How plants cope with complete submergence.

Flooding is a widespread phenomenon that drastically reduces the growth and survival of terrestrial plants. The dramatic decrease of gas diffusion in water compared with in air is a major problem for terrestrial plants and limits the entry of CO(2) for photosynthesis and of O(2) for respiration. Responses to avoid the adverse effects of submergence are the central theme in this review. These include underwater photosynthesis, aerenchyma formation and enhanced shoot elongation. Aerenchyma facilitates gas diffusion inside plants so that shoot-derived O(2) can diffuse to O(2)-deprived plant parts, such as the roots. The underwater gas-exchange capacity of leaves can be greatly enhanced by a thinner cuticle, reorientation of the chloroplasts towards the epidermis and increased specific leaf area (i.e. thinner leaves). At the same time, plants can outgrow the water through increased shoot elongation, which in some species is preceded by an adjustment of leaf angle to a more vertical position. The molecular regulatory networks involved in these responses, including the putative signals to sense submergence, are discussed and suggestions made on how to unravel the mechanistic basis of the induced expression of various adaptations that alleviate O(2) shortage underwater.

Expression of alpha-expansin genes during root acclimations to O2 deficiency in Rumex palustris.

Thirteen alpha-expansin genes were isolated from Rumex palustris , adding to the six already documented for this species. Five alpha-expansin genes were selected for expression studies in various organs/tissues of R. palustris , with a focus on roots exposed to aerated or O2)-deficient conditions, using real-time RT-PCR. Several cases of differential expression of alpha-expansin genes in the various root types of R. palustris were documented, and the identity of the dominant transcript differed between root types (i.e., tap root vs. lateral roots vs. adventitious roots). Several genes were expressed differentially in response to low O2. In situ hybridizations showed expansin mRNA expression in the oldest region of the tap root was localized to cells near the vascular cambium; this being the first report of expansin expression associated with secondary growth in roots. In situ hybridization also showed abundant expression of expansin mRNA in the most apical 1 mm of adventitious roots. Such early expression of expansin mRNA in cells soon after being produced by the root apex presumably enables cell wall loosening in the elongation zone of roots. In addition, expression of some expansin mRNAs increased in 'mature zones' of roots; these expansins might be involved in root hair formation or in formation of lateral root primordia. The present findings support the notion that large gene families of alpha-expansins enable flexibility in expression for various organs and tissues as a normal part of plant development, as well as in response to abiotic stress.

Interactions between plant hormones regulate submergence-induced shoot elongation in the flooding-tolerant dicot Rumex palustris.

Rumex palustris has the capacity to respond to complete submergence with hyponastic (upward) growth and stimulated elongation of petioles. These adaptive responses allow survival of this plant in habitats with sustained high water levels by re-establishing contact with the aerial environment. Accumulated ethylene in submerged petioles interacts with ethylene receptor proteins and operates as a reliable sensor for the under-water environment. Further downstream in the transduction pathway, a fast and substantial decrease of the endogenous abscisic acid concentration and a certain threshold level of endogenous auxin and gibberellin are required for hyponastic growth and petiole elongation. Interactions of these plant hormones results in a significant increase of the in vitro cell wall extensibility in submerged petioles. Furthermore, the pattern of transcript accumulation of a R. palustris alpha-expansin gene correlated with the pattern of petiole elongation upon submergence.