CDPKs (Ca²⁺-Dependent Protein Kinases) are very important regulators in plant response to abiotic stress. The molecular regulatory mechanism of CDPKs involved in salt stress tolerance remains unclear, although some CDPKs have been identified in salt-stress signaling. Here, we investigated the function of an Arabidopsis CDPK, CPK12, in salt-stress signaling. The CPK12-RNA interference (RNAi) mutant was much more sensitive to salt stress than the wild-type plant GL1 in terms of seedling growth. Under NaCl treatment, Na⁺ levels in the roots of CPK12-RNAi plants increased and were higher than levels in GL1 plants. In addition, the level of salt-elicited H₂O₂ production was higher in CPK12-RNAi mutants than in wild-type GL1 plants after NaCl treatment. Collectively, our results suggest that CPK12 is required for plant adaptation to salt stress.

Extracellular ATP (eATP) is now recognized as an important signaling agent in plant growth and defense response to environmental stimuli. eATP has dual functions in plant cell signaling, which is largely dependent on its concentration in the extracellular matrix (ECM). A lethal level of eATP (extremely low or high) causes cell death, whereas a moderate level of eATP benefits plant growth and development. Ecto-apyrases (Nucleoside Triphosphate-Diphosphohydrolase) help control the eATP concentrations in the ECM, and thus contributing to the mediation of plant growth and defense response upon environmental stress. In this review, we summarize eATP signaling in plants and highlight the correlation between eATP homeostasis control and programmed cell death.

Sodium chloride (NaCl) induced expression of a jacalin-related mannose-binding lectin (JRL) gene in leaves, roots, and callus cultures of Populus euphratica (salt-resistant poplar). To explore the mechanism of the PeJRL in salinity tolerance, the full length of PeJRL was cloned from P. euphratica and was transformed into Arabidopsis. PeJRL was localized to the cytoplasm in mesophyll cells. Overexpression of PeJRL in Arabidopsis significantly improved the salt tolerance of transgenic plants, in terms of seed germination, root growth, and electrolyte leakage during seedling establishment. Under NaCl stress, transgenic plants retained K⁺ and limited the accumulation of Na⁺. PeJRL-transgenic lines increased Na⁺ extrusion, which was associated with the upward regulation of SOS1, AHA1, and AHA2 genes encoding plasma membrane Na⁺/proton (H⁺) antiporter and H⁺-pumps. The activated H⁺-ATPases in PeJRL-overexpressed plants restricted the channel-mediated loss of K⁺ that was activated by NaCl-induced depolarization. Under salt stress, PeJRL–transgenic Arabidopsis maintained reactive oxygen species (ROS) homeostasis by activating the antioxidant enzymes and reducing the production of O₂⁻ through downregulation of NADPH oxidases. Of note, the PeJRL-transgenic Arabidopsis repressed abscisic acid (ABA) biosynthesis, thus reducing the ABA-elicited ROS production and the oxidative damage during the period of salt stress. A schematic model was proposed to show the mediation of PeJRL on ABA response, and ionic and ROS homeostasis under NaCl stress.

Stomatal regulation is crucial to reduce water consumption under drought conditions. Extracellular ATP (eATP) serves as a signaling agent in stomatal regulation; however, it is less known whether the eATP mediation of stomatal aperture is linked to apyrases (APYs), the principal enzymes that control the concentration of eATP. To clarify the role of APYs in stomatal control, PeAPY1 and PeAPY2 were isolated from Populus euphratica and transferred into Arabidopsis. Compared with the wild-type Arabidopsis and loss-of-function mutants (Atapy1 and Atapy2), PeAPY1- and PeAPY2-transgenic plants decreased stomatal aperture under mannitol treatment (200 mM, 2 h) and reduced water loss during air exposure (90 min). The role of apyrase in stomatal regulation resulted from its control in eATP-regulated stomatal movements and increased stomatal sensitivity to ABA. The bi-phasic dose-responses to applied nucleotides, i.e., the low ATP (0.3–1.0 mM)-promoted opening and high ATP (>2.0 mM)-promoted closure, were both restricted by P. euphratica apyrases. It is noteworthy that eATP at a low concentration (0.3 mM) counteracted ABA action in the regulation of stomatal aperture, while overexpression of PeAPY1 or PeAPY2 effectively diminished eATP promotion in opening, and consequently enhanced ABA action in closure. We postulate a speculative model of apyrase signaling in eATP- and ABA-regulated stomatal movements under drought.

The NF-YA gene family is a class of conserved transcription factors that play important roles in plant growth and development and the response to abiotic stress. Poplar is a model organism for studying the rapid growth of woody plants that need to consume many nutrients. However, studies on the response of the NF-YA gene family to nitrogen in woody plants are limited. In this study, we conducted a systematic and comprehensive bioinformatic analysis of the NF-YA gene family based on Populus × canescens genomic data. A total of 13 PcNF-YA genes were identified and mapped to 6 chromosomes. According to the amino acid sequence characteristics and genetic structure of the NF-YA domains, the PcNF-YAs were divided into five clades. Gene duplication analysis revealed five pairs of replicated fragments and one pair of tandem duplicates in 13 PcNF-YA genes. The PcNF-YA gene promoter region is rich in different cis-acting regulatory elements, among which MYB and MYC elements are the most abundant. Among the 13 PcNF-YA genes, 9 contained binding sites for P. × canescens miR169s. In addition, RT-qPCR data from the roots, wood, leaves and bark of P. × canescens showed different spatial expression profiles of PcNF-YA genes. Transcriptome data and RT-qPCR analysis showed that the expression of PcNF-YA genes was altered by treatment with different nitrogen forms. Furthermore, the functions of PcNF-YA genes in transgenic poplar were analyzed, and the potential roles of PcNF-YA genes in the response of poplar roots to different nitrogen forms were revealed, indicating that these genes regulate root growth and development.