Coalescence and directed anisotropic growth of starch granule initials in subdomains of Arabidopsis thaliana chloroplasts

Abstract

Living cells orchestrate enzyme activities to produce myriads of biopolymers but cell-biological understanding of such processes is scarce. Starch, a plant biopolymer forming discrete, semi-crystalline granules within plastids, plays a central role in glucose storage, which is fundamental to life. Combining complementary imaging techniques and Arabidopsis genetics we reveal that, in chloroplasts, multiple starch granules initiate in stromal pockets between thylakoid membranes. These initials coalesce, then grow anisotropically to form lenticular granules. The major starch polymer, amylopectin, is synthesized at the granule surface, while the minor amylose component is deposited internally. The non-enzymatic domain of STARCH SYNTHASE 4, which controls the protein’s localization, is required for anisotropic growth. These results present us with a conceptual framework for understanding the biosynthesis of this key nutrient.

Rising rates of starch degradation during daytime and trehalose 6-phosphate optimize carbon availability

“…a decrease in the amount of 12 C starch) would show that some of the preexisting starch is being degraded. This is plausible given the recent demonstration that Arabidopsis leaf starch granules grow anisotropically ( Bürgy et al, 2021 ), such that parts of the granule surface could be elaborated with newly fixed C, while other parts of the granule surface are degraded, releasing preexisting starch. Nevertheless, the results of these experiments must be interpreted with caution because cycling of 13 C out of nascent glucans or newly synthesized starch would lead to underestimation of the rates of both synthesis and degradation (see Fernandez et al, 2017 for more extensive discussion)…”

AtFZL is required for correct starch granule morphology in Arabidopsis chloroplasts

“…In Arabidopsis, each chloroplast is reported to contain 5-7 starch granules that are approx. 1-2 µm in diameter at the end of the day (Crumpton-Taylor et al ., 2012) and are formed within stromal pockets between thylakoid membranes (Seung et al ., 2018; Bürgy et al ., 2021). However, factors involved in determining the number, shape and size of starch granules within leaf chloroplasts are still poorly understood.…”

“…In Arabidopsis, the key enzyme that mediates granule initiation is STARCH SYNTHASE 4 ( At SS4) – a glucosyltransferase that may perform the initial glucan elongation steps that prime granule formation (Roldán et al ., 2007). Mutants defective in At SS4 have drastic reductions in the number of starch granules in chloroplasts, most containing zero or one granule (Roldán et al ., 2007; Bürgy et al ., 2021; Lu et al ., 2018). Several additional proteins are proposed to act in granule initiation, by influencing At SS4 activity and/or localisation.…”

“…Most chloroplasts in mutants defective in At PTST2 or At MFP1 have a single, large granule, but the granules retain a normal flattened disc shape (Seung et al ., 2018; Thieme et al ., 2022). Interestingly, in Atss4 mutants, starch granules are round rather than lenticular, suggesting AtSS4 is also required for correct granule morphology (Roldán et al ., 2007; Bürgy et al ., 2021; Lu et al ., 2018).…”

“…It has also been speculated that the flattened shape of leaf starch granules may be influenced by space available for granule growth within the chloroplast, in between layers of thylakoid membranes (Zeeman et al ., 2002). Indeed, recent work demonstrated that granules form within stromal pockets between thylakoids, in which they initiate as multiple small granules that eventually fuse into a single granule (Bürgy et al ., 2021). There is also evidence for associations between granule initiation proteins and thylakoid membranes.…”