Mechanisms of adaption of plants to abiotic stresses

Starch is the main storage carbohydrate in plants, providing the energy and the carbon skeletons necessary for the metabolism of the plant during the dark periods, when the photosynthesis is inactive. The synthesis of starch is a coordinated process that involves different enzymatic activities: elongation of the glucose chain (starch synthase), branching of the chains (starch branching enzyme) and removal of excess branches (isoamylases or debranching enzymes). Each activity is carried out by different isoenzymes that display unique characteristic, such as length of the glucose chain used as substrate, localization in the chloroplast or expression pattern. In addition to those enzymes, there are other proteins involved in the synthesis of starch performing different functions such as facilitating the interaction of some enzymes with the polymer or modulating the activity of the enzymes. During the last years, our Group has studied the function of the different starch synthases in the model plant Arabidopsis thaliana. There are 5 starch synthases in this plant. One of them (GBSSI) is involved in the synthesis of amylose (minor component of the starch, scarcely branched), whereas the other 4 (SS1-4) are involved in the synthesis of amylopectin (major component of the starch and highly branched). We have shown that each SS has a specific localization in the chloroplast. SS1 is found soluble in the stroma, whereas SS2 has a dual localization, soluble in the stroma and tightly bound to the starch granule. SS3 is localized surrounding the starch granule but the interaction with the polymer is weak as it is not found in isolated granules. And SS4 is localized in discrete dots in the chloroplasts, associated to a plastidial organelles denominated plastoglobules. Our Group has shown that SS4 does not participate in the elongation of the amylopectin branches, and it is involved in a process that has not been well characterized: the initiation of the starch granule. SS4 is a big and complex protein that appears as a dimer in vivo and that interacts with fibrillins 1a and 1b, two proteins associated to the plastoglobules. The monomer is inactive and does not interact with the fibrillins. Currently, we are analysing a possible phosphorylation of the protein that could modulate the activity of SS4 through the formation/elimination of the dimer.
We have shown that the SS4 from Brachypodium distachyon, a model for cereals, complements the absence of SS4 in an Arabidopsis mutant lacking this protein, which indicates that the function of SS4 has been evolutionally conserved. At present, our Group is analysing the effect of changes in the expression levels of SS4 in the synthesis of the storage starch and other aspects of the metabolism of potato tubers. We are modifying the levels of expression of SS4 both in the aerial part and the tuber of potato plants.

Adaptive mechanism of plants to light stress

More recently, our Group has initiated a new research topic leading to characterize the function of the fibrillins in the adaptation of plants to light stress. Fibrillins (FBNs) are a multigene family (there are 13 fibrillins in Arabidopsis) found in all plants and other photosynthetic organism such as algae and cyanobacteria. Some of these proteins are found associated to plastoglobules, lipoprotein vesicles originated from thylakoids membranes that remain physically attached to them, whereas other fibrillins are found soluble in the stroma, bound to the thylakoids membranes or they display a dual localization. It was though that fibrillins played a structural function in the accumulation of carotenoids in chromoplasts. However, it has been found that these proteins are involved in different metabolic processes such synthesis of vitamin E, synthesis of jasmonic acid or protecting the photosystems under high intense illumination, although their mechanism of action have not been determined.
Our Group is characterizing four fibrillins (FBN1a, 1b, 2 and 4) from Arabidopsis. We have shown that FBN1a and 1b interact through a “tail-head” mechanism, which allow the coalescence of plastoglobules. Nowadays, we are performing experiments to identify the transduction signal pathways mediated by these fibrillins involved in the adaptation of plants to light stress, and to identify other proteins that interact with these fibrillins.
 

- Regulation of the synthesis of starch in Arabidopsis thaliana

- Identification of elements involved in the initiation of the starch granule in long-term storage organs: endosperm of Brachypodium and potato tubers.

Dirección General de Investigación, Consejería de Economía, Innovación y Ciencia (Junta de Andalucía), Ministerio de Ciencia e Innovación
Selected Publications
Type: Publications in journals
The N-terminal part of Arabidopsis starch synthase 4 determines the localization and activity of the enzyme. Journal of Biological Chemistry
Authors: Raynaud, S., Ragel, P., Rojas, T., Mérida, A.
Publication Year: 2016
Pages: 10759-10771
Journal: Journal of Biol. Chem.
Volume: 291
Type: Publications in journals
Starch synthase 4 is located in the thylakoid membrane and interacts with plastoglobule-associated proteins in Arabidopsis
Authors: Gámez-Arjona, F.F., Raynaud, S., Ragel, P., Mérida, A.
Publication Year: 2014
Pages: 305-316
Journal: Plant Journal
Volume: 80