MicroRNA172 Regulation of HvAPETALA2 Controls the Extent of Internode Elongation in Barley

Cereals develop through a series of growth phases that culminate in grain production. Initially, cereals grow vegetatively, giving rise to stems of leafy nodes separated by compressed internodes. Following the switch to reproductive growth, the shoot tip forms a spike inflorescence of flowering nodes, pushed upward as the stem internodes underneath dramatically elongate. Remarkable architectural diversity found within this overall cereal body plan has been continually exploited to select for variants with improved grain yield, yet the genetic regulation underlying variation remains largely unknown, especially in the Triticeae.

We employed a powerful combination of genome-wide association, biparental mapping and mutant analysis to reveal that interactions between microRNA172 (miR172) and its target encoding an AP2-like transcription factor, HvAPETALA2 (HvAP2), underpin observed variation in barley grain density, a trait reflecting the extent of internode elongation between the grain-bearing nodes along the spike. Using comparative molecular and ontogenetic analyses between an extreme-dense spike mutant and wild type, we further show that miR172 downregulation of HvAP2 promotes the developmental transition to a crucial early spike differentiation phase associated with increased internode length. Inhibition of HvAP2 by miR172 also increases the rate and final extent of elongation in the stem, suggesting that HvAP2 is a general repressor of internode expansion. Ongoing research in my laboratory is focused on understanding the mechanism of HvAP2 control on developmental timing and internode growth. Preliminary evidence suggests an exciting scenario whereby HvAP2 delays maturation and elongation by influencing multiple hormone signalling pathways.