An alternative pathway to starch granule initiation unraveled in <i>Chlamydomonas reinhardtii</i>

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 characterization studies revealed the involvement of both a specific starch synthase isoform (SS4) and several nonenzymatic 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 characterization studies 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 from that described in Arabidopsis and that further efforts will be required to identify the proteins involved in this process.