BvBZR1 improves parenchyma cell development and sucrose accumulation in sugar beet (Beta vulgaris L.) taproot

BRASSINAZOLE-RESISTANT (BZR) transcription factors, key elements of brassinolide (BR) signal transduction, play an important role in regulating plant growth and development. However, little is known about the molecular regulatory mechanism of BZR in sugar beet taproot growth. In this study, <i>BvBZR1</i> expression was significantly induced by exogenous BR treatment. Transgenic sugar beet overexpressing <i>BvBZR1</i> exhibited a higher taproot diameter compared with the wild type, mainly due to a significant enhancement in the spacing between cambial rings by increasing the size and layers of parenchyma cells. <i>BvBZR1</i> regulated the expression of <i>BvCESA6</i>, <i>BvXTH33</i>, <i>BvFAD3</i>, and <i>BvCEL1</i> and enhanced cell wall metabolism to promote sugar beet taproot growth in parenchyma cells and the development of each cambium ring. In addition, <i>BvBZR1</i> overexpression significantly increased the accumulation of sucrose and soluble sugars in the taproot, which was attributed to its ability to regulate the expression of <i>BvSPS</i> and <i>BvINV</i> and improve the activity of BvSPS, BvSS-S, BvSS-C, and BvINV enzymes in each cambium ring and parenchyma cell in the sugar beet taproot. These results suggest that <i>BvBZR1</i> can regulate the expression of genes related to cell wall and sucrose metabolism, improve corresponding enzyme activity, and promote the development of each cambium ring and parenchyma cell, thereby promoting the growth and development of sugar beet taproots.