In this report, we investigate the altered APX2 expression 13 (alx13) mutation of Arabidopsis thaliana, a mutation in glutamine phosphoribosyl pyrophosphate amidotransferase 2 (ATASE2), the primary isoform of the enzyme mediating the first committed step of purine biosynthesis. Light-dependent leaf variegation was exhibited by alx13 plants, with partial shading of alx13 rosettes revealing that the development of chlorosis in emerging leaves is influenced by the growth irradiance of established leaves. Chlorotic sectors arose from emerging green alx13 leaves during a phase of rapid cell division and expansion, which shows that each new cell's fate is independent of its progenitor. In conjunction with the variegated phenotype, alx13 plants showed altered high light stress responses, including changed expression of genes encoding proteins with antioxidative functions, impaired anthocyanin production and over-accumulation of reactive oxygen species. These characteristics were observed in both photosynthetically-normal green tissues and chlorotic tissues. Chlorotic tissues of alx13 leaves accumulated mRNAs of nuclear-encoded photosynthesis genes that are repressed in other variegated mutants of Arabidopsis. Thus, defective purine biosynthesis impairs chloroplast biogenesis in a light-dependent manner and alters the induction of high light stress pathways and nuclear-encoded photosynthesis genes.

As the sun tracks daily through the sky from east to west, different parts of the canopy are exposed to high light (HL). The extent of and mechanisms by which a systemic acquired acclimation (SAA) response might preacclimate shaded leaves that will be subsequently exposed to full sunlight is largely undefined. We investigated the role of an Arabidopsis thaliana zinc finger transcription factor, ZAT10, in SAA. ZAT10 overexpression resulted in enhanced tolerance to photoinhibitory light and exogenous H2O2, increased expression of antioxidative genes whose products are targeted to multiple subcellular compartments. Partial HL exposure of a leaf or leaves rapidly induced ZAT10 mRNA in distal, shaded photosynthetic tissues, including the floral stem, cauline leaves, and rosette, but not in roots. Fully 86% of fivefold HL-upregulated and 71% of HL-downregulated genes were induced and repressed, respectively, in distal, shaded leaves. Between 15 and 23% of genes whose expression changed in the HL and/or distal tissues were coexpressed in the ZAT10 overexpression plants, implicating ZAT10 in modulating the expression of SAA-regulated genes. The SAA response was detectable in plants with mutations in abscisic acid, methyl jasmonate, or salicylic acid synthesis or perception, and systemic H2O2 diffusion was not detected. Hence, SAA is distinct from pathogen-stimulated systemic acquired resistance and apparently involves a novel signal or combination of signals that preacclimate photosynthetic tissues to HL.

Molecular analyses of plants have revealed a number of genes whose expression changes in response to high light (HL), including the H2O2 scavenger, ASCORBATE PEROXIDASE 2 (APX2). We carried out a screen in Arabidopsis thaliana for lesions that alter HL-induced expression of APX2 to identify components in abiotic stress signalling pathways. High light was used as it can be instantaneously applied or removed and accurately measured. We identified a number of alx mutations causing altered APX2 expression. Here we describe the gain-of-function mutant, alx8, which has constitutively higher APX2 expression and higher levels of foliar abscisic acid (ABA) than wild type. In fact, exogenous ABA increased APX2 expression and the APX2 promoter contains ABA response elements. Furthermore, we have shown that HL stress increases ABA in wild-type plants, implicating ABA in the regulation of HL-inducible genes. The alx8 mutant is drought tolerant, exhibits improved water-use efficiency and a number of drought-tolerance genes are upregulated. Additionally, alx8 demonstrates the complexity of ABA-dependent and ABA-independent transcriptional networks as some components in both pathways are upregulated in alx8. This study provides evidence for common steps in drought and HL stress response pathways.

A range of environmental conditions can lead to oxidative stress; thus, a prompt and effective response to oxidative stress is crucial for the survival of plants. Microarray and northern-blot analyses were performed toward the identification of the factors and signaling pathways that enable plants to limit oxidative damage caused by exposure to high light (HL). Arabidopsis plants grown under moderate light (100 μmol m−2 s−1) were exposed to HL (1,000 μmol m−2 s−1) for 1 h. The microarray analyses revealed that exposure of Arabidopsis to HL caused an increase in known antioxidant genes, as well as several unknown genes. Some of these unknown genes had homologies to possible regulatory genes and metabolic enzymes. Furthermore, it was found that a range of chaperones were up-regulated in the HL treatment and that this induction was specifically due to the HL stress. The temporal expression under HL and different oxidative stress conditions of a subset of HL-responsive genes was confirmed via northern-blot analysis. Results from the arrays were also compared with publicly available microarray data sets from a range of different stress conditions at the Arabidopsis Functional Genomics Consortium. This cross comparison enabled the identification of genes that may be induced by changes in redox poise. Finally, to determine if the genes that were differentially expressed by HL stress were under similar transcriptional control, we analyzed the promoter sequences for the presence of common motifs.