Oxidative damage and DNA repair in desiccated recalcitrant embryonic axes of Acer pseudoplatanus L

BACKGROUND

Most plants encounter water stress at one or more different stages of their life cycle. The maintenance of genetic stability is the integral component of desiccation tolerance that defines the storage ability and long-term survival of seeds. Embryonic axes of desiccation-sensitive recalcitrant seeds of Acer pseudoplatnus L. were used to investigate the genotoxic effect of desiccation. Alkaline single-cell gel electrophoresis (comet assay) methodology was optimized and used to provide unique insights into the onset and repair of DNA strand breaks and 8-oxo-7,8-dihydroguanine (8-oxoG) formation during progressive steps of desiccation and rehydration.

RESULTS

The loss of DNA integrity and impairment of damage repair were significant predictors of the viability of embryonic axes. In contrast to the comet assay, automated electrophoresis failed to detect changes in DNA integrity resulting from desiccation. Notably, no significant correlation was observed between hydroxyl radical (^٠OH) production and 8-oxoG formation, although the former is regarded to play a major role in guanine oxidation.

CONCLUSIONS

The high-throughput comet assay represents a sensitive tool for monitoring discrete changes in DNA integrity and assessing the viability status in plant germplasm processed for long-term storage.

Bromodomain-containing subunits BRD1, BRD2, and BRD13 are required for proper functioning of SWI/SNF complexes in Arabidopsis

SWI/SNF chromatin remodelers are evolutionarily conserved multiprotein complexes that use the energy of ATP hydrolysis to change chromatin structure. A characteristic feature of SWI/SNF remodelers is the occurrence in both the catalytic ATPase subunit and some auxiliary subunits, of bromodomains, the protein motifs capable of binding acetylated histones. Here, we report that the Arabidopsis bromodomain-containing proteins BRD1, BRD2, and BRD13 are likely true SWI/SNF subunits that interact with the core SWI/SNF components SWI3C and SWP73B. Loss of function of each single BRD protein caused early flowering but had a negligible effect on other developmental pathways. By contrast, a brd triple mutation (brdx3) led to more pronounced developmental abnormalities, indicating functional redundancy among the BRD proteins. The brdx3 phenotypes, including hypersensitivity to abscisic acid and the gibberellin biosynthesis inhibitor paclobutrazol, resembled those of swi/snf mutants. Furthermore, the BRM protein level and occupancy at the direct target loci SCL3, ABI5, and SVP were reduced in the brdx3 mutant background. Finally, a brdx3 brm-3 quadruple mutant, in which SWI/SNF complexes were devoid of all constituent bromodomains, phenocopied a loss-of-function mutation in BRM. Taken together, our results demonstrate the relevance of BRDs as SWI/SNF subunits and suggest their cooperation with the bromodomain of BRM ATPase.

Two-step d-ononitol epimerization pathway in Medicago truncatula

Methylated inositol, d-pinitol (3-O-methyl-d-chiro-inositol), is a common constituent in legumes. It is synthesized from myo-inositol in two reactions: the first reaction, catalyzed by myo-inositol-O-methyltransferase (IMT), consists of a transfer of a methyl group from S-adenosylmethionine to myo-inositol with the formation of d-ononitol, while the second reaction, catalyzed by d-ononitol epimerase (OEP), involves epimerization of d-ononitol to d-pinitol. To identify the genes involved in d-pinitol biosynthesis in a model legume Medicago truncatula, we conducted a BLAST search on its genome using soybean IMT cDNA as a query and found putative IMT (MtIMT) gene. Subsequent co-expression analysis performed on publicly available microarray data revealed two potential OEP genes: MtOEPA, encoding an aldo-keto reductase and MtOEPB, encoding a short-chain dehydrogenase. cDNAs of all three genes were cloned and expressed as recombinant proteins in E. coli. In vitro assays confirmed that putative MtIMT enzyme catalyzes methylation of myo-inositol to d-ononitol and showed that MtOEPA enzyme has NAD+ -dependent d-ononitol dehydrogenase activity, while MtOEPB enzyme has NADP+ -dependent d-pinitol dehydrogenase activity. Both enzymes are required for epimerization of d-ononitol to d-pinitol, which occurs in the presence of NAD+ and NADPH. Introduction of MtIMT, MtOEPA, and MtOEPB genes into tobacco plants resulted in production of d-ononitol and d-pinitol in transformants. As this two-step pathway of d-ononitol epimerization is coupled with a transfer of reducing equivalents from NADPH to NAD+ , we speculate that one of the functions of this pathway might be regeneration of NADP+ during drought stress.

Jasmonic acid and ethylene are involved in the accumulation of osmotin in germinating tomato seeds

The expression of SlNP24 encoding osmotin was studied in germinating tomato seeds Solanum lycopersicum L. cv. Moneymaker. The results show that the accumulation of the transcripts of SlNP24 and its potential upstream regulator TERF1 encoding an ethylene response factor was induced by ethylene and methyl jasmonate in germinating tomato seeds. There was no effect of gibberellins on the expression of the genes studied. The expression of SlNP24 was localized in the micropylar region of the endosperm of tomato seeds. The promoter of tomato osmotin was active in the endosperm cells of transgenic Arabidopsis thaliana seeds, which contain reporter genes under control of SlNP24 promoter. The activity of SlNP24 promoter in A. thaliana reporter line seeds was visible when the expression of its ortholog gene in A. thaliana (AtOMS34) was observed. The mechanism of induction and a possible role of NP24 in germinating tomato seeds are discussed.

HD2C histone deacetylase and a SWI/SNF chromatin remodelling complex interact and both are involved in mediating the heat stress response in Arabidopsis

Studies in yeast and animals have revealed that histone deacetylases (HDACs) often act as components of multiprotein complexes, including chromatin remodelling complexes (CRCs). However, interactions between HDACs and CRCs in plants have yet to be demonstrated. Here, we present evidence for the interaction between Arabidopsis HD2C deacetylase and a BRM-containing SWI/SNF CRC. Moreover, we reveal a novel function of HD2C as a regulator of the heat stress response. HD2C transcript levels were strongly induced in plants subjected to heat treatment, and the expression of selected heat-responsive genes was up-regulated in heat-stressed hd2c mutant, suggesting that HD2C acts to down-regulate heat-activated genes. In keeping with the HDAC activity of HD2C, the altered expression of HD2C-regulated genes coincided in most cases with increased histone acetylation at their loci. Microarray transcriptome analysis of hd2c and brm mutants identified a subset of commonly regulated heat-responsive genes, and the effect of the brm hd2c double mutation on the expression of these genes was non-additive. Moreover, heat-treated 3-week-old hd2c, brm and brm hd2c mutants displayed similar rates of growth retardation. Taken together, our findings suggest that HD2C and BRM act in a common genetic pathway to regulate the Arabidopsis heat stress response.

Mechanisms of hormonal regulation of endosperm cap-specific gene expression in tomato seeds

The micropylar region of endosperm in a seed, which is adjacent to the radicle tip, is called the 'endosperm cap', and is specifically activated before radicle emergence. This activation of the endosperm cap is a widespread phenomenon among species and is a prerequisite for the completion of germination. To understand the mechanisms of endosperm cap-specific gene expression in tomato seeds, GeneChip analysis was performed. The major groups of endosperm cap-enriched genes were pathogenesis-, cell wall-, and hormone-associated genes. The promoter regions of endosperm cap-enriched genes contained DNA motifs recognized by ethylene response factors (ERFs). The tomato ERF1 (TERF1) and its experimentally verified targets were enriched in the endosperm cap, suggesting an involvement of the ethylene response cascade in this process. The known endosperm cap enzyme endo-β-mannanase is induced by gibberellin (GA), which is thought to be the major hormone inducing endosperm cap-specific genes. The mechanism of endo-β-mannanase induction by GA was also investigated using isolated, embryoless seeds. Results suggested that GA might act indirectly on the endosperm cap. We propose that endosperm cap activation is caused by the ethylene response of this tissue, as a consequence of mechanosensing of the increase in embryonic growth potential by GA action.