Conserved structure and varied expression reveal key roles of phosphoglucan phosphatase gene starch excess 4 in barley

As one of the phosphoglucan phosphatases, starch excess 4 (SEX4) encoded by SEX4 gene has recently been intensively studied because of its vital role in the degradation of leaf starch. In this study, we isolated and chromosomally mapped barley SEX4, characterized its gene and protein structure, predicted the cis-elements of its promoter, and analysed its expression based on real-time quantitative PCR and publically available microarray data. The full length of barely SEX4 (HvSEX4) was 4,598 bp and it was mapped on the long arm of chromosome 4H (4HL). This gene contained 14 exons and 13 introns in all but two of the species analysed, Arabidopsis (13 exons and 12 introns) and Oryza brachyantha (12 exons and 11 introns). An exon–intron junction composed of intron 4 to intron 7 and exon 5 to exon 8 was highly conserved among the analysed species. SEX4 is characterized with conserved functional domains (dual specificity phosphatase domain and carbohydrate-binding module 48) and varied chloroplast transit peptide and C-terminal. Expression analyses indicated that: (1) SEX4 was mainly expressed in anthers of barley, young leaf and anthers of rice, and leaf of Arabidopsis; (2) it exhibited a diurnal pattern in barley, rice and Arabidopsis; (3) significant difference in the expression of SEX4 was not detected for either barley or rice under any of the investigated stresses; and (4) it was significantly down-regulated at middle stage and up-regulated at late stage under cold treatment, down-regulated at early stage under heat treatment, and up-regulated at late stage under salt treatment in Arabidopsis. The strong relationships detected in the current study between SEX4 and glucan, water dikinases (GWD) or phosphoglucan, water dikinases (PWD) were discussed. Collectively, our results provide insights into genetic manipulation of SEX4, especially in monocotyledon and uncovering the possible roles of SEX4 in plant development.