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Biochemical analysis of plastidial phosphorylase (Pho1)-deficient mutants of rice strongly suggests that Pho1 plays an important role in the initiation of starch biosynthesis in the endosperm. The present study examined structures and amounts of maltodextrins, malto-oligosaccharides (MOS) as well as sugars in maturing rice endosperm and compared between a pho1 -mutant line EM633 and its wild-type cultivar Taichung-65 (TC65). By using a gel permeation HPLC from developing rice endosperm of both lines, branched maltodextrins (BMD) with distinct fine structure could be isolated for the first time, a possible intermediate in the initiation process of starch biosynthesis, whereas its amount was much lower than starch content in both endosperms. This suggests that BMD as a main intermediate in the de novo synthesis of starch molecules is kept to be at a very low level during starch biosynthesis. The amounts of maltose, and MOS as well as linear maltodextrins (LMD) were greatly increased by the pho1 mutation. These results indicate that Pho1 would play an essential role in the step of mobility of maltose to MOS and then to LMD in the de novo synthesis of amylopectin molecules in developing rice endosperm. The present study has succeeded to isolate the branched maltodextrins (BMD), a possible intermediate in the initiation of starch biosynthesis, for the first time from an early developmental stage of rice endosperm, and determined the structural features of BMD. We also observed that in the plastidial starch phosphorylase (Pho1)-deficient mutant endosperm, the levels of maltose and linear dextrins (LMD) were elevated compared with those in wild-type endosperm, suggesting that Pho1 plays an essential role in the synthesis of BMD from maltose and LMD. The present study established the biochemical basis for our understanding of the molecular mechanism for the initiation of starch biosynthesis in cereal endosperm. [Display omitted] [ABSTRACT FROM AUTHOR]

The initiation of starch granule synthesis remains a relatively poorly understood phenomenon. Recent advances enabled the establishment of a model explaining the synthesis of new starch granules in Arabidopsis thaliana. These characterizations revealed the involvement of both a specific starch synthase isoform (SS4) and of several non-enzymatic proteins in this process. In this work, we investigated whether the initiation of starch synthesis in the green microalgae Chlamydomonas reinhardtii involves the same machinery as those uncovered in the plant model. Our extensive phylogenetic analysis revealed that most of the key players that were identified in higher plants are not found in microalgae suggesting that a different pathway is taking place. We showed that restoration of ADP-glucose synthesis in a mutant strain devoid of any endogenous primers allowed normal starch synthesis, revealing the existence of an initiation mechanism in Chlamydomonas. Our biochemical characterizations revealed that starch synthase isoform 3 possesses the intrinsic capacity to initiate polysaccharide synthesis in vitro and could be one of the functions involved in starch initiation. Our work suggests that the initiation of starch synthesis in Chlamydomonas involves a different pathway to that described in Arabidopsis and that further efforts will be required to identify the proteins involved in this process., (© The Author(s) 2025. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site–for further information please contact journals.permissions@oup.com.)