Light is important for plants as an energy source and a developmental signal, but it can also cause stress to plants and modulates responses to stress. Excess and fluctuating light result in photoinhibition and reactive oxygen species (ROS) accumulation around photosystems II and I, respectively. Ultraviolet light causes photodamage to DNA and a prolongation of the light period initiates the photoperiod stress syndrome. Changes in light quality and quantity, as well as in light duration are also key factors impacting the outcome of diverse abiotic and biotic stresses. Short day or shady environments enhance thermotolerance and increase cold acclimation. Similarly, shade conditions improve drought stress tolerance in plants. Additionally, the light environment affects the plants' responses to biotic intruders, such as pathogens or insect herbivores, often reducing growth‐defence trade‐offs. Understanding how plants use light information to modulate stress responses will support breeding strategies to enhance crop stress resilience. This review summarizes the effect of light as a stressor and the impact of the light environment on abiotic and biotic stress responses. There is a special focus on the role of the different light receptors and the crosstalk between light signalling and stress response pathways.

In higher plants, pluripotent stem cells reside in the specialized microenvironment called stem cell niches (SCNs) harbored at the shoot apical meristem (SAM) and root apical meristem (RAM), which give rise to the aerial and underground parts of a plant, respectively. The model plant Arabidopsis thaliana (Arabidopsis) has been extensively studied to decipher the intricate regulatory mechanisms involving some key transcriptions factors and phytohormones that play pivotal roles in stem cell homeostasis, meristem maintenance, and organ formation. However, there is increasing evidence to show the epigenetic regulation of the chromatin architecture, gene expression exerting an influence on an innate balance between the self-renewal of stem cells, and differentiation of the progeny cells to a specific tissue type or organ. Post-translational histone modifications, ATP-dependent chromatin remodeling, and chromatin assembly/disassembly are some of the key features involved in the modulation of chromatin architecture. Here, we discuss the major epigenetic regulators and illustrate their roles in the regulation of stem cell activity, meristem maintenance, and related organ patterning in Arabidopsis.

Light is important for plant life as a source of energy to drive photosynthesis but also as an environmental signal regulating development or cellular events such as resetting of the circadian clock. Light itself can cause stress such as excess light, fluctuating light, photoperiod and ultraviolet light stress. Light quality, quantity and light duration are important sources of information to prepare plants for future light stress events. Recurring light stress results in acclimation processes to the changing light environment. Furthermore, light regulates the responses of plants to diverse biotic and abiotic stresses. For example, short day conditions or shady environments prime thermotolerance and increase cold acclimation. Similarly, during drought stress, light signaling is important for the plant s stress response. Additionally, the light environment affects the plant s responses to biotic intruders such as pathogens or insect herbivores. Light influence many stress responses resulting in positive growth-defense trade-offs. Under shade, however, plants prioritize growth over defense and stress responses. In this review, we summarize the impact of light as a stressor and its influences on abiotic and biotic stress responses with special focus on the role of the different light receptors and the crosstalk between light signaling components and stress response pathways. ACKNOWLEDGEMENTSWe acknowledge funding by Deutsche Forschungsgemeinschaft (DFG) through Collaborative Research Centre 973 (www.sfb.973) and project Schm 814/29-1. There are no conflicts of interests.