Catalytic integrity of plastidial phosphorylase (PHO1) is required for L80-mediated regulation of photosystem I

The rice plastidial phosphorylase (Pho1) is essential for optimal starch synthesis and plays an additional role in modifying photosystem I (PSI) activity via its interaction with PsaC. Pho1 contains an intrinsically disordered region, L80, which acts as a negative modulator of plant growth and development as well as starch biosynthesis and PSI activity. The exact mechanism by which Pho1 lacking the L80 domain (Pho1ΔL80) stimulates starch synthesis and modulates PSI activity remains unresolved. To determine whether the enzymatic activity of Pho1 is required for starch synthesis and PSI modulation, we introduced catalytically inactive Pho1^K823A and Pho1ΔL80^K823A variants into the pho1^- mutant BMF136. Both cat^- mutants exhibited the same proportion of shrunken, white-core grain phenotype as the parent BMF136, demonstrating that Pho1 catalytic enzyme activity is indispensable for starch biosynthesis. The cat^- Pho1 proteins exhibited the same quaternary structure as WT but possessed varying protein interactive capabilities. The cat^- proteins interacted with PsaC, consistent with their localization on PSI containing thylakoid membranes, but bound very poorly with the starch disproportionating enzyme, Dpe1. While Pho1ΔL80^K823A retained early seedling growth stimulation, albeit lower than that seen for Pho1ΔL80, its PSI properties [Y(ND) and Y(NA)] were unlike Pho1ΔL80 but similar to WT. These results indicate that the stimulatory impact on plant growth, development, and productivity via the coupling of starch metabolism with photosynthesis by Pho1ΔL80 requires Pho1 catalysis.