The SIAMESE family of cell-cycle inhibitors in the response of plants to environmental stresses.

Environmental abiotic constraints are known to reduce plant growth. This effect is largely due to the inhibition of cell division in the leaf and root meristems caused by perturbations of the cell cycle machinery. Progression of the cell cycle is regulated by CDK kinases whose phosphorylation activities are dependent on cyclin proteins. Recent results have emphasized the role of inhibitors of the cyclin-CDK complexes in the impairment of the cell cycle and the resulting growth inhibition under environmental constraints. Those cyclin-CDK inhibitors (CKIs) include the KRP and SIAMESE families of proteins. This review presents the current knowledge on how CKIs respond to environmental changes and on the role played by one subclass of CKIs, the SIAMESE RELATED proteins (SMRs), in the tolerance of plants to abiotic stresses. The SMRs could play a central role in adjusting the balance between growth and stress defenses in plants exposed to environmental stresses.

The response of Arabidopsis to the apocarotenoid ß-cyclocitric acid reveals a role for SIAMESE-RELATED 5 in root development and drought tolerance.

New regulatory functions in plant development and environmental stress responses have recently emerged for a number of apocarotenoids produced by enzymatic or nonenzymatic oxidation of carotenoids. ß-Cyclocitric acid (ß-CCA) is one such compound derived from ß-carotene, which triggers defense mechanisms leading to a marked enhancement of plant tolerance to drought stress. We show here that this response is associated with an inhibition of root growth affecting both root cell elongation and division. Remarkably, ß-CCA selectively induced cell cycle inhibitors of the SIAMESE-RELATED (SMR) family, especially SMR5, in root tip cells. Overexpression of the SMR5 gene in Arabidopsis induced molecular and physiological changes that mimicked in large part the effects of ß-CCA. In particular, the SMR5 overexpressors exhibited an inhibition of root development and a marked increase in drought tolerance which is not related to stomatal closure. SMR5 up-regulation induced changes in gene expression that strongly overlapped with the ß-CCA-induced transcriptomic changes. Both ß-CCA and SMR5 led to a down-regulation of many cell cycle activators (cyclins, cyclin-dependent kinases) and a concomitant up-regulation of genes related to water deprivation, cellular detoxification, and biosynthesis of lipid biopolymers such as suberin and lignin. This was correlated with an accumulation of suberin lipid polyesters in the roots and a decrease in nonstomatal leaf transpiration. Taken together, our results identify the ß-CCA-inducible and drought-inducible SMR5 gene as a key component of a stress-signaling pathway that reorients root metabolism from growth to multiple defense mechanisms leading to drought tolerance.