BrEXLB1, a Brassica rapa Expansin-Like B1 Gene Is Associated with Root Development, Drought Stress Response, and Seed Germination

Abstract

Expansins are structural proteins prevalent in cell walls, participate in cell growth and stress responses by interacting with internal and external signals perceived by the genetic networks of plants. Herein, we investigated the <i>Brassica rapa</i> expansin-like B1 <i>(BrEXLB1)</i> interaction with phytohormones (IAA, ABA, Ethephon, CK, GA3, SA, and JA), genes (<i>Bra001852, Bra001958</i>, and <i>Bra003006</i>), biotic (Turnip mosaic Virus (TuMV), <i>Pectobacterium carotovorum,</i> clubroot disease), and abiotic stress (salt, oxidative, osmotic, and drought) conditions by either cDNA microarray or qRT-PCR assays. In addition, we also unraveled the potential role of <i>BrEXLB1</i> in root growth, drought stress response, and seed germination in transgenic Arabidopsis and <i>B. rapa</i> lines. The qRT-PCR results displayed that <i>BrEXLB1</i> expression was differentially influenced by hormones, and biotic and abiotic stress conditions; upregulated by IAA, ABA, SA, ethylene, drought, salt, osmotic, and oxidative conditions; and downregulated by clubroot disease, <i>P. carotovorum,</i> and TuMV infections. Among the tissues, prominent expression was observed in roots indicating the possible role in root growth. The root phenotyping followed by confocal imaging of root tips in Arabidopsis lines showed that <i>BrEXLB1</i> overexpression increases the size of the root elongation zone and induce primary root growth. Conversely, it reduced the seed germination rate. Further analyses with transgenic <i>B. rapa</i> lines overexpressing <i>BrEXLB1</i> sense (OX) and antisense transcripts (OX-AS) confirmed that <i>BrEXLB1</i> overexpression is positively associated with drought tolerance and photosynthesis during vegetative growth phases of <i>B. rapa</i> plants. Moreover, the altered expression of <i>BrEXLB1</i> in transgenic lines differentially influenced the expression of predicted BrEXLB1 interacting genes like Bra001852 and Bra003006. Collectively, this study revealed that <i>BrEXLB1</i> is associated with root development, drought tolerance, photosynthesis, and seed germination.