Coding and long non-coding RNAs provide evidence of distinct transcriptional reprogramming for two ecotypes of the extremophile plant Eutrema salsugineum undergoing water deficit stress.

BACKGROUND: The severity and frequency of drought has increased around the globe, creating challenges in ensuring food security for a growing world population. As a consequence, improving water use efficiency by crops has become an important objective for crop improvement. Some wild crop relatives have adapted to extreme osmotic stresses and can provide valuable insights into traits and genetic signatures that can guide efforts to improve crop tolerance to water deficits. Eutrema salsugineum, a close relative of many cruciferous crops, is a halophytic plant and extremophyte model for abiotic stress research. RESULTS: Using comparative transcriptomics, we show that two E. salsugineum ecotypes display significantly different transcriptional responses towards a two-stage drought treatment. Even before visibly wilting, water deficit led to the differential expression of almost 1,100 genes for an ecotype from the semi-arid, sub-arctic Yukon, Canada, but only 63 genes for an ecotype from the semi-tropical, monsoonal, Shandong, China. After recovery and a second drought treatment, about 5,000 differentially expressed genes were detected in Shandong plants versus 1,900 genes in Yukon plants. Only 13 genes displayed similar drought-responsive patterns for both ecotypes. We detected 1,007 long non-protein coding RNAs (lncRNAs), 8% were only expressed in stress-treated plants, a surprising outcome given the documented association between lncRNA expression and stress. Co-expression network analysis of the transcriptomes identified eight gene clusters where at least half of the genes in each cluster were differentially expressed. While many gene clusters were correlated to drought treatments, only a single cluster significantly correlated to drought exposure in both ecotypes. CONCLUSION: Extensive, ecotype-specific transcriptional reprogramming with drought was unexpected given that both ecotypes are adapted to saline habitats providing persistent exposure to osmotic stress. This ecotype-specific response would have escaped notice had we used a single exposure to water deficit. Finally, the apparent capacity to improve tolerance and growth after a drought episode represents an important adaptive trait for a plant that thrives under semi-arid Yukon conditions, and may be similarly advantageous for crop species experiencing stresses attributed to climate change.

Evidence that tolerance of Eutrema salsugineum to low phosphate conditions is hard-wired by constitutive metabolic and root-associated adaptations.

MAIN CONCLUSION: The extremophyte Eutrema salsugineum (Yukon ecotype) has adapted to an environment low in available phosphate through metabolic and root-associated traits that enables it to efficiently retrieve, use, and recycle phosphorus. Efficient phosphate (Pi) use by plants would increase crop productivity under Pi-limiting conditions and reduce our reliance on Pi applied as fertilizer. An ecotype of Eutrema salsugineum originating from the Yukon, Canada, shows no evidence of decreased relative growth rate or biomass under low Pi conditions and, as such, offers a promising model for identifying mechanisms to improve Pi use by crops. We evaluated traits associated with efficient Pi use by Eutrema (Yukon ecotype) seedlings and 4-week-old plants, including acquisition, remobilization, and the operation of metabolic bypasses. Relative to Arabidopsis, Eutrema was slower to remobilize phosphorus (P) from senescing leaves, primary and lateral roots showed a lower capacity for rhizosphere acidification, and root acid phosphatase activity was more broadly distributed and not Pi responsive. Both species produced long root hairs on low Pi media, whereas Arabidopsis root hairs were well endowed with phosphatase activity. This capacity was largely absent in Eutrema. In contrast to Arabidopsis, maximal in vitro rates of pyrophosphate-dependent phosphofructokinase and phosphoenolpyruvate carboxylase activities were not responsive to low Pi conditions suggesting that Eutrema has a constitutive and likely preferential capacity to use glycolytic bypass enzymes. Rhizosphere acidification, exudation of acid phosphatases, and rapid remobilization of leaf P are unlikely strategies used by Eutrema for coping with low Pi. Rather, equipping an entire root system for Pi acquisition and utilizing a metabolic strategy suited to deficient Pi conditions offer better explanations for how Eutrema has adapted to thrive on alkaline, highly saline soil that is naturally low in available Pi.

Transcriptome analysis of response to Plasmodiophora brassicae infection in the Arabidopsis shoot and root.

BACKGROUND: Clubroot is an important disease caused by the obligate parasite Plasmodiophora brassicae that infects the Brassicaceae. As a soil-borne pathogen, P. brassicae induces the generation of abnormal tissue in the root, resulting in the formation of galls. Root infection negatively affects the uptake of water and nutrients in host plants, severely reducing their growth and productivity. Many studies have emphasized the molecular and physiological effects of the clubroot disease on root tissues. The aim of the present study is to better understand the effect of P. brassicae on the transcriptome of both shoot and root tissues of Arabidopsis thaliana. RESULTS: Transcriptome profiling using RNA-seq was performed on both shoot and root tissues at 17, 20 and 24 days post inoculation (dpi) of A. thaliana, a model plant host for P. brassicae. The number of differentially expressed genes (DEGs) between infected and uninfected samples was larger in shoot than in root. In both shoot and root, more genes were differentially regulated at 24 dpi than the two earlier time points. Genes that were highly regulated in response to infection in both shoot and root primarily were involved in the metabolism of cell wall compounds, lipids, and shikimate pathway metabolites. Among hormone-related pathways, several jasmonic acid biosynthesis genes were upregulated in both shoot and root tissue. Genes encoding enzymes involved in cell wall modification, biosynthesis of sucrose and starch, and several classes of transcription factors were generally differently regulated in shoot and root. CONCLUSIONS: These results highlight the similarities and differences in the transcriptomic response of above- and below-ground tissues of the model host Arabidopsis following P. brassicae infection. The main transcriptomic changes in root metabolism during clubroot disease progression were identified. An overview of DEGs in the shoot underlined the physiological changes in above-ground tissues following pathogen establishment and disease progression. This study provides insights into host tissue-specific molecular responses to clubroot development and may have applications in the development of clubroot markers for more effective breeding strategies.

Exogenous allantoin increases Arabidopsis seedlings tolerance to NaCl stress and regulates expression of oxidative stress response genes.

Allantoin is a nitrogenous compound derived from purine catabolism that contributes to nitrogen recycling in plants. Accumulation of allantoin in plant tissues and a potential role in protection of plants from abiotic stress conditions has been identified. The present work shows that application of exogenous allantoin increased stress tolerance of Arabidopsis seedlings when germinated on, or subjected to the media containing NaCl. Allantoin-induced tolerance to NaCl stress was associated with decreased production of superoxide and hydrogen peroxide in seedlings. To understand the molecular mechanism, the effect of exogenous allantoin treatment on expression of several stress-related genes was investigated. Exogenous allantoin altered the expression of several antioxidant encoding genes and upregulated the expression of two genes involved in oxidative stress tolerance, SOS1 and RCD1, in the presence or absence of NaCl. Allantoin increased the NaCl tolerance of abscisic acid insensitive mutants, suggesting that it can function independently of abscisic acid signaling. These results provide additional evidence for the role of allantoin in enhancing plants tolerance to oxidative stress.

Ureide metabolism under abiotic stress in Arabidopsis thaliana.

Ureides are nitrogenous compounds derived from purine catabolism which contribute to nitrogen recycling in plants. Accumulation of ureide compounds has been reported in a number of plant species under stress conditions, suggesting their involvement in plants' response to stress. In this research a biochemical and molecular approach was applied to address the ureide accumulation under abiotic stress conditions in Arabidopsis thaliana. Ureide concentration and changes in expression of ureide metabolic genes were examined in response to drought, NaCl and mannitol treatments. Additionally, an Arabidopsis allantoinase (ALN) mutant with constitutive accumulation of a ureide compound, allantoin, was used to investigate the impact of high levels of this compound on drought and NaCl stress responses. In the leaf tissue of adult plants allantoin accumulated in response to soil drying. Transcription of urate oxidase (UO), involved in allantoin production, was highly up-regulated under the same conditions. Allantoin and allantoate also accumulated in seedlings following treatment with NaCl or mannitol. aln mutants with enhanced levels of allantoin exhibited higher tolerance to drought and NaCl. Hydrogen peroxide and superoxide did not accumulate in the aln mutant leaves to the same degree in response to drought when compared to the wild-type. Our results suggest that ureide metabolism and accumulation contribute to the abiotic stress response which is regulated, at least in part, at the transcriptional level. Higher concentrations of allantoin in the mutant elevates abiotic stress tolerance, possibly by reducing oxidative damage.