The effect of exogenous sugar supplementation on the dynamics of microRNA156 and SPL gene expression in apple (Malus × domestica Borkh.) in vitro cultures

The juvenile phase refers to the period in vegetative development during which plants cannot flower even though environmental conditions are suitable for flower induction. This period can last up to 12 years in specific apple varieties, and thus strongly delays the apple breeding process by impairing rapid accumulation of genetic gain via repetitive cross-breeding. Studies in plant model systems have identified microRNA156 (miR156) as the major regulator of juvenility and vegetative phase change (VPC). In higher plants, miR156 targets transcripts of several Squamosa Promoter Binding Protein-like (SPLs) genes, many of which promote VPC and flower development. This central miR156-SPL cascade is upstream regulated by sugar accumulation, with mainly glucose acting as an age-dependent signaling factor that represses MIR156 expression, thereby providing a molecular link between the plant’s nutrient status and developmental timing of reproduction. In this study, we used in vitro microshoots of a half-sib (HS) ‘Fuji’ genotype to investigate the effect of exogenous sugar supplementation on juvenility dynamics by quantifying MdmiR156 abundance and MdSPL2/9 gene expression at three different time points. We show that MdmiR156 abundance decrease abruptly under in vitro conditions. Finally, we found specific effects of glucose and sorbitol on MdSPL2 and MdSPL9 dynamics, however without a clear correlation with MdmiR156 abundance. Validation of MdmiR156 targets and inclusion of other MdSPL genes into the analysis remain of vital importance in future apple in vitro experiments.