The complex regulatory properties of the plastidial phosphorylase L80 intrinsically disordered region

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 L80 IDR boosts seedling growth, flowering time, overall biomass, and grain yields and alters PSI redox properties, the precise underlying regulatory basis remains unclear. To pinpoint IDR regulatory elements, four Pho1 partial-IDR deletion variants (ΔN _1–42 , ΔC _42–80 , ΔM _21–59, and ΔN _1–20 ΔC _61–80 ) were evaluated in the Pho1-deficient mutant BMF136 . Lines carrying ΔM _21–59 and ΔC _42–80 exhibited enhanced starch metabolism, yielding larger, heavier grains, and flowered 5–10 days earlier than wild type (WT). Conversely, ΔN _1–42 and ΔN _1–20 ΔC _61–80 had no impact on grain traits and delayed flowering by 3–5 days compared with WT. Both ΔM _21–59 and ΔC _42–80 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. Although ΔN _1–42 and ΔN _1–20 ΔC _61–80 reduced donor-side limitation under low actinic light (250 PPFD) like Pho1ΔL80, they failed to reproduce the increased acceptor-side limitation. ΔM _21–59 and ΔC _42–80 PSI redox properties remained indistinguishable from WT. 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.