Overexpressing BrWRKY22 Delays Flowering and Leaf Senescence via Inhibition of GA Biosynthesis in Brassica rapa

WRKY transcription factors play a predominant role in plant stress responses, as well as growth and development. Although <i>WRKY</i> genes have been extensively studied in model plants, little is known about them in <i>Brassica rapa</i>. In this study, the <i>BrWRKY22</i> gene was isolated and characterized. BrWRKY22 is nuclear localized and has self-activation and dimerization activity. <i>BrWRKY22</i> was highly expressed in young leaves, roots, and stems. The overexpressed <i>BrWRKY22 Arabidopsis</i> and <i>Brassica rapa</i> lines exhibited a dwarfish, delayed flowering and leaf senescence phenotype compared to the wild-type (WT). Molecular evidence showed that the transcript levels of <i>BrCHLP</i> are increased, whereas those of <i>BrLFY</i>, <i>BrSOC1</i>, <i>BrGA20OX2</i>, <i>BrGA3OX1</i>, and <i>BrGASA6</i> are significantly decreased in <i>BrWRKY22</i> overexpressing plants compared to the WT. BrWRKY22 can bind directly to the promoters of <i>BrCHLP</i> and <i>BrGA20OX2</i>, activating <i>BrCHLP</i> and repressing <i>BrGA20OX2</i> gene transcription. The chlorophyll b and tocopherol levels are increased, whereas the GA and ABA levels are significantly decreased, in three-week-old <i>BrWRKY22</i> overexpressing <i>Brassica</i> lines compared to the WT. Collectively, our results suggest that BrWRKY22 directly controls chlorophyll b and GA biosynthesis and plays a repressive role in leaf senescence and the initiation of flowering in <i>Brassica rapa</i> plant development.