Defining the components of the miRNA156-SPL-miR172 aging pathway in pea and their expression relative to changes in leaf morphology

The timing of developmental phase transitions is crucial for plant reproductive success, and two microRNAs (miRNA), miR156 and miR172, are implicated in the control of these changes, together with their respective SQUAMOSA promoter binding-like (SPL) and APETALA2 (AP2)-like targets. While their patterns of regulation have been studied in a growing range of species, to date they have not been examined in pea (Pisum sativum), an important legume crop and model species. We analysed the recently-released pea genome and defined nine miR156, 21 SPL, four miR172, and five AP2-like genes. Phylogenetic analysis of the SPL genes in pea, Medicago and Arabidopsis confirmed the eight previously defined clades, and identified a ninth potentially legume-specific SPL clade in pea and Medicago. Among the PsSPL, 14 contain a miR156 binding site and all five AP2-like transcription factors in pea include a miR172 binding site. Phylogenetic relationships, expression levels and temporal expression changes identified PsSPL2a/3a/3c/6b/9a/9b/13b/21, PsmiR156d/j and PsmiR172a/d as the most likely of these genes to participate in phase change in pea. Comparisons with leaf morphology suggests that vegetative phase change is unlikely to be definitively marked by a change in leaflet number. In addition, the timing of FT gene induction suggests that the shift from the juvenile to the adult vegetative phase may occur within fourteen days in plants grown under inductive conditions, and calls into question the contribution of miR172/AP2 to the floral transition. This work provides the first insight into the nature of vegetative phase change in pea, and an important foundation for future functional studies.