Modulation of redox homeostasis under suboptimal conditions by Arabidopsis nudix hydrolase 7

<p>Abstract</p> <p>Background</p> <p>Nudix hydrolases play a key role in maintaining cellular homeostasis by hydrolyzing various nuceloside diphosphate derivatives and capped mRNAs. Several independent studies have demonstrated that <it>Arabidopsis nudix hydrolase 7 </it>(AtNUDT7) hydrolyzes NADH and ADP-ribose. Loss of function <it>Atnudt7-1 </it>mutant plants (SALK_046441) exhibit stunted growth, higher levels of reactive oxygen species, enhanced resistance to pathogens. However, using the same T-DNA line, two other groups reported that mutant plants do not exhibit any visible phenotypes. In this study we analyze plausible factors that account for differences in the observed phenotypes in <it>Atnudt7</it>. Secondly, we evaluate the biochemical and molecular consequences of increased NADH levels due to loss of function of AtNUDT7 in Arabidopsis.</p> <p>Results</p> <p>We identified a novel conditional phenotype of <it>Atnudt7-1 </it>knockout plants that was contingent upon nutrient composition of potting mix. In nutrient-rich Metro-Mix, there were no phenotypic differences between mutant and wild-type (WT) plants. In the nutrient-poor mix (12 parts vermiculite: 3 parts Redi-earth and 1 part sand), mutant plants showed the characteristic stunted phenotype. Compared with WT plants, levels of glutathione, NAD⁺, NADH, and in turn NADH:NAD⁺ratio were higher in <it>Atnudt7-1 </it>plants growing in 12:3:1 potting mix. Infiltrating NADH and ADP-ribose into WT leaves was sufficient to induce AtNUDT7 protein. Constitutive over-expression of <it>AtNudt7 </it>did not alter NADH levels or resistance to pathogens. Transcriptome analysis identified nearly 700 genes differentially expressed in the <it>Atnudt7-1 </it>mutant compared to WT plants grown in 12:3:1 potting mix. In the <it>Atnudt7-</it>1 mutant, genes associated with defense response, proteolytic activities, and systemic acquired resistance were upregulated, while gene ontologies for transcription and phytohormone signaling were downregulated.</p> <p>Conclusions</p> <p>Based on these observations, we conclude that the differences observed in growth phenotypes of the <it>Atnudt7-1 </it>knockout mutants can be due to differences in the nutrient composition of potting mix. Our data suggests AtNUDT7 plays an important role in maintaining redox homeostasis, particularly for maintaining NADH:NAD⁺balance for normal growth and development. During stress conditions, rapid induction of AtNUDT7 is important for regulating the activation of stress/defense signaling and cell death pathways.</p

A Humidity-Sensitive Arabidopsis Copine Mutant Exhibits Precocious Cell Death and Increased Disease Resistance

The copines are a newly identified class of calcium-dependent, phospholipid binding proteins that are present in a wide range of organisms, including Paramecium, plants, Caenorhabditis elegans, mouse, and human. However, the biological functions of the copines are unknown. Here, we describe a humidity-sensitive copine mutant in Arabidopsis. Under nonpermissive, low-humidity conditions, the cpn1-1 mutant displayed aberrant regulation of cell death that included a lesion mimic phenotype and an accelerated hypersensitive response (HR). However, the HR in cpn1-1 showed no increase in sensitivity to low pathogen titers. Low-humidity-grown cpn1-1 mutants also exhibited morphological abnormalities, increased resistance to virulent strains of Pseudomonas syringae and Peronospora parasitica, and constitutive expression of pathogenesis-related (PR) genes. Growth of cpn1-1 under permissive, high-humidity conditions abolished the increased disease resistance, lesion mimic, and morphological mutant phenotypes but only partially alleviated the accelerated HR and constitutive PR gene expression phenotypes. The disease resistance phenotype of cpn1-1 suggests that the CPN1 gene regulates defense responses. Alternatively, the primary function of CPN1 may be the regulation of plant responses to low humidity, and the effect of the cpn1-1 mutation on disease resistance may be indirect