Rapid colour changes in Euglena sanguinea (Euglenophyceae) caused by internal lipid globule migration

The accumulation of red pigments under chronic stress is a response observed in most groups of oxygenic photoautotrophs. It is thought that the red pigments in the cell shield the chlorophyll located underneath from the light. Among these red pigments, the accumulation of carotenoids is one of the most frequent cases. However, the synthesis or degradation of carotenoids is a slow process and this response is usually only observed when the stress is maintained over a period of time. In the Euglenophyte Euglena sanguinea, this is due to the accumulation of a large amount of free and esterified astaxanthin (representing 80% of the carotenoid pool). While reddening is a slow and sometimes irreversible process in other phototrophs, reducing the efficiency of light harvesting by chlorophyll, in E. sanguinea it is highly dynamic, capable of shifting from red to green (and vice-versa) in 10-20 min. This change is not due to de novo carotenogenesis, but to the relocation of cytoplasmic lipid globules where astaxanthin accumulates. Thus, red globules migrate from the centre of the cell to peripheral locations when photoprotection is demanded. This protective system seems to be so efficient that other classical mechanisms are not operative in this species. For example, despite the presence and operation of the diadino-diatoxanthin cycle, nonphotochemical quenching (NPQ) is almost undetectable. Since E. sanguinea forms extensive floating colonies, reddening can be observed at much greater scale than at a cellular level, the mechanism described here being one of the fastest and most dramatic colour changes attributable to photosynthetic organisms at cell and landscape level. In sum, these data indicate an extremely dynamic and efficient photoprotective mechanism based on organelle migration more than on carotenoid biosynthesis that prevents excess light absorption by chlorophylls reducing the need for other protective processes related to energy dissipation.This work was supported by the Basque Government [UPV/EHU-GV IT-1018-16] [UPV/EHU PPG17/67 – GV IT-1040-16], and by the Spanish Ministry of Economy and Competitiveness (MINECO) and the European Research and Development Foundation (FEDER) through (i) [CTM2014-53902-C2-2-P] national grant and (ii) a “Juan de la Cierva-Incorporación” postdoctoral grant [IJCI-2014-22489] to BFM

Evolution, biosynthesis and protective roles of oligogalactolipids: Key molecules for terrestrial photosynthesis?

14 p.Galactolipids (GLs) are the main lipids in chloroplast membranes and by default are also the most abundant polar lipids on earth. GLs with one or two galactose residues, monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG), are ubiquitous and essential for photosynthesis. GLs with a headgroup formed by three to five galactoses, the so-called oligogalactolipids (OGLs), are only detected in some taxa, organs and environmental conditions. OGLs can be synthesized by two metabolic pathways: successive galactosylation by DGDG synthase (DGD) or transgalactosylation from MGDG by the GL:GL galactosyltransferase (GGGT/SFR2). While the first route appeared early in the evolution (cyanobacteria), the second evolved associated to the process of terrestrialization in the streptophytes. Both routes also differ on the anomeric type of glycosidic linkages formed: ?-type in DGD and ?-type in GGGT/SFR2. Despite functional differences between both configurations, the anomeric analysis of OGLs allows tracking their biosynthetic origin. While ?-OGLs are constitutive and present in some algae and non-vegetative organs of vascular plants, ?-OGLs are typically stressinducible in photosynthetic tissues. Land colonization by plants involved new challenges, such as the risk of dehydration, which required developing biochemical and physiological strategies to stabilize chloroplast membranes and safeguard their functioning. Based on the integrated assessment of data available we propose that the appearance of OGLs was one of those adaptations that simultaneously could have provided advantages against other environmental constraints such as freezingUniversidad del País Vasco ; Ministerio de Economía y Competitividad ; FEDER ; Universidad de Alcal

Acclimation of antioxidant pools to the light environment in a natural forest canopy

Leaf growth irradiance determines the pools of photoprotective molecules. We asked whether the potential for acclimation of antioxidant pool size to changes in the leaf light environment is affected by the position of the leaf within the canopy profile. The study was conducted in a mixed canopy formed by Tilia cordata at the lower level and Populus tremula at the upper level. Leaves were either exposed to extra light or enclosed in shade bags. Ascorbate, glutathione and α-tocopherol pools increased with growth irradiance. Only α-tocopherol increased in leaves of both species in response to extra light. The slope of tocopherol changes was positively correlated with growth irradiance in both species. It also correlated with the slope of xanthophyll cycle (VAZ, sum of violaxanthin, antheraxanthin and zeaxanthin) pool changes with cumulative extra light in T. cordata. We conclude that α-tocopherol is the key antioxidant altering tolerance to high light, and that it may cooperate with zeaxanthin. The pools of hydrophilic antioxidants either acclimate more slowly, or their pools are large enough not to limit the overall acclimation to altered light environment