The plastidial α-glucan phosphorylase modulates maltodextrin metabolism and affects starch parameters in Arabidopsis thaliana.

The plastidial α-glucan phosphorylase (PHS1) can catalyze the elongation and degradation of glucans, but its exact physiological role in plants is not completely deciphered. Many studies have indicated that PHS1 is involved in transitory starch turnover both in photosynthetic tissues and reserve starch accumulation in sink organs, by exerting its effects on the plastidial maltodextrin pools. Recent studies have also established its role in the mobilization of short maltooligosaccharides, thereby assisting in starch granule initiation. Here we report findings from studying four constitutive double knockout mutants related to plastidial maltodextrin metabolism, namely phs1dpe1, phs1ptst2, phs1pgm1, and phs1isa3 of Arabidopsis thaliana. We observed different effects on carbon partitioning in these double mutants. Carbon allocation between starch and sucrose in different double mutants varied with respect to time and light conditions, with significant overall changes in phs1dpe1. Furthermore, we uncovered a potential time-specific function of PHS1 in maltodextrin metabolism. Changes in maltodextrin turnover exerted effects on the starch granule number and size in the double mutants, especially phs1dpe1. We further assessed the double mutants in terms of photosynthetic efficiency and starch parameters, such as internal structure and morphology, in detail. We found that the different photosynthetic parameters in pgm1 and its corresponding double mutant were affected relative to the wild type and phs1. However, other double mutants were not impaired in terms of photosynthetic efficiency despite alterations in their maltooligosaccharide levels.