Molecular identification of the key starch branching enzyme-encoding gene SBE2.3 and its interacting transcription factors in banana fruits

Starch branching enzyme(SBE)has rarely been studied in common starchy banana fruits.For the first time,we report here the molecular characterization of seven SBE(MaSBE)and six SBE(MbSBE)genes in the banana A-and B-genomes,respectively,which could be classified into three distinct subfamilies according to genome-wide identification.Systematic transcriptomic analysis revealed that six MaSBEs and six MbSBEs were expressed in the developing banana fruits of two different genotypes,BaXi Jiao(BX,AAA)and Fen Jiao(FJ,AAB),among which MaSBE2.3 and MbSBE2.3 were highly expressed.Transient silencing of MaSBE2.3 expression in banana fruit discs led to a significant decrease in its transcription,which coincides with significant reductions in total starch and amylopectin contents compared to those of empty vector controls.The suggested functional role of MaSBE2.3 in banana fruit development was corroborated by its transient overexpression in banana fruit discs,which led to significant enhancements in total starch and amylopectin contents.A number of transcription factors,including three auxin response factors(ARF2/12/24)and two MYBs(MYB3/308),that interact with the MaSBE2.3 promoter were identified by yeast one-hybrid library assays.Among these ARFs and MYBs,MaARF2/MaMYB308 and MaARF12/MaARF24/MaMYB3 were demonstrated via a luciferase reporter system to upregulate and downregulate the expression of MaSBE2.3,respectively.

Genome-wide identification and expression analysis of the β-amylase genes strongly associated with fruit development,ripening, and abiotic stress response in two banana cultivars

β-amylase(BAM) is an important enzyme involved in conversion of starch to maltose in multiple biological processes in plants. However, there is currently insufficient information on the BAM gene family in the important fruit crop banana. This study identified 16 BAM genes in the banana genome. Phylogenetic analysis showed that Ma BAMs were classified into four subfamilies. Most Ma BAMs in each subfamily shared similar gene structures. Conserved motif analysis showed that all identified Ma BAM proteins had the typical glyco hydro14 domains. Comprehensive transcriptomic analysis of two banana genotypes revealed the expression patterns of Ma BAMs in different tissues, at various stages of fruit development and ripening, and in responses to abiotic stresses. Most Ma BAMs showed strong transcript accumulation changes during fruit development and late-stage ripening. Some Ma BAMs showed significant changes under cold, salt, and osmotic stresses. This finding indicated that Ma BAMs might be involved in regulating fruit development, ripening, and responses to abiotic stress. Analysis of five hormone-related and seven stressrelevant elements in the promoters of Ma BAMs further revealed that BAMs participated in various biological processes. This systemic analysis provides new insights into the transcriptional characteristics of the BAM genes in banana and may serve as a basis for further functional studies of such genes.

Cold stress responsive microRNAs and their targets in Musa balbisiana

Cold stress is an environmental factor affecting plant development and production. Recently,micro RNAs(mi RNAs) have been found to be involved in several plant processes such as growth regulation and stress responses. Although mi RNAs and their targets have been identified in several banana species, their participation during cold accumulation in banana remains unknown. In this study, two small RNA libraries were generated from micropropagated plantlets of Musa balbisiana grown at normal and low temperature(5°C).A total of 69 known mi RNAs and 32 putative novel mi RNAs were detected in the libraries by Solexa sequencing. Sixty-four cold-inducible mi RNAs were identified through differentially expressed mi RNAs analysis. Among 43 mi RNAs belonging to 26 conserved mi RNA families with altered expression, 18 were upregulated and 25 downregulated under cold stress. Of21 putative novel mi RNAs with altered expression, four were downregulated and 17 upregulated. Furthermore,eight mi RNAs were validated by stem-loop q RT-PCR and their dynamic differential expression was analyzed. In addition, 393 target genes of 58 identified cold-responsive mi RNAs were predicted and categorized by function.These results provide important information for further characterization and functional analysis of cold-responsive mi RNAs in banana.