Genome-wide identification of the expansin gene family and differences in transcriptional responses to boron deficiency in Brassica napusL.

Boron (B) is an essential micronutrient for higher plant growth and development. The most prominent symptoms of B deficiency are associated with primary cell walls. Oilseed rape (Brassica napus L.) is a major oil crop worldwide and shows sensitivity to B deficiency. Expansins mediate plant growth by catalyzing the loosening of cell walls without lysing wall polymers, allowing plants to maintain growth and resist different environmental stresses. However, comprehensive studies on expansins in B. napus and their expression in response to B deficiency are lacking. In the present research, 109 B. napus expansins were identified, which could be classified into four subfamilies, including 79 BnaEXPAs, 21 BnaEXPBs, five BnaEXLAs, and four BnaEXLBs. Most BnaEXPs clustered in the same subfamily shared relatively conserved exon-intron organization and motif composition. The 109 expansin genes were distributed on 19 chromosomes, and 10 were located in the intervals of B-efficient quantitative trait loci (QTLs). RNA-Seq results showed that, under low B, 40, 18, and 30 BnaEXPs were significantly up- or down-regulated in the roots, juvenile leaves, and old leaves of B-efficient cultivar ‘QY10’, respectively; in contrast, 27, 24, and 41 BnaEXPs were significantly up-or down-regulated in the roots, juvenile leaves, and old leaves of B-inefficient cultivar ‘W10’, respectively. Among them, the expression levels of BnaC04.EXPA6a in roots, BnaA09.EXPA5 in juvenile leaves, and BnaA09.EXPA16, BnaC04.EXPA3, BnaCnn.EXPA5b, and BnaA03.EXPA8 in old leaves of ‘QY10’ were significantly higher than that of ‘W10’. This preliminary analysis of expansin genes provides basic data to help reveal the B-efficiency mechanism in B.napus.