Plants under low temperature (LT) stress exhibit a C-repeat binding factor (CBF)-dependent responsive pathway. The transcription factors in the CBF family, existing in multiple plant species, are the key regulators of the cold-responsive (COR) genes. CBF1 and CBF3 are regulated in a different way from CBF2, and CBF4 is the only known CBF gene definitely involved in abscisic acid (ABA)-dependent signaling pathways. RAP2.1 and RAP2.6 are the downstream regulators under CBFs. The upstream regulators of the CBF named inducer of CBF expression (ICE) acts as a positive regulator of CBFs. Meanwhile, these CBF signaling pathway components could associate with many other transcription activators and repressors in regulating gene expression when plants are under LT stress. HOS1 negatively regulates ICE1, which down regulates MYB15, an upstream repressor of CBFs. ZAT12 participates in the repression of CBFs, while ZAT10 and FRY2 negatively regulate the CBF-target genes. ADF5 was recently also found to repress CBFs. LOS2 works against ZAT10, and LOS4 positively regulates CBFs. SFR6 is involved in the modification of CBFs to activate the COR genes, and SIZ1-dependent sumoylation plays a positive role in the regulation of ICE1. The utilization of CBF-dependent signaling components has a broad perspective in the field of plant breeding for enhancing crop LT tolerance.

Nicotinamide adenine dinucleotide (NAD+) participates in intracellular and extracellular signaling events unrelated to metabolism. In animals, purinergic receptors are required for extracellular NAD+ (eNAD+) to evoke biological responses, indicating that eNAD+ may be sensed by cell-surface receptors. However, the identity of eNAD+-binding receptors still remains elusive. Here, we identify a lectin receptor kinase (LecRK), LecRK-I.8, as a potential eNAD+ receptor in Arabidopsis. The extracellular lectin domain of LecRK-I.8 binds NAD+ with a dissociation constant of 436.5 ± 104.8 nM, although much higher concentrations are needed to trigger in vivo responses. Mutations in LecRK-I.8 inhibit NAD+-induced immune responses, whereas overexpression of LecRK-I.8 enhances the Arabidopsis response to NAD+. Furthermore, LecRK-I.8 is required for basal resistance against bacterial pathogens, substantiating a role for eNAD+ in plant immunity. Our results demonstrate that lectin receptors can potentially function as eNAD+-binding receptors and provide direct evidence for eNAD+ being an endogenous signaling molecule in plants.DOI: http://dx.doi.org/10.7554/eLife.25474.001

BackgroundPlants adapted to diverse environments on Earth throughout their evolutionary history, and developed mechanisms to thrive in a variety of terrestrial habitats. When plants are grown in the novel environment of spaceflight aboard the International Space Station (ISS), an environment completely outside their evolutionary history, they respond with unique alterations to their gene expression profile. Identifying the genes important for physiological adaptation to spaceflight and dissecting the biological processes and pathways engaged by plants during spaceflight has helped reveal spaceflight adaptation, and has furthered understanding of terrestrial growth processes. However, the underlying regulatory mechanisms responsible for these changes in gene expression patterns are just beginning to be explored. Epigenetic modifications, such as DNA methylation at position five in cytosine, has been shown to play a role in the physiological adaptation to adverse terrestrial environments, and may play a role in spaceflight as well.ResultsWhole Genome Bisulfite Sequencing of DNA of Arabidopsis grown on the ISS from seed revealed organ-specific patterns of differential methylation compared to ground controls. The overall levels of methylation in CG, CHG, and CHH contexts were similar between flight and ground DNA, however, thousands of specifically differentially methylated cytosines were discovered, and there were clear organ-specific differences in methylation patterns. Spaceflight leaves had higher methylation levels in CHG and CHH contexts within protein-coding genes in spaceflight; about a fifth of the leaf genes were also differentially regulated in spaceflight, almost half of which were associated with reactive oxygen signaling.ConclusionsThe physiological adaptation of plants to spaceflight is likely nuanced by epigenomic modification. This is the first examination of differential genomic methylation from plants grown completely in the spaceflight environment of the ISS in plant growth hardware developed for informing exploration life support strategies. Yet even in this optimized plant habitat, plants respond as if stressed. These data suggest that gene expression associated with physiological adaptation to spaceflight is regulated in part by methylation strategies similar to those engaged with familiar terrestrial stress responses. The differential methylation maps generated here provide a useful reference for elucidating the layers of regulation of spaceflight responses.Electronic supplementary materialThe online version of this article (10.1186/s12864-019-5554-z) contains supplementary material, which is available to authorized users.

The C-repeat binding factor (CBF) is crucial for regulation of cold response in higher plants. In Arabidopsis, the mechanism of CBF3-caused growth retardation is still unclear. Our present work shows that CBF3 shares the similar repression of bioactive gibberellin (GA) as well as upregulation of DELLA proteins with CBF1 and -2. Genetic analysis reveals that DELLAs play an essential role in growth reduction mediated by CBF1, -2, -3 genes. The in vivo and in vitro evidences demonstrate that GA2-oxidase 7 gene is a novel CBF3 regulon. Meanwhile, DELLAs contribute to cold induction of CBF1, -2, -3 genes through interaction with jasmonate (JA) signaling. We conclude that CBF3 promotes DELLAs accumulation through repressing GA biosynthesis and DELLAs positively regulate CBF3 involving JA signaling. CBFs and DELLAs collaborate to retard plant growth in response to low temperature.

INDUCER OF CBF EXPRESSION 1 (ICE1) encodes a MYC-like basic helix-loop-helix (bHLH) transcription factor playing a critical role in plant responses to chilling and freezing stresses and leaf stomata development. However, no information connecting ICE1 and reproductive development has been reported. In this study, we show that ICE1 controls plant male fertility via impacting anther dehydration. The loss-of-function mutation in ICE1 gene in Arabidopsis caused anther indehiscence and decreased pollen viability as well as germination rate. Further analysis revealed that the anthers in the mutant of ICE1 (ice1-2) had the structure of stomium, though the epidermis did not shrink to dehisce. The anther indehiscence and influenced pollen viability as well as germination in ice1-2 were due to abnormal anther dehydration, for most of anthers dehisced with drought treatment and pollen grains from those dehydrated anthers had similar viability and germination rates compared with wild type. Accordingly, the sterility of ice1-2 could be rescued by ambient dehydration treatments. Likewise, the stomatal differentiation of ice1-2 anther epidermis was disrupted in a different manner compared with that in leaves. ICE1 specifically bound to MYC-recognition elements in the promoter of FAMA, a key regulator of guard cell differentiation, to activate FAMA expression. Transcriptome profiling in the anther tissues further exhibited ICE1-modulated genes associated with water transport and ion exchange in the anther. Together, this work reveals the key role of ICE1 in male fertility control and establishes a regulatory network mediated by ICE1 for stomata development and water movement in the anther.