Abstract
Fasciclin-like arabinogalactan proteins (FLAs) are a subclass of arabinogalactan proteins (AGPs) involved in plant growth, development and response to abiotic stress. Although many studies have been performed to identify molecular functions of individual family members, little information is available on genome-wide identification and characterization of FLAs in the genus Populus. Based on genome-wide analysis, we have identified 35 Populus FLAs which were distributed on 16 chromosomes and phylogenetically clustered into four major groups. Gene structure and motif composition were relatively conserved in each group. All the members contained N-terminal signal peptide, 23 of which included predicted glycosylphosphatidylinositol (GPI) modification sites and were anchored to plasma membranes. Subcellular localization analysis showed that PtrFLA2/20/26 were localized in cell membrane and cytoplasm of protoplasts from Populus stem-differentiating xylem. The Ka/Ks ratios showed that purifying selection has played a leading role in the long-term evolutionary period which greatly maintained the function of this family. The expression profiles showed that 32 PtrFLAs were differentially expressed in four tissues at four seasons based on publicly available microarray data. 18 FLAs were further verified with qRT-PCR in different tissues, which indicated that PtrFLA1/2/3/7/11/12/20/21/22/24/26/30 were significantly expressed in male and female flowers, suggesting close correlations with the reproductive development. In addition, PtrFLA1/9/10/11/17/21/23/24/26/28 were highly expressed in the stems and differentiating xylem, which may be involved in stem development. To determine salt response of FLAs, qRT-PCR was performed to analyze the expression of 18 genes under salinity stress across two time points. Results demonstrated that all the 18 FLAs were expressed in root tissues; especially, PtrFLA2/12/20/21/24/30 were significantly induced at different time points. In summary, this study may lay the foundation for further investigating the biological functions of FLA genes in Populus trichocarpa.
Highlights
Arabinogalactan proteins (AGPs) are heavily glycosylated proteoglycans that affect various processes such as plant growth, development, nutrition, reproduction, and adaptation to environmental changes (Showalter, 2001; Tan et al, 2012)
35 PtrFLA nonredundant genes were identified in the Populus genome and named as PtrFLA1-PtrFLA35 based on their chromosomal positions from top to bottom successively
Most researchers focused on the function of fasciclin-like AGPs (FLAs) genes in annual herbaceous plants, especially Arabidopsis, while neglected this family in perennial woody plant species
Summary
Arabinogalactan proteins (AGPs) are heavily glycosylated proteoglycans that affect various processes such as plant growth, development, nutrition, reproduction, and adaptation to environmental changes (Showalter, 2001; Tan et al, 2012). Based on the protein structure, AGPs can be subdivided into six main categories: classical AGPs, Lys-rich AGPs, AG peptides, fasciclin-like AGPs (FLAs), non-classical AGPs, and chimeric AGPs (Schultz et al, 2002; Seifert and Roberts, 2007; Showalter et al, 2010). Fasciclin domains contain about 110–150 amino acid residues with low sequence similarity. A common denominator among fasciclin domains is that they all have two highly conserved regions (H1 and H2) and one [Phe/Tyr]-His motif, with approximately 10 amino acids in each conserved region (Kawamoto et al, 1998; Johnson et al, 2003; Huang et al, 2008). AGPs are mostly located in cellular membrane, cell walls and extracellular matrix, which may be involved in cell interactions, cell adhesion and cell wall biosynthesis (Kawamoto et al, 1998; Johnson et al, 2003; Shi et al, 2003)
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