HvPHO1 Controls a Glucose 1-Phosphate-SnRK1 Signalling Module That Coordinates Starch Accumulation in Barley Endosperm.

Developing barley grains must balance the use of imported sucrose between soluble sugars and storage starch, but how this balance is controlled remains unclear. Here we show that loss of the plastidial α-glucan phosphorylase HvPHO1 causes starch and soluble sugars to accumulate concurrently in barley grains. Developing pho1 grains showed sustained increases in glucose 1-phosphate (G1P) and adenosine diphosphate glucose (ADP-glucose), and exogenous G1P also promoted starch accumulation, supporting a stimulatory effect of elevated G1P on starch synthesis. G1P directly bound sucrose non-fermenting 1-related protein kinase 1 (HvSnRK1), suppressing its kinase activity and its phosphorylation of sucrose synthase 2 (HvSUSY2); the metabolic phenotypes of snrk1 and susy2 mutants supported a role for this phosphorylation module in sucrose utilization and soluble-sugar homeostasis. HvPHO1 also interacted with the starch-granule initiation factor starch synthase 4 (HvSS4). Loss of HvPHO1 increased the abundance of B-type starch granules and shifted their size distribution, indicating a parallel role in storage-starch organization. pho1 lines showed greater grain dry weight and test weight. Thus, loss of HvPHO1 initiates G1P-mediated metabolic feedback that reconfigures the balance between sucrose utilization and starch accumulation and is associated with enhanced grain carbon deposition and grain weight. These findings reveal G1P-dependent feedback linking starch-precursor status to SnRK1-mediated carbon allocation in barley endosperm.