Singlet oxygen is known to be produced by cells in response to photo-oxidative stresses and wounding. Due to singlet oxygen being highly reactive, it is thought to have a very short half-life in biological systems and, consequently, it is difficult to detect. A new commercially available reagent (singlet oxygen sensor green, SOSG), which is highly selective for singlet oxygen, was applied to a range of biological systems that are known to generate singlet oxygen. Induction of singlet oxygen production by the addition of myoglobin to liposome preparations demonstrated that the singlet oxygen-induced increases in SOSG fluorescence closely followed the increase in the concentration of conjugated dienes, which is stoichiometrically related to singlet oxygen production. Applications of photo-oxidative stresses to diatom species and leaves, which are known to result in the production of singlet oxygen, produced large increases in SOSG fluorescence, as did the addition of 3-(3′,4′-dichlorophenyl)1,1-dimethylurea (DCMU) to these systems, which inhibits electron transport in photosystem II and stimulates singlet oxygen production. The conditional fluorescent (flu) mutant of Arabidopsis produces singlet oxygen when exposed to light after a dark period, and this coincided with a large increase in SOSG fluorescence. Wounding of leaves was followed by an increase in SOSG fluorescence, even in the dark. It is concluded that SOSG is a useful in vivo probe for the detection of singlet oxygen.

The mutant regulator of APX2 1-1 (rax1-1) was identified in Arabidopsis thaliana that constitutively expressed normally photooxidative stress-inducible ASCORBATE PEROXIDASE2 (APX2) and had ≥50% lowered foliar glutathione levels. Mapping revealed that rax1-1 is an allele of γ-GLUTAMYLCYSTEINE SYNTHETASE 1 (GSH1), which encodes chloroplastic γ-glutamylcysteine synthetase, the controlling step of glutathione biosynthesis. By comparison of rax1-1 with the GSH1 mutant cadmium hypersensitive 2, the expression of 32 stress-responsive genes was shown to be responsive to changed glutathione metabolism. Under photo-oxidative stress conditions, the expression of a wider set of defense-related genes was altered in the mutants. In wild-type plants, glutathione metabolism may play a key role in determining the degree of expression of defense genes controlled by several signaling pathways both before and during stress. This control may reflect the physiological state of the plant at the time of the onset of an environmental challenge and suggests that changes in glutathione metabolism may be one means of integrating the function of several signaling pathways.

Over the last 470 Ma, plant evolution has seen major evolutionary transitions, such as the move from water to land and the origins of vascular tissues, seeds, and flowers [1]. These have resulted in the evolution of terrestrial flora that has shaped modern ecosystems and the diversification of the Plant Kingdom, Viridiplantae, into over 374,000 described species [2]. Each of these transitions was accompanied by the gain and loss of genes in plant genomes. For example, whole-genome duplications are known to be fundamental to the origins of both seed and flowering plants [3, 4]. With the ever-increasing quality and quantity of whole-genome data, evolutionary insight into origins of distinct plant groups using comparative genomic techniques is now feasible. Here, using an evolutionary genomics pipeline to compare 208 complete genomes, we analyze the gene content of the ancestral genomes of the last common ancestor of land plants and all other major groups of plant. This approach reveals an unprecedented level of fundamental genomic novelties in two nodes related to the origin of land plants: the first in the origin of streptophytes during the Ediacaran and another in the ancestor of land plants in the Ordovician. Our findings highlight the biological processes that evolved with the origin of land plants and emphasize the importance of conserved gene novelties in plant diversification. Comparisons to other eukaryotic studies suggest a separation of the genomic origins of multicellularity and terrestrialization in plants.

In Arabidopsis thaliana, changes in metabolism and gene expression drive increased drought tolerance and initiate diverse drought avoidance and escape responses. To address regulatory processes that link these responses, we set out to identify genes that govern early responses to drought. To do this, a high-resolution time series transcriptomics data set was produced, coupled with detailed physiological and metabolic analyses of plants subjected to a slow transition from well-watered to drought conditions. A total of 1815 drought-responsive differentially expressed genes were identified. The early changes in gene expression coincided with a drop in carbon assimilation, and only in the late stages with an increase in foliar abscisic acid content. To identify gene regulatory networks (GRNs) mediating the transition between the early and late stages of drought, we used Bayesian network modeling of differentially expressed transcription factor (TF) genes. This approach identified AGAMOUS-LIKE22 (AGL22), as key hub gene in a TFGRN. It has previously been shown that AGL22 is involved in the transition from vegetative state to flowering but here we show that AGL22 expression influences steady state photosynthetic rates and lifetime water use. This suggests that AGL22 uniquely regulates a transcriptional network during drought stress, linking changes in primary metabolism and the initiation of stress responses.

Peptide methionine sulfoxide reductase (PMSR) is a ubiquitous enzyme that repairs oxidatively damaged proteins. In Arabidopsis (Arabidopsis thaliana), a null mutation in PMSR2 (pmsr2-1), encoding a cytosolic isoform of the enzyme, exhibited reduced growth in short-day conditions. In wild-type plants, a diurnally regulated peak of total PMSR activity occurred at the end of the 16-h dark period that was absent in pmsr2-1 plants. This PMSR activity peak in the wild-type plant coincided with increased oxidative stress late in the dark period in the mutant. In pmsr2-1, the inability to repair proteins resulted in higher levels of their turnover, which in turn placed an increased burden on cellular metabolism. This caused increased respiration rates, leading to the observed higher levels of oxidative stress. In wild-type plants, the repair of damaged proteins by PMSR2 at the end of the night in a short-day diurnal cycle alleviates this potential burden on metabolism. Although PMSR2 is not absolutely required for viability of plants, the observation of increased damage to proteins in these long nights suggests the timing of expression of PMSR2 is an important adaptation for conservation of their resources.