Figure 8: A possible model of the stress tolerance mechanism conferred by RSS1.

RSS1 ensures cell cycle progression, and thereby contributes to the maintenance of meristematic cells under undesirable environmental conditions. RSS1 stability is controlled by the APC/C-26S proteasome pathway, leading to RSS1 accumulation from G1/S to M phase. RSS1 functions primarily at the G1–S transition, possibly through the regulation of protein phosphatase 1 (PP1), which antagonizes the cyclin-dependent protein kinase (CDK) activity. The phosphorylation states of the substrate X are the determinants of G1–S transition (and subsequent cell cycle progression). Under stressful conditions, the G1/S checkpoint is conducted by cyclin-dependent kinase inhibitor (CKI) to inhibit the CDK activity that leads to a restrained transition to S phase, whereas RSS1 sustains cell cycle progression to balance cell division with cell differentiation. Defects in the cell cycle progression in rss1 plants affect, at least, the maintenance of proliferative tissues in the roots and the number of dividing cells and the organization of the shoot apical meristems. The latter interferes with cytokinin metabolism, which in turn influences cell division control and anti-senescence activity. Taken together, the role of RSS1 is essential for growth and viability under stressful conditions.