Effects of Sulfur Assimilation in Pseudomonas fluorescens SS101 on Growth, Defense, and Metabolome of Different Brassicaceae

Genome-wide analysis of plant-growth-promoting Pseudomonas fluorescens strain SS101 (PfSS101) followed by site-directed mutagenesis previously suggested that sulfur assimilation may play an important role in growth promotion and induced systemic resistance in Arabidopsis. Here, we investigated the effects of sulfur metabolism in PfSS101 on growth, defense, and shoot metabolomes of Arabidopsis and the Brassica crop, Broccoli. Root tips of seedlings of Arabidopsis and two Broccoli cultivars were treated with PfSS101 or with a mutant disrupted in the adenylsulfate reductase cysH, a key gene in cysteine and methionine biosynthesis. Phenotyping of plants treated with wild-type PfSS101 or its cysH mutant revealed that sulfur assimilation in PfSS101 was associated with enhanced growth of Arabidopsis but with a reduction in shoot biomass of two Broccoli cultivars. Untargeted metabolomics revealed that cysH-mediated sulfur assimilation in PfSS101 had significant effects on shoot chemistry of Arabidopsis, in particular on chain elongation of aliphatic glucosinolates (GLSs) and on indole metabolites, including camalexin and the growth hormone indole-3-acetic acid. In Broccoli, PfSS101 sulfur assimilation significantly upregulated the relative abundance of several shoot metabolites, in particular, indolic GLSs and phenylpropanoids. These metabolome changes in Broccoli plants coincided with PfSS101-mediated suppression of leaf infections by Xanthomonas campestris. Our study showed the metabolic interconnectedness of plants and their root-associated microbiota.

A Low Level of NaCl Stimulates Plant Growth by Improving Carbon and Sulfur Assimilation in Arabidopsis thaliana

High-salinity stress represses plant growth by inhibiting various metabolic processes. In contrast to the well-studied mechanisms mediating tolerance to high levels of salt, the effects of low levels of salts have not been well studied. In this study, we examined the growth of Arabidopsis thaliana plants under different NaCl concentrations. Interestingly, both shoot and root biomass increased in the presence of 5 mM NaCl, whereas more than 10 mM NaCl decreased plant biomass. To clarify the biological mechanism by which a low level of NaCl stimulated plant growth, we analyzed element accumulation in plants grown under different NaCl concentrations. In addition to the Na and Cl contents, C, S, Zn, and Cu contents were increased under 5 mM NaCl in shoots; this was not observed at higher NaCl concentrations. Adverse effects of high salinity, such as decreased levels of nitrate, phosphate, sulfate, and some cations, did not occur in the presence of 5 mM NaCl. An increase in C was possibly attributed to increased photosynthesis supported by Cl, Zn, and Cu, which also increased in shoots after NaCl application. Salt stress-responsive gene expression was enhanced under 20 mM NaCl but not at lower doses. Among the S metabolites analyzed, cysteine (Cys) was increased by 5 mM NaCl, suggesting that S assimilation was promoted by this dose of NaCl. These results indicate the usefulness of NaCl for plant growth stimulation.

The Enhancement of Drought Tolerance in Arabidopsis Plants Induced by Pretreatment with Sulfur Dioxide

Sulfur dioxide (SO2) is a common air pollutant that has multiple effects on plants. Here, the effect of prior exposure to SO2 on the improvements of drought tolerance and possible regulation mechanisms were investigated in Arabidopsis plants. The experimental results showed that pre-exposure to 30 mg/m3 SO2 for 72 h could reduce leaf water loss, and enhance the drought tolerance of Arabidopsis plants. SO2 pre-exposure decreased leaf stomatal conductance (Gs) and transpiration rate (Tr) but increased net photosynthetic rate (Pn), water use efficiency (iWUE) and photosynthetic pigment contents under drought conditions. Importantly, the activities of superoxide dismutase (SOD) and peroxidase (POD) were significantly increased, while the contents of hydrogen peroxide (H2O2) and malondialdehyde (MDA) were decreased in SO2-pretreated Arabidopsis plants under drought stress. Additionally, the activity of o-acetylserine(thio)lyase (OASTL) and the content of cysteine (Cys), the rate-limiting enzyme and the first organic product of sulfur assimilation, were increased significantly in drought-stressed plants after SO2 pretreatment, along with the increases of other thiol-containing compounds glutathione (GSH) and non-protein thiol (NPT). Meanwhile, SO2 pre-exposure induced a higher level of proline accumulation, accompanied by the increased activity of proline synthase P5CS, the decreased activity of proline dehydrogenase ProDH and the corresponding alteration of gene transcription. Collectively, the enhanced drought tolerance afforded by SO2 might be related to the improvement of plant photosynthesis, antioxidant defense, sulfur assimilation and osmotic adjustment. These findings provide new insights in understanding the role of SO2 in plant adaptation to environmental stress.

Photorespiration: The Futile Cycle?

Photorespiration, or C2 photosynthesis, is generally considered a futile cycle that potentially decreases photosynthetic carbon fixation by more than 25%. Nonetheless, many essential processes, such as nitrogen assimilation, C1 metabolism, and sulfur assimilation, depend on photorespiration. Most studies of photosynthetic and photorespiratory reactions are conducted with magnesium as the sole metal cofactor despite many of the enzymes involved in these reactions readily associating with manganese. Indeed, when manganese is present, the energy efficiency of these reactions may improve. This review summarizes some commonly used methods to quantify photorespiration, outlines the influence of metal cofactors on photorespiratory enzymes, and discusses why photorespiration may not be as wasteful as previously believed.

Genetic Variation of the Serine Acetyltransferase Gene Family for Sulfur Assimilation in Maize

Improving sulfur assimilation in maize kernels is essential due to humans and animals’ inability to synthesize methionine. Serine acetyltransferase (SAT) is a critical enzyme that controls cystine biosynthesis in plants. In this study, all SAT gene members were genome-wide characterized by using a sequence homology search. The RNA-seq quantification indicates that they are highly expressed in leaves, other than root and seeds, consistent with their biological functions in sulfur assimilation. With the recently released 25 genomes of nested association mapping (NAM) founders representing the diverse maize stock, we had the opportunity to investigate the SAT genetic variation comprehensively. The abundant transposon insertions into SAT genes indicate their driving power in terms of gene structure and genome evolution. We found that the transposon insertion into exons could change SAT gene transcription, whereas there was no significant correlation between transposable element (TE) insertion into introns and their gene expression, indicating that other regulatory elements such as promoters could also be involved. Understanding the SAT gene structure, gene expression and genetic variation involved in natural selection and species adaption could precisely guide genetic engineering to manipulate sulfur assimilation in maize and to improve nutritional quality.

The SLIM1 transcription factor regulates arsenic sensitivity in Arabidopsis thaliana

The transcriptional regulators of arsenic-induced gene expression remain largely unknown; however, arsenic exposure rapidly depletes cellular glutathione levels increasing demand for thiol compounds from the sulfur assimilation pathway. Thus, sulfur assimilation is tightly linked with arsenic detoxification. To explore the hypothesis that the key transcriptional regulator of sulfur assimilation, SLIM1, is involved in arsenic-induced gene expression, we evaluated the response of slim1 mutants to arsenic treatments. We found that slim1 mutants were sensitive to arsenic in root growth assays. Furthermore, arsenic treatment caused high levels of oxidative stress in the slim1 mutants, and slim1 mutants were impaired in both thiol and sulfate accumulation. We also found enhanced arsenic accumulation in the roots of slim1 mutants. Furthermore, microarray analyses identified genes from a highly co-regulated gene cluster (the O-acetylserine gene cluster), as being significantly upregulated in the slim1-1 mutant background in response to arsenic exposure. The present study identified the SLIM1 transcription factor as an important component in arsenic-induced gene expression and arsenic tolerance. Our results suggest that the severe arsenic sensitivity of the slim1 mutants is a result of both altered redox status as well as mis-regulation of key genes.HighlightWe identify a critical function of the SLIM1 transcription factor in regulating arsenic transcriptional responses and propose that SLIM1 acts as both a transcriptional activator and repressor.

The SLIM1 transcription factor regulates arsenic sensitivity in Arabidopsis thaliana

The transcriptional regulators of arsenic-induced gene expression remain largely unknown; however, arsenic exposure rapidly depletes cellular glutathione levels increasing demand for thiol compounds from the sulfur assimilation pathway. Thus, sulfur assimilation is tightly linked with arsenic detoxification. To explore the hypothesis that the key transcriptional regulator of sulfur assimilation, SLIM1, is involved in arsenic-induced gene expression, we evaluated the response of slim1 mutants to arsenic treatments. We found that slim1 mutants were sensitive to arsenic in root growth assays. Furthermore, arsenic treatment caused high levels of oxidative stress in the slim1 mutants, and slim1 mutants were impaired in both thiol and sulfate accumulation. We also found enhanced arsenic accumulation in the roots of slim1 mutants. Furthermore, microarray analyses identified genes from a highly co-regulated gene cluster (the O-acetylserine gene cluster), as being significantly upregulated in the slim1-1 mutant background in response to arsenic exposure. The present study identified the SLIM1 transcription factor as an important component in arsenic-induced gene expression and arsenic tolerance. Our results suggest that the severe arsenic sensitivity of the slim1 mutants is a result of both altered redox status as well as mis-regulation of key genes.