On the Inside

Claeys et al. (pp. 519–527) have conducted a literature review that reveals that a majority of previous studies examining abiotic stress have used very high stress levels and parameters such as germination, plant survival, or the development of visual symptoms such as bleaching. The authors assert that these parameters, at least in Arabidopsis (Arabidopsis thaliana), are indicators of extreme stress and thus provide only incomplete information about stress sensitivity. Even at the molecular level, typical stressmarkers are induced only at high stress levels in young seedlings. However, in natural and field conditions, plants often experience a wide variety of stress levels, requiring a range of different response mechanisms. Therefore, tools are needed to study the effects of mild stress. To address this issue, the authors surveyed the scientific literature concerning Arabidopsis stress physiology studies that have used in vitro assays. For each relevant publication, they determined which stress level and what types of phenotypingwere used to assess stress responses. Based on the results of this survey, they exposed Arabidopsis seedlings to a wide range of salt, osmotic, and oxidative stresses, tracking germination and growth over time and measuring the expression of marker genes. These experiments confirmed that there is a highly dose-dependent response of plants to stress, the nature of which depends on the type of stress, and that shoot growth is a very sensitive indicator of stress. Furthermore, current marker genes for stress are induced only at very severe stress levels, and novel mild stress markers are needed. Finally, their data suggest that care should be taken when using mannitol as an osmoticum. Plant growth is extremely sensitive to mannitol, and growth rates drop rapidly when plants are exposed to low concentrations, but this effect levels off asmannitol levels increase. However, for sorbitol, the other osmoticum they tested, this was not the case. Recently, it was reported that Arabidopsis contains a pair of specific receptors formannitol that trigger growth arrest. This is thought to be a biotic stress response because many fungi produce mannitol; thus, it is distinct from generic osmotic stress effects. Mannitol thus induces a combination of biotic and osmotic stress responses.