Integrated analysis of small RNA, transcriptome and degradome sequencing provides new insights into floral development and abscission in yellow lupine (Lupines luteus L.)

Background Yellow lupine (Lupinus luteus L., Taper c.) is an important legume crop. However, its flower development and pod formation are often affected by excessive abscission. Organ detachment occurs within the abscission zone (AZ) and in L. luteus primarily affects flowers formed at the top of the inflorescence. The top flowers’ fate appears determined before anthesis. The organ development and abscission mechanisms utilize a complex molecular network, not yet not fully understood, especially as to the role of miRNAs and siRNAs. We aimed at identifying differentially expressed (DE) small ncRNAs in lupine by comparing small RNA-seq libraries generated from developing upper and lower raceme flowers, and flower pedicels with active and inactive AZs. Their target genes were also identified using transcriptome and degradome sequencing. Results Within all the libraries, 394 known and 28 novel miRNAs and 316 phased siRNAs were identified. In flowers at different stages of development, 30 miRNAs displayed DE in the upper and 29 in the lower parts of the raceme. In comparisons between upper and lower raceme flowers, a total of 46 DE miRNAs were identified. miR393 and miR160 were related to the upper and miR396 to the lower flowers and pedicels of non-abscising flowers. In flower pedicels we identified 34 DE miRNAs, with miR167 being the most abundant of all. Most siRNAs seem to play a marginal role in the processes studied herein, with the exception of tasiR-ARFs, which were DE in the developing flowers. The target genes of these miRNAs were predominantly categorized into the following Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways: ‘Metabolism’ (15,856), ‘Genetic information processing’ (5,267), and ‘Environmental information processing’ (1,517). Over 700 putative targets were categorized as belonging to the ‘Plant Hormone Signal Transduction pathway’. 26,230 target genes exhibited Gene Ontology (GO) terms: 23,092 genes were categorized into the ‘Cellular component’, 23,501 into the ‘Molecular function’, and 22,939 into the ’Biological process’. Conclusion Our results indicate that miRNAs and siRNAs in yellow lupine may influence the development of flowers and, consequently, their fate by repressing their target genes, particularly those associated with the homeostasis of phytohormones, mainly auxins.