While working on a project to unravel the role of Vitamin E, Jana Cela, from the University of Barcelona, measured the expression of genes involved in ethylene biosynthesis as well as the perception (receptors) and signalling in plants with altered Vitamin E composition. “My idea was to examine the role, if any, of Vitamin E beyond its well-known antioxidant function in plants”, Jana says. “First, I investigated how ethylene signalling alters Vitamin E biosynthesis in plants. Later, I wondered if Vitamin E could also affect ethylene-related genes”. This idea was developed together with me (Sergi Munné-Bosch), a Senior Lecturer at the University of Barcelona. Caren Chang, Senior Lecturer of the University of Maryland, also contributed to this research project.
Plants help us understand complex Vitamin E functions. Since vitamins are compounds that we humans cannot synthesize, they need to be replenished in our diets. Vitamins have a complex biochemistry and play an essential role in human nutrition and health. Vitamin deficiencies cause diseases that can be severe and even lethal in some cases. Plants, on the other hand, consist of a wide range of vitamins that are essential not only for human metabolism but also for the plants themselves; because of their role as cofactors (compounds/substances favouring several enzymatic reactions), some of them, such as Vitamin E, also have strong antioxidant potential.
Naturally Accumulating In the Oldest Leaves
Recent investigations have focused on whether or not these compounds can play roles beyond their antioxidant function, such as cell signalling effects in both animals and plants. Plants are very useful in this respect since they are, together with algae and cyanobacteria, the only organisms capable of synthesizing this interesting group of compounds. Therefore, the researchers expected to find particular phenotypes in those organisms that have developed the capacity to synthesize these compounds.
Investigating the response of tocopherol mutants (which are plants with altered gene expression in Vitamin E biosynthesis genes) to water and salt stress by imposing deficit irrigation and by watering plants with a saline solution, respectively, it was discovered that while Vitamin E deficiency does not alter plant response to stress, the accumulation of ɣ-tocopherol instead of α-tocopherol leads to altered gene expression in ethylene biosynthesis, perception and signalling genes, particularly in the oldest leaves. It is interesting to note that ɣ-tocopherol accumulates in a natural way in the oldest leaves of plants and that ethylene is involved in the regulation of programmed cell death, among other processes.
Therefore, it is likely that ɣ-tocopherol accumulation in such mutants is sensed as a stress signal by old leaves only. This study therefore demonstrates, for the first time, a link between the Vitamin E composition of leaves and ethylene-related responses. This study suggests that the levels of ɣ- and α-tocopherol may be finely regulated to prevent any alteration at the gene expression level. In other words, if ɣ-tocopherol accumulates instead of α-tocopherol, the plant responds as though it were exposed to a stress situation, thus suggesting that Vitamin E acts as a stress signal. If a similar process occurs in animals, then, the intake of different tocopherol forms should be carefully considered in human nutrition.