Plastidial starch phosphorylase regulates maltodextrin turnover during starch granule initiation in Arabidopsis leaves.

PLASTIDIAL STARCH PHOSPHORYLASE 1 (PHS1) is considered integral to starch synthesis, yet its role in transient starch synthesis in photosynthetic tissues remains unclear, as mutation of PHS1 in Arabidopsis (Arabidopsis thaliana) does not affect the metabolic profile of leaves. PHS1 activity is elevated in the starch branching enzyme sbe2.1 sbe2.2 double mutant, which lacks starch granules but retains intact genes encoding granule initiation proteins, making it an ideal plant material for exploring PHS1 function. We generated a triple mutant, sbe2.1 sbe2.2 phs1-1, which showed additional accumulation of soluble maltodextrins, a loss of insoluble linear α-glucans in the leaves, and substantially retarded plant growth, compared to the sbe2.1 sbe2.2 double mutant. STARCH SYNTHASE 3 (SS3) and SS4 activities increased in the sbe2.1 sbe2.2 phs1-1 triple mutant relative to the sbe2.1 sbe2.2 double mutant. Additional loss of SS4 in the sbe2.1 sbe2.2 phs1-1 background partially reversed phenotypes observed in the triple mutant: maltodextrin content decreased, insoluble α-glucans reappeared, and plant growth improved. Principal component analysis revealed that the metabolite profile of the sbe2.1 sbe2.2 ss4 and sbe2.1 sbe2.2 phs1-1 ss4 mutants, particularly the levels of organic acids from the tricarboxylic acid cycle, more closely resembled that of the wild type than that of sbe2.1 sbe2.2 and sbe2.1 sbe2.2 phs1-1. These findings suggest that PHS1 plays a critical role in maltodextrin turnover and carbon regulation in chloroplasts, maintaining a coordinated balance of synthetic and degradative activities. We propose that PHS1 functions as a metabolic buffer, with its role becoming more crucial when starch synthesis pathways are disrupted.