Plant nitrate supply regulates Erwinia amylovora virulence gene expression in Arabidopsis.

We showed previously that nitrogen (N) limitation decreases Arabidopsis resistance to Erwinia amylovora (Ea). We show that decreased resistance to bacteria in low N is correlated with lower apoplastic reactive oxygen species (ROS) accumulation and lower jasmonic acid (JA) pathway expression. Consistently, pretreatment with methyl jasmonate (Me-JA) increased the resistance of plants grown under low N. In parallel, we show that in planta titres of a nonvirulent type III secretion system (T3SS)-deficient Ea mutant were lower than those of wildtype Ea in low N, as expected, but surprisingly not in high N. This lack of difference in high N was consistent with the low expression of the T3SS-encoding hrp virulence genes by wildtype Ea in plants grown in high N compared to plants grown in low N. This suggests that expressing its virulence factors in planta could be a major limiting factor for Ea in the nonhost Arabidopsis. To test this hypothesis, we preincubated Ea in an inducing medium that triggers expression of hrp genes in vitro, prior to inoculation. This preincubation strongly enhanced Ea titres in planta, independently of the plant N status, and was correlated to a significant repression of JA-dependent genes. Finally, we identify two clusters of metabolites associated with resistance or with susceptibility to Ea. Altogether, our data showed that high susceptibility of Arabidopsis to Ea, under low N or following preincubation in hrp-inducing medium, is correlated with high expression of the Ea hrp genes in planta and low expression of the JA signalling pathway, and is correlated with the accumulation of specific metabolites.

Methionine Sulfoxide Reductases Contribute to Anaerobic Fermentative Metabolism in Bacillus cereus.

Reversible oxidation of methionine to methionine sulfoxide (Met(O)) is a common posttranslational modification occurring on proteins in all organisms under oxic conditions. Protein-bound Met(O) is reduced by methionine sulfoxide reductases, which thus play a significant antioxidant role. The facultative anaerobe Bacillus cereus produces two methionine sulfoxide reductases: MsrA and MsrAB. MsrAB has been shown to play a crucial physiological role under oxic conditions, but little is known about the role of MsrA. Here, we examined the antioxidant role of both MsrAB and MrsA under fermentative anoxic conditions, which are generally reported to elicit little endogenous oxidant stress. We created single- and double-mutant Δmsr strains. Compared to the wild-type and ΔmsrAB mutant, single- (ΔmsrA) and double- (ΔmsrAΔmsrAB) mutants accumulated higher levels of Met(O) proteins, and their cellular and extracellular Met(O) proteomes were altered. The growth capacity and motility of mutant strains was limited, and their energy metabolism was altered. MsrA therefore appears to play a major physiological role compared to MsrAB, placing methionine sulfoxides at the center of the B. cereus antioxidant system under anoxic fermentative conditions.

Every cloud has a silver lining: how abiotic stresses affect gene expression in plant-pathogen interactions.

Current environmental and climate changes are having a pronounced influence on the outcome of plant-pathogen interactions, further highlighting the fact that abiotic stresses strongly affect biotic interactions at various levels. For instance, physiological parameters such as plant architecture and tissue organization together with primary and specialized metabolism are affected by environmental constraints, and these combine to make an individual plant either a more or less suitable host for a given pathogen. In addition, abiotic stresses can affect the timely expression of plant defense and pathogen virulence. Indeed, several studies have shown that variations in temperature, and in water and mineral nutrient availability affect the expression of plant defense genes. The expression of virulence genes, known to be crucial for disease outbreak, is also affected by environmental conditions, potentially modifying existing pathosystems and paving the way for emerging pathogens. In this review, we summarize our current knowledge on the impact of abiotic stress on biotic interactions at the transcriptional level in both the plant and the pathogen side of the interaction. We also perform a metadata analysis of four different combinations of abiotic and biotic stresses, which identifies 197 common modulated genes with strong enrichment in Gene Ontology terms related to defense . We also describe the multistress-specific responses of selected defense-related genes.

Nitrogen Limitation Alters the Response of Specific Genes to Biotic Stress.

In their natural environment, plants are generally confronted with multiple co-occurring stresses. However, the interaction between stresses is not well known and transcriptomic data in response to combined stresses remain scarce. This study aims at characterizing the interaction between transcriptomic responses to biotic stress and nitrogen (N) limitation. Plants were grown in low or full N, infected or not with Erwinia amylovora (Ea) and plant gene expression was analyzed through microarray and qRT-PCR. Most Ea-responsive genes had the same profile (induced/repressed) in response to Ea in low and full N. In response to stress combination, one third of modulated transcripts responded in a manner that could not be deduced from their response to each individual stress. Many defense-related genes showed a prioritization of their response to biotic stress over their response to N limitation, which was also observed using Pseudomonas syringae as a second pathosystem. Our results indicate an interaction between transcriptomic responses to N and biotic stress. A small fraction of transcripts was prioritized between antagonistic responses, reflecting a preservation of the plant defense program under N limitation. Furthermore, this interaction also led to a complex and specific response in terms of metabolism and cellular homeostasis-associated genes.

The bile acid deoxycholate elicits defences in Arabidopsis and reduces bacterial infection.

Disease has an effect on crop yields, causing significant losses. As the worldwide demand for agricultural products increases, there is a need to pursue the development of new methods to protect crops from disease. One mechanism of plant protection is through the activation of the plant immune system. By exogenous application, 'plant activator molecules' with elicitor properties can be used to activate the plant immune system. These defence-inducing molecules represent a powerful and often environmentally friendly tool to fight pathogens. We show that the secondary bile acid deoxycholic acid (DCA) induces defence in Arabidopsis and reduces the proliferation of two bacterial phytopathogens: Erwinia amylovora and Pseudomonas syringae pv. tomato. We describe the global defence response triggered by this new plant activator in Arabidopsis at the transcriptional level. Several induced genes were selected for further analysis by quantitative reverse transcription-polymerase chain reaction. We describe the kinetics of their induction and show that abiotic stress, such as moderate drought or nitrogen limitation, does not impede DCA induction of defence. Finally, we investigate the role in the activation of defence by this bile acid of the salicylic acid biosynthesis gene SID2, of the receptor-like kinase family genes WAK1-3 and of the NADPH oxidase-encoding RbohD gene. Altogether, we show that DCA constitutes a promising molecule for plant protection which can induce complementary lines of defence, such as callose deposition, reactive oxygen species accumulation and the jasmonic acid and salicylic acid signalling pathways.

Nitrogen metabolism meets phytopathology.

Nitrogen (N) is essential for life and is a major limiting factor of plant growth. Because soils frequently lack sufficient N, large quantities of inorganic N fertilizers are added to soils for crop production. However, nitrate, urea, and ammonium are a major source of global pollution, because much of the N that is not taken up by plants enters streams, groundwater, and lakes, where it affects algal production and causes an imbalance in aquatic food webs. Many agronomical data indicate that the higher use of N fertilizers during the green revolution had an impact on the incidence of crop diseases. In contrast, examples in which a decrease in N fertilization increases disease severity are also reported, indicating that there is a complex relationship linking N uptake and metabolism and the disease infection processes. Thus, although it is clear that N availability affects disease, the underlying mechanisms remain unclear. The aim of this review is to describe current knowledge of the mechanisms that link plant N status to the plant's response to pathogen infection and to the virulence and nutritional status of phytopathogens.