Starch Synthase 4 and Plastidal Phosphorylase Differentially Affect Starch Granule Number and Morphology

Abstract

The process of starch granule formation in leaves of Arabidopsis () is obscure. Besides STARCH SYNTHASE4 (SS4), the PLASTIDIAL PHOSPHORYLASE (PHS1) also seems to be involved, since double mutants lacking both PHS1 and the cytosolic DISPROPORTIONATING ENZYME2 (DPE2) displayed only one starch granule per chloroplast under normal growth conditions. For further studies, a triple mutant and various combinations of double mutants were generated and metabolically analyzed with a focus on starch metabolism. The mutant revealed a massive starch excess phenotype. Furthermore, these plants grown under 12 h of light/12 h of dark harbored a single large and spherical starch granule per plastid. The number of starch granules was constant when the light/dark regime was altered, but this was not observed in the parental lines. With regard to growth, photosynthetic parameters, and metabolic analyses, the triple mutant additionally displayed alterations in comparison with and The results clearly illustrate that PHS1 and SS4 are differently involved in starch granule formation and do not act in series. However, SS4 appears to exert a stronger influence. In connection with the characterized double mutants, we discuss the generation of starch granules and the observed formation of spherical starch granules.

Distinct Functions of STARCH SYNTHASE 4 Domains in Starch Granule Formation

“…PTST2 was proposed to bind long malto-oligosaccharides with its carbohydrate-binding module and provide them as substrates to SS4 (Seung et al, 2017). This idea is broadly consistent with other observations that mutations affecting malto-oligosaccharide metabolism affect starch granule initiation (Malinova et al, 2017;Satoh et al, 2008;Seung et al, 2016).…”

“…Rather, it appears to play a critical role in starch granule initiation: the Arabidopsis (Arabidopsis thaliana) ss4 mutant usually has zero, one, or rarely two or three granules per chloroplast (Roldán et al, 2007;Malinova et al, 2017), instead of five to seven granules found in most wild-type chloroplasts (Crumpton-Taylor et al, 2012). The total starch content at dusk is lower in this mutant, yet this starch is incompletely metabolized at night.…”

Starch Biosynthesis in the Developing Endosperms of Grasses and Cereals

“…Moreover, recent studies with the rice and barley Pho1 indicate that the enzyme is capable of producing and extending MOS solely from Glc1P (i.e., in the absence of α-glucan primer [302]. If SP plays a role in granule initiation, as indicated by these studies (and those of Malinova et al [254] in the above Section 5.2.2), it is not yet clear whether this is in concert with the SSIII/SSIV-mediated pathway or if the two pathways operate separately under different circumstances. It has been argued that SP and SBE activities acting in concert produce branched glucans resistant to phosphorolysis in the early stages of endosperm development and potential initiation points for granule synthesis to continue [302,362].…”

“…For a review of starch granule initiation the reader is referred to a review by D'Hulst and Mérida [252]. Recently, a role for plastidial starch phosphorylase (SP, see below) has been proposed for starch granule initiation in chloroplasts, in addition to SSIII and SSIV [253,254]. However, studies with ssIV mutants in the endosperm of japonica rice show very different effects on storage starch synthesis.…”

Auxin‐mediated statolith production for root gravitropism

“…Of these genes, SS4 seems to play a predominant role in starch granule formation within the root apex and in root gravitropism. Recent work has also shown that the SS4 gene is important for regulating starch granule number in leaves (Malinova et al, 2017). Further work revealed that among SS isoforms, only SS4 contains a unique Nterminal region with several long coiled-coil motifs that have been implicated in protein-protein interactions.…”