Mapping quantitative trait loci for freezing tolerance in a recombinant inbred line population of Arabidopsis thaliana accessions Tenela and C24 reveals REVEILLE1 as negative regulator of cold acclimation.

The ability to increase freezing tolerance when exposed to low temperatures is a property of many plant species from temperate climates and involves a wide array of metabolic adjustments and changes in gene expression. In Arabidopsis thaliana, natural accessions show high variation in their acclimation capacity, and freezing tolerance correlates with natural habitat temperatures. To investigate the genetic basis of this variation, a recombinant inbred line population from reciprocal crosses between the accessions C24 and Tenela (Te), showing large variation in tolerance, was established. Over 250 recombinant inbred lines were genotyped for 69 single nucleotide polymorphism markers in a linkage map with 391.9 centimorgans (cM) and phenotyped for their freezing tolerance using the electrolyte leakage method that reports cell damage after a freeze-thaw cycle. Mapping of quantitative trait loci (QTL) for acclimated plants revealed three QTL regions on chromosomes 2, 4 and 5. Based on gene expression data, QTL regions were screened for genes differentially responding to low temperature in C24 and Te. Among the candidate genes, the Myb family transcription factor REVEILLE1 (At5g17300) on chromosome 5 was identified as a novel negative regulator of freezing tolerance in Arabidopsis.

NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1) and some NPR1-related proteins are sensitive to salicylic acid.

NONEXPRESSOR OF PATHOGENESIS-RELATED PROTEINS1 (NPR1; also known as NIM1) is a master regulator of systemic acquired resistance (SAR). SAR is induced by salicylic acid (SA), leading to the expression of PATHOGENESIS-RELATED (PR) genes. Current evidence suggests that NPR1 is part of a transcription complex tethered to activation sequence-1 (as-1)-like cis-acting elements in PR-1 gene promoters through TGA transcription factors, and that SA-dependent PR-1 gene expression is regulated by NIM1-INTERACTING (NIMIN) proteins. In Arabidopsis, NPR1 is active only after SA induction. Regulation of Arabidopsis NPR1 activity has been proposed to comprise cysteine-156 (Cys-156), mediating SA-induced cytoplasmic oligomer-nuclear monomer exchange, and Cys-521 and Cys-529, mediating SA-dependent transcriptional activation. Tobacco NPR1 does not harbour these residues. To understand the function of tobacco NPR1, we analysed its biochemical capabilities in a heterologous system: yeast. Tobacco NPR1 differs from Arabidopsis NPR1 in its subcellular localization and its transactivation potential. Yet, both tobacco and Arabidopsis NPR1, as well as tobacco NIM1-like1, alter some of their biochemical activities in response to SA. Whereas the addition of SA to yeast growth medium induces transcriptional activity in tobacco NPR1, its interaction with NIMIN2-type proteins is suppressed. The effects of SA are specific, sensitive and occur coordinately. They are abolished completely by mutation of the arginine residue within the invariable penta-amino acid motif LENRV, as present in the nonfunctional Arabidopsis nim1-4 allele. Furthermore, NPR1 proteins with the LENRV domain coincidently harbour a broad and strongly conserved NIMIN1/NIMIN2 binding site. Our data suggest that NPR1 and some NPR1-like proteins are sensitive to the plant hormone SA, altering some of their biochemical capabilities to enable stimulus-dependent gene expression. The sensitivity of NPR1 proteins to SA, together with their differential interaction with diverse NIMIN proteins, seems a plausible molecular basis for the timely and coordinated activation of PR genes during SAR.

A systems biology approach for the analysis of carbohydrate dynamics during acclimation to low temperature in Arabidopsis thaliana.

Low temperature is an important environmental factor affecting the performance and distribution of plants. During the so-called process of cold acclimation, many plants are able to develop low-temperature tolerance, associated with the reprogramming of a large part of their metabolism. In this study, we present a systems biology approach based on mathematical modelling to determine interactions between the reprogramming of central carbohydrate metabolism and the development of freezing tolerance in two accessions of Arabidopsis thaliana. Different regulation strategies were observed for (a) photosynthesis, (b) soluble carbohydrate metabolism and (c) enzyme activities of central metabolite interconversions. Metabolism of the storage compound starch was found to be independent of accession-specific reprogramming of soluble sugar metabolism in the cold. Mathematical modelling and simulation of cold-induced metabolic reprogramming indicated major differences in the rates of interconversion between the pools of hexoses and sucrose, as well as the rate of assimilate export to sink organs. A comprehensive overview of interconversion rates is presented, from which accession-specific regulation strategies during exposure to low temperature can be derived. We propose this concept as a tool for predicting metabolic engineering strategies to optimize plant freezing tolerance. We confirm that a significant improvement in freezing tolerance in plants involves multiple regulatory instances in sucrose metabolism, and provide evidence for a pivotal role of sucrose-hexose interconversion in increasing the cold acclimation output.