On the role of guard cells in sensing environmental signals and memorising stress periods

The capacity to perceive and memorise adverse environmental conditions is pivotal for the survival of any biological system. Despite being sessile, plants do it with maestri. Plants can rapidly respond to changes in environmental cues and memorise stress conditions, as a consequence of their modularity and the presence of highly complex cell types, such as the guard cells. These cells integrate endogenous and environmental signals to regulate the opening of the stomatal pore, mainly found at leaf epidermis. Whilst the stomatal opening enables the influx of CO2 for photosynthesis, stomatal closure is important to reduce water loss during drought stress. Guard cell is thus crucial for the perception of environmental signals and a master regulator of water use efficiency (WUE). Furthermore, recent results indicate that guard cell gene expression precedes those observed in mesophyll cells when plants are subjected to drought and that guard cell is an important hub for stress memory. Here, we highlight these recent findings and provide an updated overview regarding the intrinsic complexity of guard cell structure and the signalling networks related to the perception and response to different environmental signals. We explored recently published guard cell transcriptomics data from plants under drought and discussed their implication for plant stress acclimation and metabolic engineer toward plant drought tolerance improvement. We further highlight the possible interplay among the mechanisms that regulate stress memory and stomatal speediness and how they can be used to improve WUE and/or stress tolerance in plants.