Regulation of Submergence-induced Enhanced Shoot Elongation in Oryza sativa L.

Rice (Oryza sativa L.) is the only cereal that can be cultivatedin the frequently flooded river deltas of South-East and SouthAsia. The survival strategies used by rice have been studiedquite extensively and the role of several phytohormones in theelongation response has been established. Deep-water rice cultivarscan diminish flooding stress by rapid elongation of their submergedtissues to keep up with the rising waters. Other rice cultivarsmay react by mechanisms of submergence tolerance. Aerenchymaand aerenchymatous adventitious roots are formed that facilitateoxygen diffusion to prevent anaerobic conditions in the submergedtissues. This paper discusses the molecular aspects of the mechanismthat leads to shoot elongation (leaves of seedlings and internodes),the regulation of which involves metabolism of, and interactionsbetween, ethylene, gibberellins and abscisic acid. Finally,the importance of new techniques in future research is assessed.Current molecular technology can reveal subtle differences ingene activity between tolerant and non-tolerant cultivars, andidentify genes that are involved in the regulation of submergenceavoidance and tolerance.     

The role of oxygen in submergence-induced petiole elongation in Rumex palustris: in situ measurements of oxygen in petioles of intact plants using micro-electrodes

In a study on the mechanism of stimulated petiole elongation in submerged plants, oxygen concentrations in petioles of the flood-tolerant plant Rumex palustris were measured with micro-electrodes. Short-term submergence lowered petiole partial oxygen pressure to c . 19 kPa whereas prolonged submergence under continuous illumination depressed oxygen levels to c . 8–12 kPa after 24 h. Oxygen levels in petioles depended on the presence of the lamina, even in submerged conditions, and on available light. In darkness, petiole oxygen levels in submerged plants dropped quickly to values as low as 0.5–4 kPa. It is hypothesized that prolonged submergence in the light is accompanied by a decrease in carbon dioxide in the petiole. Submergence-enhanced petiolar elongation rate was compared with emergent plants. Peak daily elongation rates occurred at the end of the dark period in emergent plants, but in the middle of the light period in submerged plants. We suggest that this shift in daily elongation pattern is induced by dependence of growth on photosynthetically derived oxygen in submerged plants. Implications of reduced oxygen for ethylene production are raised. Levels of 1- aminocyclopropane-1-carboxylic acid synthase and 1-aminocyclopropane-1-carboxylic acid oxidase and ethylene sensitivity are cited as potential factors in hypoxia-induced ethylene release.     

Sensitivity to ethylene: the key factor in submergence-induced shoot elongation of Rumex

Rosettes of flooding-resistant Rumex palustris plants show a submergence-induced stimulation of elongation, which is confined to the petioles of young leaves. This response increases the probability of survival. It is induced by ethylene that accumulates in submerged tissues. Flooding-intolerant Rumex acetosella plants do not show this response. We investigated whether differences in shoot elongation between the species, between old and young leaves and between the petiole and leaf blade of a R. palustris plant result from differences in internal ethylene concentration or in sensitivity to the gas. Concentrations of free and conjugated ACC in petioles and leaf blades of R. palustris indicated that ethylene is synthesized throughout the submerged shoot, although production rates varied locally. Nevertheless, no differences in ethylene concentration were found between submerged leaves of various ages. In contrast, dose-response curves showed that only elongation of young petioles of R. palustris was sensitive to ethylene. In R. acetosella, elongation of all leaves was insensitive to ethylene. We conclude that variation in ethylene sensitivity rather than content explains the differences in submergence-induced shoot elongation between the two Rumex species and between leaves of R. palustris.