, suggesting that the BraRGL1-BraSOC1 module regulates bolting and flowering of <i>B. rapa</i> through GA signal transduction. Thus, we hypothesized that BraRGL1 is degraded, and BraSOC1 is released in the presence of GA₃, which promotes the expression of <i>BraXTH3</i> and <i>BraLFY</i>, thereby inducing stalk development in <i>B. rapa</i>. Further, the <i>BraRGL1-M</i> mutant promoted the flower bud differentiation without affecting the stalk quality. Thus, <i>BraRGL1</i> can serve as a valuable target for the molecular breeding of early maturing varieties.