Identification of a sequence motif of the rice Pho1 L80 that modulates grain starch metabolism and floral timing

Abstract

Plastidial α-glucan phosphorylase (Pho1) regulates starch metabolism and also modulates photosystem I (PSI) activity. In rice, Pho1 contains an 80-amino-acid intrinsically disordered region (IDR), L80, that acts as a negative regulator. While total removal of the IDR boosts seedling growth, flowering time, overall biomass, and grain yield and alters PSI redox properties, the precise underlying regulatory basis remains unclear. To pinpoint IDR regulatory elements, four Pho1 partial-IDR deletion variants (ΔN1–41, ΔC42–80, ΔM21–59, and ΔN1–20 ΔC60–80) were evaluated. Lines carrying ΔM21–59 and ΔC42–80 exhibited enhanced starch metabolism, yielding larger, heavier grains, and flowered 5–10 days earlier than wild type. Both ΔM21–59 and ΔC42–80 share an acidic motif, (E/V)SEEV, identifying it as a primary negative regulator of starch synthesis and floral timing. By contrast, no partial deletion fully mirrored the PSI redox modifications of Pho1ΔL80. These results demonstrate that starch biosynthesis and flowering are governed by one or more discrete sequence motifs within the middle and C-terminal regions, whereas PSI modulation likely requires the structural integrity of the intact IDR.