Indoleacetic Acid Levels in Wheat and Rice Seedlings under Oxygen Deficiency and Subsequent Reoxygenation

The lack of oxygen and post-anoxic reactions cause significant alterations of plant growth and metabolism. Plant hormones are active participants in these alterations. This study focuses on auxin-a phytohormone with a wide spectrum of effects on plant growth and stress tolerance. The indoleacetic acid (IAA) content in plants was measured by ELISA. The obtained data revealed anoxia-induced accumulation of IAA in wheat and rice seedlings related to their tolerance of oxygen deprivation. The highest IAA accumulation was detected in rice roots. Subsequent reoxygenation was accompanied with a fast auxin reduction to the control level. A major difference was reported for shoots: wheat seedlings contained less than one-third of normoxic level of auxin during post-anoxia, while IAA level in rice seedlings rapidly recovered to normoxic level. It is likely that the mechanisms of auxin dynamics resulted from oxygen-induced shift in auxin degradation and transport. Exogenous IAA treatment enhanced plant survival under anoxia by decreased electrolyte leakage, production of hydrogen peroxide and lipid peroxidation. The positive effect of external IAA application coincided with improvement of tolerance to oxygen deprivation in the 35S:iaaM × 35S:iaaH lines of transgene tobacco due to its IAA overproduction.

Kinetics of ethanol and acetaldehyde release suggest a role for acetaldehyde production in tolerance of rice seedlings to micro-aerobic conditions

BACKGROUND AND AIMS

This paper examines the basis of the greater tolerance of an indica rice cultivar FR13A to complete submergence compared with relatively intolerant japonica rice CT6241. We study whether this superior tolerance is related to its greater tolerance to O2 shortage and to an ability to run a more favourable rate of alcoholic fermentation during and after O2 deprivation. METHODS Fermentation products were analysed using sensitive laser-based photoacoustics at high time resolution to establish patterns and rates of ethanol and acetaldehyde emission by intact rice seedlings exposed to micro-aerobic (0.05-0.5 % O2) or zero O2 supply, and also during their return to air. Oxygen and CO2 emission or uptake was also quantified.

KEY RESULTS

In the dark, no acetaldehyde and ethanol emission was observed until external O2 concentration in a gas phase decreased to <or=0.3% O2. The ethanol production rate was maximal in 0% O2, similar in both cultivars and gradually diminished with increasing O2 concentration. Lag time for induction of fermentation increased with O2 up to 0.3% and was shorter in CT6241. Light strongly suppressed fermentation. In contrast to that of ethanol, emission of acetaldehyde in the dark under micro-aerobic conditions (<or=0.15% O2 gas phase) exceeded that under anaerobiosis, was maximal in 0.05% O2 and was greater in FR13A than in CT6241. A drop in acetaldehyde emission to about half its value immediately followed a switch to anaerobic conditions after 6.5 h treatment under 0.05% O2, while ethanol release showed a further increase. A large peak in acetaldehyde emission immediately followed the return of seedlings to air after treatment with <or=0.15% O2. The emission from FR13A was up to three times larger than from CT6241.

CONCLUSIONS

Tolerance to submergence in FR13A appears not to be connected to its rate of ethanol production during anaerobiosis, but to the increased acetaldehyde output during and after experiencing micro-aerobic conditions (0.05-0.15% O2). Extra acetaldehyde production from ethanol may be a consequence of diversion of the reactive oxygen species away from the damaging lipid peroxidation pathway.