Cloning and Phylogenetic Analysis of <i>Brassica napus</i> L. <i>Caffeic Acid O-Methyltransferase 1</i> Gene Family and Its Expression Pattern under Drought Stress

Abstract

<div><p>For many plants, regulating lignin content and composition to improve lodging resistance is a crucial issue. Caffeic acid O-methyltransferase (COMT) is a lignin monomer-specific enzyme that controls S subunit synthesis in plant vascular cell walls. Here, we identified 12 <i>BnCOMT1</i> gene homologues, namely <i>BnCOMT1-1</i> to <i>BnCOMT1-12</i>. Ten of 12 genes were composed of four highly conserved exons and three weakly conserved introns. The length of intron I, in particular, showed enormous diversification. Intron I of homologous <i>BnCOMT1</i> genes showed high identity with counterpart genes in <i>Brassica rapa</i> and <i>Brassica oleracea</i>, and intron I from positional close genes in the same chromosome were relatively highly conserved. A phylogenetic analysis suggested that <i>COMT</i> genes experience considerable diversification and conservation in <i>Brassicaceae</i> species, and some <i>COMT1</i> genes are unique in the <i>Brassica</i> genus. Our expression studies indicated that <i>BnCOMT1</i> genes were differentially expressed in different tissues, with <i>BnCOMT1-4</i>, <i>BnCOMT1-5</i>, <i>BnCOMT1-8</i>, and <i>BnCOMT1-10</i> exhibiting stem specificity. These four <i>BnCOMT1</i> genes were expressed at all developmental periods (the bud, early flowering, late flowering and mature stages) and their expression level peaked in the early flowering stage in the stem. Drought stress augmented and accelerated lignin accumulation in high-lignin plants but delayed it in low-lignin plants. The expression levels of <i>BnCOMT1s</i> were generally reduced in water deficit condition. The desynchrony of the accumulation processes of total lignin and <i>BnCOMT1</i>s transcripts in most growth stages indicated that <i>BnCOMT1s</i> could be responsible for the synthesis of a specific subunit of lignin or that they participate in other pathways such as the melatonin biosynthesis pathway.</p></div