Abscisic acid (ABA) is an important phytohormone regulating plant growth, development, and stress responses. It has an essential role in multiple physiological processes of plants, such as stomatal closure, cuticular wax accumulation, leaf senescence, bud dormancy, seed germination, osmotic regulation, and growth inhibition among many others. Abscisic acid controls downstream responses to abiotic and biotic environmental changes through both transcriptional and posttranscriptional mechanisms. During the past 20 years, ABA biosynthesis and many of its signaling pathways have been well characterized. Here we review the dynamics of ABA metabolic pools and signaling that affects many of its physiological functions.

Drought stress is a condition that in specific climate contexts results in insufficient water availability and often limits plant productivity through perturbing development and reducing plant growth and survival. Plants use senescence of old leaves and dormancy of buds and seeds to survive extreme environmental conditions. The plant hormone ABA accumulates after drought stress, and increases plant survival by inducing quick responses such as stomatal closure, and long-term responses such as extended growth inhibition, osmotic regulation, accumulation of cuticular wax, senescence, abscission and dormancy. Here we focus on how the long-term ABA responses contribute to plant survival during severe drought stress. Leaf senescence and abscission of older leaves reduce total plant transpirational water loss and increase the transfer of nutrients to meristems and to some storage tissues. Osmotic regulation favors water consumption in sink tissues, and accumulation of cuticular wax helps to seal the plant surface and limits non-stomatal water loss.

Summary Environmental stimuli evoke transient increases of the cytosolic Ca 2+ level. To identify upstream components of Ca 2+ signaling, we have optimized two forward genetic screening systems based on Ca 2+ reporter aequorin. AEQ sig6 and AEQ ub plants were used for generating ethyl methanesulfonate (EMS)-mutagenized libraries. The AEQ sig6 EMS-mutagenized library was preferably used to screen the mutants with reduced Ca 2+ signal response due to its high effectiveness, while the AEQ ub EMS-mutagenized library was used for screening of the mutants with altered Ca 2+ signal response. For complete details on the use and execution of this protocol, please refer to Chen et al. (2020) and Zhu et al. (2013) .