Xenobiotic sensing and signalling in higher plants.

Anthropogenic changes and chemical pollution confront plant communities with various xenobiotic compounds or combinations of xenobiotics, involving chemical structures that are at least partially novel for plant species. Plant responses to chemical challenges and stimuli are usually characterized by the approaches of toxicology, ecotoxicology, and stress physiology. Development of transcriptomics and proteomics analysis has demonstrated the importance of modifications to gene expression in plant responses to xenobiotics. It has emerged that xenobiotic effects could involve not only biochemical and physiological disruption, but also the disruption of signalling pathways. Moreover, mutations affecting sensing and signalling pathways result in modifications of responses to xenobiotics, thus confirming interference or crosstalk between xenobiotic effects and signalling pathways. Some of these changes at gene expression, regulation and signalling levels suggest various mechanisms of xenobiotic sensing in higher plants, in accordance with xenobiotic-sensing mechanisms that have been characterized in other phyla (yeast, invertebrates, vertebrates). In higher plants, such sensing systems are difficult to identify, even though different lines of evidence, involving mutant studies, transcription factor analysis, or comparative studies, point to their existence. It remains difficult to distinguish between the hypothesis of direct xenobiotic sensing and indirect sensing of xenobiotic-related modifications. However, future characterization of xenobiotic sensing and signalling in higher plants is likely to be a key element for determining the tolerance and remediation capacities of plant species. This characterization will also be of interest for understanding evolutionary dynamics of stress adaptation and mechanisms of adaptation to novel stressors.