Sylvain Jeandroz

Nitric oxide production mediates oligogalacturonide‐triggered immunity and resistance to Botrytis cinerea in Arabidopsis thaliana

Nitric oxide (NO) regulates a wide range of plant processes from development to environmental adaptation. In this study, we investigated the production and/or function of NO in Arabidopsis thaliana leaf discs and plants elicited by oligogalacturonides (OGs) and challenged with Botrytis cinerea. We provided evidence that OGs triggered a fast and long lasting NO production which was Ca(2+) dependent and involved nitrate reductase (NR). Accordingly, OGs triggered an increase of both NR activity and transcript accumulation. NO production was also sensitive to the mammalian NO synthase inhibitor L-NAME. Intriguingly, we showed that L-NAME affected NO production by interfering with NR activity, thus questioning the mechanisms of how this compound impairs NO synthesis in plants. We further demonstrated that NO modulates RBOHD-mediated reactive oxygen species (ROS) production and participates in the regulation of OG-responsive genes such as anionic peroxidase (PER4) and a β-1,3-glucanase. Mutant plants impaired in PER4 and β-1,3-glucanase, as well as Col-0 plants treated with the NO scavenger cPTIO, were more susceptible to B. cinerea. Taken together, our investigation deciphers part of the mechanisms linking NO production, NO-induced effects and basal resistance to B. cinerea.

Protein S-nitrosylation: What's going on in plants?

Nitric oxide (NO) is now recognized as a key regulator of plant physiological processes. Understanding the mechanisms by which NO exerts its biological functions has been the subject of extensive research. Several components of the signaling pathways relaying NO effects in plants, including second messengers, protein kinases, phytohormones, and target genes, have been characterized. In addition, there is now compelling experimental evidence that NO partly operates through posttranslational modification of proteins, notably via S-nitrosylation and tyrosine nitration. Recently, proteome-wide scale analyses led to the identification of numerous protein candidates for S-nitrosylation in plants. Subsequent biochemical and in silico structural studies revealed certain mechanisms through which S-nitrosylation impacts their functions. Furthermore, first insights into the physiological relevance of S-nitrosylation, particularly in controlling plant immune responses, have been recently reported. Collectively, these discoveries greatly extend our knowledge of NO functions and of the molecular processes inherent to signal transduction in plants.

Occurrence, structure, and evolution of nitric oxide synthase–like proteins in the plant kingdom

Nitric oxide synthase is conserved in some species of algae, but not in land plants. Gloss In animals, nitric oxide (NO) is mainly produced by NO synthases (NOSes), a family of enzymes partly conserved in bacteria. In contrast, how NO is produced in plants is poorly understood, and whether these organisms have proteins homologous to animal NOSes is unknown. In this Review, with three figures, two tables, and 60 references, we describe a search for the presence of NOSes in over 1300 species of land plants and algae, which showed that NOSes are present in a few algal species but appear to not be conserved in land plants. Thus, land plants seem to have evolved mechanisms for NO synthesis different from those present in animals. These mechanisms rely mainly on nitrite reduction. Nitric oxide (NO) signaling regulates various physiological processes in both animals and plants. In animals, NO synthesis is mainly catalyzed by NO synthase (NOS) enzymes. Although NOS-like activities that are sensitive to mammalian NOS inhibitors have been detected in plant extracts, few bona fide plant NOS enzymes have been identified. We searched the data set produced by the 1000 Plants (1KP) international consortium for the presence of transcripts encoding NOS-like proteins in over 1000 species of land plants and algae. We also searched for genes encoding NOS-like enzymes in 24 publicly available algal genomes. We identified no typical NOS sequences in 1087 sequenced transcriptomes of land plants. In contrast, we identified NOS-like sequences in 15 of the 265 algal species analyzed. Even if the presence of NOS enzymes assembled from multipolypeptides in plants cannot be conclusively discarded, the emerging data suggest that, instead of generating NO with evolutionarily conserved NOS enzymes, land plants have evolved finely regulated nitrate assimilation and reduction processes to synthesize NO through a mechanism different than that in animals.

NO signaling in plant immunity: A tale of messengers

Nitric oxide (NO) is a free radical gas involved in a myriad of plant physiological processes including immune responses. How NO mediates its biological effects in plant facing microbial pathogen attack is an unresolved question. Insights into the molecular mechanisms by which it propagates signals reveal the contribution of this simple gas in complex signaling pathways shared with reactive oxygen species (ROS) and the second messenger Ca(2+). Understanding of the subtle cross-talks operating between these signals was greatly improved by the recent identification and the functional analysis of proteins regulated through S-nitrosylation, a major NO-dependent post-translational protein modification. Overall, these findings suggest that NO is probably an important component of the mechanism coordinating and regulating Ca(2+) and ROS signaling in plant immunity.

Nitric oxide synthase in plants: Where do we stand?

Over the past twenty years, nitric oxide (NO) has emerged as an important player in various plant physiological processes. Although many advances in the understanding of NO functions have been made, the question of how NO is produced in plants is still challenging. It is now generally accepted that the endogenous production of NO is mainly accomplished through the reduction of nitrite via both enzymatic and non-enzymatic mechanisms which remain to be fully characterized. Furthermore, experimental arguments in favour of the existence of plant nitric oxide synthase (NOS)-like enzymes have been reported. However, recent investigations revealed that land plants do not possess animal NOS-like enzymes while few algal species do. Phylogenetic and structural analyses reveals interesting features specific to algal NOS-like proteins.

Cross-Regulation between N Metabolism and Nitric Oxide (NO) Signaling during Plant Immunity

Plants are sessile organisms that have evolved a complex immune system which helps them cope with pathogen attacks. However, the capacity of a plant to mobilize different defense responses is strongly affected by its physiological status. Nitrogen (N) is a major nutrient that can play an important role in plant immunity by increasing or decreasing plant resistance to pathogens. Although no general rule can be drawn about the effect of N availability and quality on the fate of plant/pathogen interactions, plants’ capacity to acquire, assimilate, allocate N, and maintain amino acid homeostasis appears to partly mediate the effects of N on plant defense. Nitric oxide (NO), one of the products of N metabolism, plays an important role in plant immunity signaling. NO is generated in part through Nitrate Reductase (NR), a key enzyme involved in nitrate assimilation, and its production depends on levels of nitrate/nitrite, NR substrate/product, as well as on L-arginine and polyamine levels. Cross-regulation between NO signaling and N supply/metabolism has been evidenced. NO production can be affected by N supply, and conversely NO appears to regulate nitrate transport and assimilation. Based on this knowledge, we hypothesized that N availability partly controls plant resistance to pathogens by controlling NO homeostasis. Using the Medicago truncatula/Aphanomyces euteiches pathosystem, we showed that NO homeostasis is important for resistance to this oomycete and that N availability impacts NO homeostasis by affecting S-nitrosothiol (SNO) levels and S-nitrosoglutathione reductase activity in roots. These results could therefore explain the increased resistance we noted in N-deprived as compared to N-replete M. truncatula seedlings. They open onto new perspectives for the studies of N/plant defense interactions.

Physiological and proteomic approaches to evaluate the role of sterol binding in elicitin-induced resistance

Cryptogein is a proteinaceous elicitor secreted by Phytophthora cryptogea that can induce resistance to P. parasitica in tobacco plants. On the basis of previous computer modelling experiments, by site-directed mutagenesis a series of cryptogein variants was prepared with altered abilities to bind sterols, phospholipids or both. The sterol binding and phospholipid transfer activities corresponded well with the previously reported structural data. Induction of the synthesis of reactive oxygen species (ROS) in tobacco cells in suspension and proteomic analysis of intercellular fluid changes in tobacco leaves triggered by these mutant proteins were not proportional to their ability to bind or transfer sterols and phospholipids. However, changes in the intercellular proteome corresponded to transcription levels of defence genes and resistance to P. parasitica and structure-prediction of mutants did not reveal any significant changes in protein structure. These results suggest, contrary to previous proposals, that the sterol-binding ability of cryptogein and its mutants, and the associated conformational change in the ω-loop, might not be principal factors in either ROS production or resistance induction. Nevertheless, the results support the importance of the ω-loop for the interaction of the protein with the high affinity binding site on the plasma membrane.

Nitrogen modulation of Medicago truncatula resistance to Aphanomyces euteiches depends on plant genotype

Nitrogen (N) availability can impact plant resistance to pathogens by the regulation of plant immunity. To better understand the links between N nutrition and plant defence, we analysed the impact of N availability on Medicago truncatula resistance to the root pathogen Aphanomyces euteiches. This oomycete is considered to be the most limiting factor for legume production. Ten plant genotypes were tested in vitro for their resistance to A. euteiches in either complete or nitrate-deficient medium. N deficiency led to enhanced or reduced susceptibility depending on the plant genotype. Focusing on four genotypes displaying contrasting responses, we determined the impact of N deficiency on plant growth and shoot N concentration, and performed expression analyses on N- and defence-related genes, as well as the quantification of soluble phenolics and different amino acids in roots. Our analyses suggest that N modulation of plant resistance is not linked to plant response to N deprivation or to mechanisms previously identified to be involved in plant resistance. Furthermore, our studies highlight a role of glutamine in mediating the susceptibility to A. euteiches in M. truncatula.

Structure and functions of the chaperone-like p97/CDC48 in plants

BACKGROUND The chaperone-like p97 is a member of the AAA+ ATPase enzyme family that contributes to numerous cellular activities. P97 has been broadly studied in mammals (VCP/p97) and yeasts (CDC48: Cell Division Cycle 48/p97) and numerous investigations highlighted that this protein is post-translationally regulated, is structured in homohexamer and interacts with partners and cofactors that direct it to distinct cellular signalization pathway including protein quality control and degradation, cell cycle regulation, genome stability, vesicular trafficking, autophagy and immunity. SCOPE OF REVIEW p97 is also conserved in plants (CDC48) but its functions are less understood. In the present review we intended to present the state of the art of the structure, regulation and functions of CDC48 in plants. MAJOR CONCLUSIONS Evidence accumulated underline that CDC48 plays a crucial role in development, cell cycle regulation and protein turnover in plants. Furthermore, its involvement in plant immunity has recently emerged and first interacting partners have been identified, shedding light on its putative cellular activities. GENERAL SIGNIFICANCE Identification of emerging functions of CDC48 in plants opens new roads of research in immunity and provides new insights into the mechanisms of protein quality control.

NO Signalling in Plant Immunity

The importance of nitric oxide (NO) in innate and adaptive immunity in mammals is well recognised. NO exerts antimicrobial properties against invaders but also displays immunoregulatory functions in which S-nitrosylation represents a signalling process of major importance. Over the last two decades, a growing body of evidence suggests that NO is also a major component of plant immunity. Our understanding of its role in plant defence has been enriched by the identification and functional analysis of S-nitrosylated proteins. The recent identification of new S-nitrosylated proteins including the chaperone-like enzyme cell division cycle 48 (CDC48), histone deacetylases (HDACs) and calmodulin (CaM) reveals that NO could act as a modulator of epigenetic changes and targeting of ubiquitinated proteins for degradation. These findings also expand our understanding of the mechanisms controlling NO synthesis and its crosstalks with calcium signalling in plant immunity.