Redox titration of electron acceptor Q and the plastoquinone pool in Photosystem II

The primary photochemical quencher Q and the secondary electron acceptor pool in Photosystem II have been titrated. We used particles of Scenedesmus mutant No. 8 that lack System I and allowed the system to equilibrate with external redox mediators in darkness prior to measurement of the fluorescence rise curve.The titration of Q, as indicated by the dark level of F_i, occurs in two discrete steps. The high-potential component (Q_h) has a midpoint potential of +68 mV (pH 7.2) and accounts for ～67% of Q. The pH sensitivity of the midpoint potential is ?60 mV, indicating the involvement of 1 $\text{H}{}^{\mathrm{+}}\text{e}$. The low-potential component (Q₁) accounts for the remaining 33% of Q and shows a midpoint potential near?300 mV (pH 7.2).The plastoquinone pool, assayed as the half-time of the fluorescence rise curve, titrates as a single component with a midpoint potential 30–40 mV more oxidizing than that of Q_h, i.e., at 106 mV (pH 7.2). The E_m shows a pH sensitivity of ?60 mV/pH unit, indicating the involvement of 1 $\text{H}{}^{\mathrm{+}}\text{e}$. The observation that all 12–14 electron equivalents in the pool titrate as a single component indicates that the heterogeneity otherwise observed in the secondary acceptor system is a kinetic rather than a thermodynamic property.Illumination causes peculiar, and as yet unclarified, changes of both Q and the secondary pool under anaerobic conditions that are reversed by oxygen.     

Metallochaperone-like genes in Arabidopsis thaliana

A complete inventory of metallochaperone-like proteins containing a predicted HMA domain in Arabidopsis revealed a large family of 67 proteins. 45 proteins, the HIPPs, have a predicted isoprenylation site while 22 proteins, the HPPs, do not. Sequence comparisons divided the proteins into seven major clusters (I-VII). Cluster IV is notable for the presence of a conserved Asp residue before the CysXXCys, metal binding motif, analogous to the Zn binding motif in E. coli ZntA. HIPP20, HIPP21, HIPP22, HIPP26 and HIPP27 in Cluster IV were studied in more detail. All but HIPP21 could rescue the Cd-sensitive, ycf1 yeast mutant but failed to rescue the growth of zrt1zrt2, zrc1cot1 and atx1 mutants. In Arabidopsis, single and double mutants did not show a phenotype but the hipp20/21/22 triple mutant was more sensitive to Cd and accumulated less Cd than the wild-type suggesting the HIPPs can have a role in Cd-detoxification, possibly by binding Cd. Promoter-GUS reporter expression studies indicated variable expression of these HIPPs. For example, in roots, HIPP22 and HIPP26 are only expressed in lateral root tips while HIPP20 and HIPP25 show strong expression in the root vasculature.     

Transgenerational response to stress in Arabidopsis thaliana

Plants exposed to stress pass the memory of exposure to stress to the progeny. Previously, we showed that the phenomenon of transgenerational memory of stress is of epigenetic nature and depends on the function of Dicer-like (DCL) 2 and DCL3 proteins. Here, we discuss a possible role of DNA methylation and function of small RNAs in establishing and maintaining transgenerational responses to stress. Our new data report that memory of stress is passed to the progeny predominantly through the female rather than male gamete. Possible evolutionary advantages of this mechanism are also discussed.Key words: transgenerational response to stress, Arabidopsis thaliana, maternal inheritance, methylation changes, homologous recombination frequency, genome instability, adaptive response, dcl2, dcl3Plants are sedentary organisms and thus can not respond to rapidly changing growth conditions by escaping to new environments as animals usually do. Moreover, since seed dispersal is rather limited in the vast majority of plants, the progeny is very likely to grow under the same environmental growth conditions as its parents did. The memory of pre-existing growth conditions can be advantageous for plant survival. The environmental experience of parents can be recorded in the form of induced epigenetic modifications that occur in somatic cell lineages. The very late, almost at the end of plant development, separation of germline cells from somatic tissues enables incorporation of acquired epigenetic changes in the gametes. Indeed, previous reports suggested that the progeny of exposed plants might have an advantage while growing in the same environment as its parents.^1–3 Despite a growing number of experimental evidences that support the existence of the phenomenon of memory of stress, the data on adaptive changes in the progeny of stressed plants are scarce.Parental exposure to stress may not only lead to adaptive effects in progeny but also introduce a certain degree of changes in genome stability.^4–9 Our early report showed that the progeny of tobacco plants infected with tobacco mosaic virus had an increased meiotic recombination frequency.⁸ A more recent report demonstrated that these progeny plants had a higher frequency of rearrangements at the loci carrying the homology to N-gene-like R-gene loci, allowing speculations about a possible role of these rearrangements in pathogen resistance evolution.⁹ Similarly, a study of Molinier et al. (2006) showed that the progeny of plants exposed to UVC or flagellin had an increased frequency of somatic homologous recombination events (HRF).⁴ The authors demonstrated that an increase in HRF triggered by a single exposure to UVC was maintained for five consecutive generations in the absence of stress. In contrast, our most recent reports demonstrated that maintaining an increase in HRF caused by ancestral exposure to heat, cold, flood, UVC or salt required exposure to stress in subsequent generations: if F1 plants were propagated for one more generation without stress, the effect diminished and HRF returned back to the level observed in the progeny of untreated plants.^6,7 This scenario seems to be more probable from an evolutionary point of view. Within a given environmental niche, plants establish certain genetic and epigenetic traits needed to cope with the expected growth conditions. Drastic environmental changes or new unusual stresses may trigger a cascade of gene expression changes in attempt to survive and adapt to new conditions. Some of these potentially advantageous changes are most probably recorded in the form of DNA methylation and chromatin modifications and are passed to progeny as memory of stress exposure.It can be further hypothesized that if these new environmental conditions are no longer present during the lifespan of future generations, the newly established methylation patterns and chromatin organization will return to the original epigenetic landscape that was the most adequate fit for this environmental niche. If the same new stresses occur in consecutive generations, the newly established epigenetic changes will be maintained and possibly stabilized after many generations of exposure.     

Identifying essential genes in Arabidopsis thaliana

Eight years after publication of the Arabidopsis genome sequence and two years before completing the first phase of an international effort to characterize the function of every Arabidopsis gene, plant biologists remain unable to provide a definitive answer to the following basic question: what is the minimal gene set required for normal growth and development? The purpose of this review is to summarize different strategies employed to identify essential genes in Arabidopsis, an important component of the minimal gene set in plants, to present an overview of the datasets and specific genes identified to date, and to discuss the prospects for future saturation of this important class of genes. The long-term goal of this collaborative effort is to facilitate basic research in plant biology and complement ongoing research with other model organisms.     

Redox regulation of free amino acid levels in Arabidopsis thaliana

Abiotic stresses induce oxidative stress, which modifies the level of several metabolites including amino acids. The redox control of free amino acid profile was monitored in wild‐type and ascorbate or glutathione deficient mutant Arabidopsis thaliana plants before and after hydroponic treatment with various redox agents. Both mutations and treatments modified the size and redox state of the ascorbate (AsA) and/or glutathione (GSH) pools. The total free amino acid content was increased by AsA, GSH and H₂O₂ in all three genotypes and a very large (threefold) increase was observed in the GSH‐deficient pad2‐1 mutant after GSH treatment compared with the untreated wild‐type plants. Addition of GSH reduced the ratio of amino acids belonging to the glutamate family on a large scale and increased the relative amount of non‐proteinogenic amino acids. The latter change was because of the large increase in the content of alpha‐aminoadipate, an inhibitor of glutamatic acid (Glu) transport. Most of the treatments increased the proline (Pro) content, which effect was due to the activation of genes involved in Pro synthesis. Although all studied redox compounds influenced the amount of free amino acids and a mostly positive, very close (r > 0.9) correlation exists between these parameters, a special regulatory role of GSH could be presumed due to its more powerful effect. This may originate from the thiol/disulphide conversion or (de)glutathionylation of enzymes participating in the amino acid metabolism.     

Elevated CO2 influences the expression of floral-initiation genes in Arabidopsis thaliana

Atmospheric CO(2) concentration ([CO(2)]) is rising on a global scale and is known to affect flowering time. Elevated [CO(2)] may be as influential as temperature in determining future changes in plant developmental timing, but little is known about the molecular mechanisms that control altered flowering times at elevated [CO(2)]. Using Arabidopsis thaliana, the expression patterns were compared of floral-initiation genes between a genotype that was selected for high fitness at elevated [CO(2)] and a nonselected control genotype. The selected genotype exhibits pronounced delays in flowering time when grown at elevated [CO(2)], whereas the control genotype is unaffected by elevated [CO(2)]. Thus, this comparison provides an evolutionarily relevant system for gaining insight into the responses of plants to future increases in [CO(2)]. Evidence is provided that elevated [CO(2)] influences the expression of floral-initiation genes. In addition, it is shown that delayed flowering at elevated [CO(2)] is associated with sustained expression of the floral repressor gene, FLOWERING LOCUS C (FLC), in an elevated CO(2)-adapted genotype. Understanding the mechanisms that account for changes in plant developmental timing at elevated [CO(2)] is critical for predicting the responses of plants to a high-CO(2) world of the near future.     

Widespread tissue expression of nepenthesin-like aspartic protease genes in Arabidopsis thaliana.

There are approximately 69 genes encoding aspartyl protease homologues in Arabidopsis thaliana, and most of the gene products constitute a novel subfamily of aspartic proteases. However, their physiological roles are largely unknown. As an initial step to shed light on the roles of these nepenthesin-like aspartic proteases (NAPs), a phylogenetic tree was constructed, which indicated that these proteases are classified into several distinct sub-sub-groups. Based on these results, specific primers were designed for genes selected from several of these groups and their tissue expression was investigated using RT-PCR. The results indicated that these genes are widely expressed in several tissues, such as leaves, stems, seeds and pods, suggesting ubiquitous occurrence and multiple functions of the corresponding proteases in the tissues of A. thaliana.