The growth of plant cells is inseparable from relaxation and expansion of cell walls. Expansins are a class of cell wall binding proteins, which play important roles in the relaxation of cell walls. Although there are many members in expansin gene family, the functions of most expansin genes in plant growth and development are still poorly understood. In this study, the functions of two expansin genes, AtEXPA4 and AtEXPB5 were characterized in Arabidopsis thaliana. AtEXPA4 and AtEXPB5 displayed consistent expression patterns in mature pollen grains and pollen tubes, but AtEXPA4 also showed a high expression level in primary roots. Two single mutants, atexpa4 and atexpb5, showed normal reproductive development, whereas atexpa4atexpb5 double mutant was defective in pollen tube growth. Moreover, AtEXPA4 overexpression enhanced primary root elongation, on the contrary, knocking out AtEXPA4 made the growth of primary root slower. Our results indicated that AtEXPA4 and AtEXPB5 were redundantly involved in pollen tube growth and AtEXPA4 was required for primary root elongation.
Expansins are a kind of structural proteins of the plant cell wall, and they enlarge cells by loosening the cell walls. Therefore, expansins are involved in many growth and development processes. The complete genomic sequences of Brassica rapa, Brassica oleracea and Brassica nigra provide effective platforms for researchers to study expansin genes, and can be compared with analogues in Arabidopsis thaliana. This study identified and characterized expansin families in B. rapa, B. oleracea, and B. nigra. Through the comparative analysis of phylogeny, gene structure, and physicochemical properties, the expansin families were divided into four subfamilies, and then their expansion patterns and evolution details were explored accordingly. Results showed that after the three species underwent independent evolution following their separation from A. thaliana, the expansin families in the three species had increased similarities but fewer divergences. By searching divergences of promoters and coding sequences, significant positive correlations were revealed among orthologs in A. thaliana and the three basic species. Subsequently, differential expressions indicated extensive functional divergences in the expansin families of the three species, especially in reproductive development. Hence, these results support the molecular evolution of basic Brassica species, potential functions of these genes, and genetic improvement of related crops.
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