Title : Microalgae: An excellent tool to study the effects of sulphur nutrition starvation on plants in general
Abstract:
Sulfur (S) deficiency is a recent but serious problem in some European countries, attributed to the decline in industrial emissions and to less intensive application of mineral fertilizers. In all plants, S is assimilated into numerous essential compounds such as the amino acids cysteine and methionine, glutathione, hormones, vitamins, cofactors, and numerous secondary products. In addition, some S-containing compounds are involved in stress responses; for example, glutathione is an important antioxidant that prevents ROS (Reactive Oxygen Species) from damaging cellular components and it is a substrate for phytochelatin synthesis. Consequently, insufficient S nutrition reduces plant growth and resistance to abiotic and biotic stresses. Vascular plants have evolved organ-specific modes of nutrition: a phototrophic shoot closely interacts with a heterotrophic root using xylem and phloem as communication highways. The complex communication between autotrophic and heterotrophic metabolism could be studied in unicellular organisms and the results could be transferred to the multiorgan system of higher plants. The utilization of unicellular algae as model system to study enzymes involved in plant metabolism is advantageous because their metabolism depends only on the nutrients that each cell takes up from the medium and not from a metabolite exchange between cells. Microalgae reproduces fast, which provides an opportunity to introduce a wide range of nutritional changes in culture and to study their effects in a short span of time. Then, the unicellular algae represent a suitable experimental system to study phenomena that occur in plant cells as a consequence of sulphur shortage or S supply. A tight regulatory cross-talk between glutathione and ascorbic acid metabolism and the activation of ROS-scavenging enzymes, in the green alga C. sorokiniana subjected to S deficiency, was demonstrated. In plants and algae, sulfate assimilation and cysteine synthesis are regulated by S availability from the environment. In addition, we present the first report detailing the effects of S starvation under both autotrophic and heterotrophic culture conditions in the microalga Galdieria phlegrea inhabiting hot springs with high S and sulphate levels. Our results show that in G. phlegrea both in autotrophic cells but especially in cells cultured in heterotrophy, the glutathione content is much higher than that found in C. sorokiniana. The results presented are discussed in the context of physiological changes that occur in an S deficient plant cell and aim to provide new insights on metabolism regulation under S limited conditions.
