Useful or merely convenient? On the issue of a suitability of enzymatic antioxidant activity as a proxy for abiotic stress tolerance.

During their lifespan, plants are often exposed to a broad range of stresses that change their redox balance and lead to accumulation of reactive oxygen species (ROS). The traditional view is that this comes with negative consequences to cells structural integrity and metabolism and, to prevent this, plants evolved a complex and well-coordinated antioxidant defence system that relies on the operation of a range of enzymatic and non-enzymatic antioxidants (AO). Due to the simplicity of measuring their activity, and in the light of the persistent dogma that stress-induced ROS accumulation is detrimental for plants, it is not surprising that enzymatic AO have often been advocated as suitable proxies for stress tolerance, as well as potential targets for improving tolerance traits. However, there is a growing number of reports showing either no changes or even downregulation of AO systems in stressed plants. Moreover, ROS are recognised now as important second messengers operating in both local and systemic signalling, synergistically interacting with the primary stressor, to regulate gene expression needed for optimal acclimatization. This work critically assesses the suitability of using enzymatic AO as a proxy for stress tolerance, or as a target for crop genetic improvement. It is concluded that constitutively higher AO activity may interfere with stress-induced ROS signalling and be of disadvantage to plant stress tolerance.