A Regulator of Calcium Signatures Revealed

Calcium (Ca²⁺) is an important cellular second messenger for diverse developmental processes and environmental responses in both plants and animals. Transient increases in cytosolic Ca²⁺ are activated in plants during a host of environmental and developmental processes, including root growth, stomatal movement, pollen growth, abiotic stress responses, and plant-microbe interactions. The secret as to how calcium transients regulate such a diversity of processes may be due to the fact that not all calcium transients are identical. Calcium transients with unique magnitude, frequency, shape, and duration in response to environmental and endogenous cues are referred as “calcium signatures,” and they are thought to encode stimulus-specific information. Calcium signatures are generated by the coordinated action of various Ca²⁺ influx and efflux systems including channels, pumps, and exchangers located at different cellular membranes. However, the molecular identities of calcium-specific influx channels remain controversial. An intriguing component involved in calcium signaling and plant immunity is the Arabidopsis BONZAI1 (BON1) gene. BON1 is a member of an evolutionarily conserved protein family found in protozoa, plants, nematodes, and mammals. BON1 is a plasma membrane localized protein that negatively regulates the expression of immune receptor genes and positively regulates stomatal closure. Yang et al. (10.1104/pp.17.00495) now provide evidence that BON1 functions together with autoinhibited calcium ATPase10 (ACA10) and ACA8 to regulate calcium signatures in Arabidopsis. aca10 loss-of-function mutants have an autoimmune phenotype similar to that of the bon1 loss-of-function mutants. Moreover, these loss-of-function mutants have altered calcium signatures and calcium homeostasis and are defective in stomatal closure and plant immunity. Most strikingly, a cytosolic calcium oscillation imposed by external calcium treatment was altered in aca10, aca8, and bon1 mutants Thus, this research has uncovered a critical role for the plasma membrane-localized calcium pumps ACA10/8 in calcium signature generation as well as their regulation by BON1 in Arabidopsis.