Cold temperatures trigger the expression of the CBF family of transcription factors, which in turn activate many downstream genes that confer chilling and freezing tolerance to plants. We report here the identification ofICE1(inducer ofCBFexpression1), an upstream transcription factor that regulates the transcription ofCBFgenes in the cold. AnArabidopsis ice1mutant was isolated in a screen for mutations that impair cold-induced transcription of aCBF3promoter-luciferase reporter gene. Theice1mutation blocks the expression ofCBF3and decreases the expression of many genes downstream ofCBFs, which leads to a significant reduction in plant chilling and freezing tolerance.ICE1encodes a MYC-like bHLH transcriptional activator. ICE1 binds specifically to the MYC recognition sequences in theCBF3promoter.ICE1is expressed constitutively, and its overexpression in wild-type plants enhances the expression of theCBFregulon in the cold and improves freezing tolerance of the transgenic plants.

To understand the gene network controlling tolerance to cold stress, we performed an Arabidopsis thaliana genome transcript expression profile using Affymetrix GeneChips that contain ∼24,000 genes. We statistically determined 939 cold-regulated genes with 655 upregulated and 284 downregulated. A large number of early cold-responsive genes encode transcription factors that likely control late-responsive genes, suggesting a multitude of transcriptional cascades. In addition, many genes involved in chromatin level and posttranscriptional regulation were also cold regulated, suggesting their involvement in cold-responsive gene regulation. A number of genes important for the biosynthesis or signaling of plant hormones, such as abscisic acid, gibberellic acid, and auxin, are regulated by cold stress, which is of potential importance in coordinating cold tolerance with growth and development. We compared the cold-responsive transcriptomes of the wild type and inducer of CBF expression 1 (ice1), a mutant defective in an upstream transcription factor required for chilling and freezing tolerance. The transcript levels of many cold-responsive genes were altered in the ice1 mutant not only during cold stress but also before cold treatments. Our study provides a global picture of the Arabidopsis cold-responsive transcriptome and its control by ICE1 and will be valuable for understanding gene regulation under cold stress and the molecular mechanisms of cold tolerance.

Two Arabidopsis thaliana extragenic mutations that suppress NaCl hypersensitivity of the sos3–1 mutant were identified in a screen of a T-DNA insertion population in the genetic background of Col-0 gl1 sos3–1 . Analysis of the genome sequence in the region flanking the T-DNA left border indicated that sos3–1 hkt1–1 and sos3–1 hkt1–2 plants have allelic mutations in AtHKT1. AtHKT1 mRNA is more abundant in roots than shoots of wild-type plants but is not detected in plants of either mutant, indicating that this gene is inactivated by the mutations. hkt1–1 and hkt1–2 mutations can suppress to an equivalent extent the Na ⁺ sensitivity of sos3–1 seedlings and reduce the intracellular accumulation of this cytotoxic ion. Moreover, sos3–1 hkt1–1 and sos3–1 hkt1–2 seedlings are able to maintain [K ⁺ ] _int in medium supplemented with NaCl and exhibit a substantially higher intracellular ratio of K ⁺ /Na ⁺ than the sos3–1 mutant. Furthermore, the hkt1 mutations abrogate the growth inhibition of the sos3–1 mutant that is caused by K ⁺ deficiency on culture medium with low Ca ²⁺ (0.15 mM) and <200 μM K ⁺ . Interestingly, the capacity of hkt1 mutations to suppress the Na ⁺ hypersensitivity of the sos3–1 mutant is reduced substantially when seedlings are grown in medium with low Ca ²⁺ (0.15 mM). These results indicate that AtHKT1 is a salt tolerance determinant that controls Na ⁺ entry and high affinity K ⁺ uptake. The hkt1 mutations have revealed the existence of another Na ⁺ influx system(s) whose activity is reduced by high [Ca ²⁺ ] _ext .

The plant hormone abscisic acid (ABA) plays a wide range of important roles in plant growth and development, including embryogenesis, seed dormancy, root and shoot growth, transpiration, and stress tolerance. ABA and various abiotic stresses also activate the expression of numerous plant genes through undefined signaling pathways. To gain insight into ABA and stress signal transduction, we conducted a genetic screen based on ABA- and stress-inducible gene transcription. Here we report the identification of anArabidopsis mutation, fiery1 (fry1), which results in super-induction of ABA- and stress-responsive genes. Seed germination and postembryonic development of fry1 are more sensitive to ABA or stress inhibition. The mutant plants are also compromised in tolerance to freezing, drought, and salt stresses. Map-based cloning revealed that FRY1 encodes an inositol polyphosphate 1-phosphatase, which functions in the catabolism of inositol 1, 4, 5-trisphosphate (IP₃). Upon ABA treatment,fry1 mutant plants accumulated more IP₃ than did the wild-type plants. These results provide the first genetic evidence indicating that phosphoinositols mediate ABA and stress signal transduction in plants and their turnover is critical for attenuating ABA and stress signaling.

Genetic and physiological data establish that Arabidopsis AtHKT1 facilitates Na⁺ homeostasis in planta and by this function modulates K⁺ nutrient status. Mutations that disrupt AtHKT1 function suppress NaCl sensitivity of sos1-1 and sos2-2, as well as of sos3-1 seedlings grown in vitro and plants grown in controlled environmental conditions. hkt1 suppression of sos3-1 NaCl sensitivity is linked to higher Na⁺ content in the shoot and lower content of the ion in the root, reducing the Na⁺ imbalance between these organs that is caused by sos3-1. AtHKT1 transgene expression, driven by its innate promoter, increases NaCl but not LiCl or KCl sensitivity of wild-type (Col-0 gl1) or of sos3-1 seedlings. NaCl sensitivity induced by AtHKT1 transgene expression is linked to a lower K⁺ to Na⁺ ratio in the root. However, hkt1 mutations increase NaCl sensitivity of both seedlings in vitro and plants grown in controlled environmental conditions, which is correlated with a lower K⁺ to Na⁺ ratio in the shoot. These results establish that AtHKT1 is a focal determinant of Na⁺ homeostasis in planta, as either positive or negative modulation of its function disturbs ion status that is manifested as salt sensitivity. K⁺-deficient growth of sos1-1, sos2-2, and sos3-1 seedlings is suppressed completely by hkt1-1. AtHKT1 transgene expression exacerbates K⁺ deficiency of sos3-1 or wild-type seedlings. Together, these results indicate that AtHKT1 controls Na⁺ homeostasis in planta and through this function regulates K⁺ nutrient status.