Systematic analysis of cotton non-specific lipid transfer proteins revealed a clade of genes that play an important role in fiber development

Systematic analysis of cotton non-specific lipid transfer proteins revealed a clade of genes that play an important role in fiber development Cheng-sheng Meng1,2, Yuan-yuan Yan1,2, Zheng-wen Liu1, Li-ting Chen1, Yan Zhang1, Xiu-xin Li1, Li-qiang Wu1, Gui-yin Zhang1, Xing-fen Wang1* and Zhi-ying Ma1* 1College of Agronomy, North China Key Laboratory for Germplasm Resources of Education Ministry, Co-Innovation Center For Cotton Industry of Hebei Province, Hebei Agricultural University, Baoding, Hebei 071001, China 2 These authors contributed equally to this work *Correspondence: mzhy@hebau.edu.cn; cotton@hebau.edu.cn It makes great sense to investigate the regulatory mechanism of fiber development of cotton as it supports an important economic industry-textile. Plant non-specific lipid transfer proteins (nsLTPs) are capable to bind in vitro to various phospholipids including phosphatidylglycerol, phosphatidylcholine, phosphatidylinositol and galactolipids with broad specificity. Multiple physiological functions of nsLTPs have been suggested, including membrane and liposome biogenesis, somatic embryogenesis, pollen development, stress resistance, defence and signal transduction. Although some nsLTPs were isolated from cotton fibers and supposed to regulate fiber development, little progress has been achieved on how nsLTPs regulate fiber development. In the present study, nsLTP genes were strictly identified from cotton genome with 138 members in G. hirsutum, 65 members in G. arboreum and 70 members in G. raimondii. These cotton nsLTP genes could be clustered into ten subgroups according to the number of flanking amino acid residues within the conserved ECM domain. Interestingly, type Ⅺ extremely expanded in G. hirsutum genome to be the largest subgroup, which is different from Arabidopsis, cacao，grape， rape and rice whose genome contain most members belonging to type Ⅰ or type Ⅱ. Sequence analysis revealed that GhLtpⅪs were evolutionally close to GhLtpⅡ12-15, and duplication pairs were indentified between GhLtpⅪs and GhLtpⅡs, indicating that type Ⅺ genes of G. hirsutum are likely to diverge from type Ⅱ genes. A large amount of tandem duplication events and non-reciprocal DNA exchanges were found within GhLtpⅪs, which might contribute to the tremendous expansion of type Ⅺ genes. Transcriptional analysis showed that GhLtps were highly transcribed in ovules and fibers, especially GhLtpⅪs whose transcription was significantly higher during fiber elongation in upland cotton cultivars with long fibers, suggesting an essential role of GhLtpⅪs in fiber elongation. Additionally, analysis of the published transcriptome data suggested that most of GhLtpⅪs, ignoring 12 genes that were scarcely detected, were significantly differentially transcribed in ovules at 10 and 20DPA compared with their orthologs in genome A or D, indicating a correlation of cotton type Ⅺ genes with fiber evolution. As fibers are trichomes of the outer epidermis of a single ovule, GhLtpⅪ17, 24, 27 and 28 were cloned and ectopically expressed in Arabidopsis. And significantly elongated trichomes of GhLtpⅪs overexpressed Arabidopsis suggested functional roles of GhLtpⅪs in promoting cell elongation. Our results implied a clade of nsLTP genes regulating fiber elongation and provide new insights into the phenotypic evolution of Gossypium species.