Massimiliano Corso

Endoplasmic reticulum-localized CCX2 is required for osmotolerance by regulating ER and cytosolic Ca2+ dynamics in Arabidopsis

Significance Given the lack of a nervous system in plants, signaling molecules, such as Ca2+, are of vital importance for perceiving environmental stimuli. However, little information on the genes that control this network is available. Our demonstration that Arabidopsis CCX2, localized to the endoplasmic reticulum, modulates osmotic stress responses through the regulation of cytosolic and endoplasmic reticulum Ca2+ concentrations is one more step toward identifying all the players involved in plant stress responses. The elucidation of these pathways will help to meet the necessary requirements to face global challenges, such as high salinity on the world’s surface, including arable land. This work sheds light on an as yet uncharacterized and key player that tunes intracellular Ca2+ homeostasis, with particular significance for plants under salt stress. Ca2+ signals in plant cells are important for adaptive responses to environmental stresses. Here, we report that the Arabidopsis CATION/Ca2+ EXCHANGER2 (CCX2), encoding a putative cation/Ca2+ exchanger that localizes to the endoplasmic reticulum (ER), is strongly induced by salt and osmotic stresses. Compared with the WT, AtCCX2 loss-of-function mutant was less tolerant to osmotic stress and displayed the most noteworthy phenotypes (less root/shoot growth) during salt stress. Conversely, AtCCX2 gain-of-function mutants were more tolerant to osmotic stress. In addition, AtCCX2 partially suppresses the Ca2+ sensitivity of K667 yeast triple mutant, characterized by Ca2+ uptake deficiency. Remarkably, Cameleon Ca2+ sensors revealed that the absence of AtCCX2 activity results in decreased cytosolic and increased ER Ca2+ concentrations in comparison with both WT and the gain-of-function mutants. This was observed in both salt and nonsalt osmotic stress conditions. It appears that AtCCX2 is directly involved in the control of Ca2+ fluxes between the ER and the cytosol, which plays a key role in the ability of plants to cope with osmotic stresses. To our knowledge, Atccx2 is unique as a plant mutant to show a measured alteration in ER Ca2+ concentrations. In this study, we identified the ER-localized AtCCX2 as a pivotal player in the regulation of ER Ca2+ dynamics that heavily influence plant growth upon salt and osmotic stress.

Transcriptomic analysis supports the role of CATION EXCHANGER 1 in cellular homeostasis and oxidative stress limitation during cadmium stress

ABSTRACT Investigation of genetic determinants of Cd tolerance in the Zn/Cd hyperaccumulator Arabidopsis halleri allowed the identification of the vacuolar Ca2+/H+ exchanger encoding CAX1 gene. CAX1 was proposed to interfere with the positive feedback loop between Reactive Oxygen Species (ROS) production and Cd-induced cytosolic Ca2+ spikes, especially at low external Ca2+ supply. In this study expression of genes involved in ROS homeostasis, cell wall composition, apoplastic pH regulation and Ca2+ homeostasis were monitored in Arabidopsis thaliana wild-type and cax1-1 knock-out mutant and in Arabidopsis halleri wild-type exposed to cadmium or in control conditions. Clustering the outputs of the expression analysis in a gene co-expression network revealed that CAX1 and genes involved in Ca2+ cellular homeostasis, apoplastic pH and oxidative stress response were highly correlated in A. thaliana, but not in A. halleri. Many of the studied genes were already highly expressed in A. halleri and/or their expression was not modified by exposure to Cd. The results further supported the role of CAX1 in the regulation of cytosolic ROS accumulation as well as the existence of different cell wall modifications strategies in response to Cd in Arabidopsis thaliana and halleri.