Abstract

Starch is a vital plant product and accumulates inside plastids of green algae and plants. The component, amylopectin, is a branched glucan polymer with similarities to glycogen made in animals, fungi, and prokaryotes. Yet unlike glycogen, starch occurs as insoluble, semi-crystalline granules, which in Arabidopsis chloroplasts always form between thylakoid membranes. Using complementary approaches (electron tomography, isotope labelling, and NanoSIMS imaging), we visualized how starch granules initiate and subsequently grow. Further, using a combination of biochemistry and genetics we identified a set of proteins involved in the initiation process. Modulation of these proteins can greatly influence the number of granules that form. One protein, MFP1 (MAR BINDING FILAMENT-LIKE PROTEIN 1) is thylakoid anchored and assembles into discrete domains within the membrane system. Remarkably, MFP1 directs where the starch granules will form, and relocalizing it is sufficient to change the site of starch formation. Many proteins involved in starch biosynthesis become embedded within the granule as it grows. Two granule-bound proteins, LIKE EARLY STARVATION 1 (LESV) and EARLY STARVATION 1 (ESV1), identified using proteomic analyses, both possess an unusual, conserved carbohydrate-binding surface. Functional studies in yeast and in plants suggest that LESV and ESV1 help promote amylopectin phase transition, and stabilize the resultant granules, respectively. This is remarkable since crystallization was long assumed to occur spontaneously. Collectively, these studies into the cell biological aspects of starch biosynthesis reveal the importance of non-enzymatic carbohydrate binding proteins and offer a range of new targets with which to modulate the amount and/or properties of starch in crops.

Dr. Zeeman has long been a world leader in the field of starch metabolism and has broad experience in many aspects of plant biochemistry.  If you would like to meet with Dr. Zeeman on Friday, 11/07 please contact Alan Myers at ammyers@iastate.edu.

Host: Alan Myers