Disentangling the Role of Transitory Starch Storages in Plant Development and in Osmotic Stress Response

Starch is a polymer of D-glucose that plants accumulate as semi-crystalline and osmotically inert granules. Besides being the major energy storage in plants, starch is of primary importance also in human and animal diet, industry and biofuels production. Two kinds of starch, structurally indistinguishable, but different for location and rates of synthesis and degradation are found in plants: secondary starch (characterized by long term-accumulation and found in storage organs) and transitory starch (mainly located in chloroplasts, produced during the day and degraded the subsequent night to meet the energy demand of the plant). Due to its structure, several enzymes are required in starch biosynthesis and degradation, controlling distinct features of starch granules and conferring different physical-chemical properties. Here, the TILLING approach was used on the barley TILLMore population to identify new alleles in five genes related to secondary starch metabolism and known to be expressed in barley seeds. Moreover, the role in Arabidopsis development of the (phospho)glucan, water dikinase proteins (GWD1, GWD2, PWD), known to be involved in nighttime transitory starch degradation, was investigated. Other enzymes, such as β-amylase 1 (BAM1) and α-amylase 3 (AMY3), were demonstrated to be involved in diurnal transitory starch degradation in specialized cells or under stress conditions. Here, we demonstrated that carbon skeletons deriving from BAM1 diurnal degradation of transitory starch support the biosynthesis of proline, a compatible solute, required to face osmotic stress. Moreover, the behaviour of Arabidopsis BAM1 and AMY3 enzymes under oxidative treatments and the possible role of glutathionylation, a redox post-translational modification occurring mainly under stress conditions and promoted by ROS, were investigated. AtBAM1 and AtAMY3 were found to be sensitive to oxidants and glutathionylated, with a modulatory and protective effect on protein activity.