Figure 13.

Model for the Role of Cytokinin under Circadian Stress in Preventing Disproportionate Stress Responses and Cell Death.

Regular (24 h) light-dark cycles adjust circadian time on a daily basis, leading to optimal synchronization with the environment. This allows the clock to coordinate diverse physiological and developmental processes, including stress responses, in a time-of-day-specific manner, thereby enhancing plant fitness (left side). In contrast, circadian stress regimes lead to a perturbation of the circadian clock, which consequently generates altered, in part detrimental, clock outputs (right side). While a normal cytokinin status can strongly alleviate this response (A), it is particularly pronounced in plants with a reduced cytokinin status exhibiting insufficient nighttime expression of CCA1/LHY (B) and in plants with reduced CCA1/LHY function (C). In these plants, circadian stress causes disproportionate stress responses, specifically oxidative stress and JA responses, eventually leading to cell death. We showed that the activation of the JA pathway, through JA-Ile, promotes cell death development. We conclude that cytokinin, primarily by supporting nighttime CCA1 and LHY expression, sustains proper clock function under circadian stress. We propose that this function reflects an activity of the hormone in promoting clock function that is also exerted under natural favorable conditions (dashed arrow) and serves as a backup mechanism under unfavorable conditions.