Insight into the molecular evolution of non-specific lipid transfer proteins via comparative analysis between rice and sorghum

Phylogenetic analysis was conducted on 9 kDa non-specific lipid transfer protein (nsLTP) genes from nine plant species. Each of the five classified types in angiosperms exhibited eight conserved cysteine patterns. The most abundant nsLTP genes fell into the type I category, which was particularly enriched in a grass-specific lineage of clade I.1. Six pairs of tandem copies of nsLTP genes on the distal region of rice chromosomes 11 and 12 were well-preserved under concerted evolution, which was not observed in sorghum. The transgenic promoter-reporter assay revealed that both rice and sorghum nsLTP genes of type I displayed a relatively conserved expression feature in the epidermis of growing tissue, supporting its functional roles in cutin synthesis or defence against phytopathogens. For type I, the frequent expression in the stigma and seed are indicative of functional involvement in pistil-pollen interactions and seed development. By way of contrast, several type V genes were observed, mainly in the vascular bundle of the rosette as well as the young shoots, which might be related with vascular tissue differentiation or defence signalling. Compared with sorghum, the highly redundant tissue-specific expression pattern among members of rice nsLTP genes in clade I.1 suggests that concerted evolution via gene conversion favours the preservation of crucial expression motifs via the homogenization of proximal promoter sequences under high selection constraints. However, extensive regulatory subfunctionalization might also have occurred under relative low selection constraints, resulting in functional divergence at the expression level.