Plants are constantly threatened by a wide array of microbial pathogens. Pathogen invasion can lead to vast yield losses and the demand for sustainable plant-protection strategies has never been ...greater. Chemical plant activators and selected strains of rhizobacteria can increase resistance against specific types of pathogens but these treatments are often ineffective or even cause susceptibility against others. Silicon application is one of the scarce examples of a treatment that effectively induces broad-spectrum disease resistance. The prophylactic effect of silicon is considered to be the result of both passive and active defences. Although the phenomenon has been known for decades, very little is known about the molecular basis of silicon-afforded disease control. By combining knowledge on how silicon interacts with cell metabolism in diatoms and plants, this review describes silicon-induced regulatory mechanisms that might account for broad-spectrum plant disease resistance. Priming of plant immune responses, alterations in phytohormone homeostasis, regulation of iron homeostasis, silicon-driven photorespiration and interaction with defence signalling components all are potential mechanisms involved in regulating silicon-triggered resistance responses. Further elucidating how silicon exerts its beneficial properties may create new avenues for developing plants that are better able to withstand multiple attackers.
Brassinosteroids (BRs) and gibberellins (GAs) are two groups of phytohormones that regulate many common developmental processes throughout the plant life cycle. Fueled by large-scale ‘omics’ ...technologies and the burgeoning field of plant computational biology, the past few years have witnessed paradigm-shifting advances in our understanding of how BRs and GA are perceived and their signals transduced. Accumulating evidence also implicates BR and GA in the coordination and integration of plant immune responses. Similarly to other growth regulators, BR and GA play ambiguous roles in molding pathological outcomes, the effects of which may depend not only on the pathogen’s lifestyle and infection strategy, but also on specialized features of each interaction. Analysis of the underpinning molecular mechanisms points to a crucial role of GA-inhibiting DELLA proteins and the BR-regulated transcription factor BZR1. Acting at the interface of developmental and defense signaling, these proteins likely serve as central hubs for pathway crosstalk and signal integration, allowing appropriate modulation of plant growth and defense in response to various stimuli. In this review, we outline the latest discoveries dealing with BR and GA modulation of plant innate immunity and highlight interactions between BR and GA signaling, plant defense, and microbial virulence.
Complex defense signaling pathways, controlled by different hormones, are involved in the reaction of plants to a wide range of biotic and abiotic stress factors. We studied the ability of salicylic ...acid, jasmonate (JA), and ethylene (ET) to induce systemic defense in rice (Oryza sativa) against the root knot nematode Meloidogyne graminicola. Exogenous ET (ethephon) and JA (methy1 jasmonate) supply on the shoots induced a strong systemic defense response in the roots, exemplified by a major up-regulation of pathogenesis-related genes OsPR1a and OsPR1b, while the salicylic acid analog BTH (benzo-1,2,3-thiadiazole-7-carbothioic acid S-methy1 ester) was a less potent systemic defense inducer from shoot to root. Experiments with JA biosynthesis mutants and ET-insensitive transgenics showed that ET-induced defense requires an intact JA pathway, while JA-induced defense was still functional when ET signaling was impaired. Pharmacological inhibition of JA and ET biosynthesis confirmed that JA biosynthesis is needed for ET-induced systemic defense, and quantitative real-time reverse transcription-polymerase chain reaction data revealed that ET application onto the shoots strongly activates JA biosynthesis and signaling genes in the roots. All data provided in this study point to the JA pathway to play a pivotal role in rice defense against root knot nematodes. The expression of defense-related genes was monitored in root galls caused by M. graminicola. Different analyzed defense genes were attenuated in root galls caused by the nematode at early time points after infection. However, when the exogenous defense inducers ethephon and methy1 jasmonate were supplied to the plant, the nematode was less effective in counteracting root defense pathways, hence making the plant more resistant to nematode infection.
The soil-borne fungus
causes serious vascular disease in a wide variety of annual crops and woody perennials. Verticillium wilt is notoriously difficult to control by conventional methods, so there ...is great potential for biocontrol to manage this disease. In this study we aimed to review the research about
biocontrol to get a better understanding of characteristics that are desirable in a biocontrol agent (BCA) against Verticillium wilt. We only considered studies in which the BCAs were tested on plants. Most biocontrol studies were focused on plants of the
, and
and within these families eggplant, cotton, and oilseed rape were the most studied crops. The list of bacterial BCAs with potential against
was dominated by endophytic
and
isolates, while non-pathogenic xylem-colonizing
and
isolates topped the fungal list. Predominant modes of action involved in biocontrol were inhibition of primary inoculum germination, plant growth promotion, competition and induced resistance. Many BCAs showed
antibiosis and mycoparasitism but these traits were not correlated with activity
and there is no evidence that they play a role
. Good BCAs were obtained from soils suppressive to Verticillium wilt, disease suppressive composts, and healthy plants in infested fields. Desirable characteristics in a BCA against
are the ability to (1) affect the survival or germination of microsclerotia, (2) colonize the xylem and/or cortex and compete with the pathogen for nutrients and/or space, (3) induce resistance responses in the plant and/or (4) promote plant growth. Potential BCAs should be screened in conditions that resemble the field situation to increase the chance of successful use in practice. Furthermore, issues such as large scale production, formulation, preservation conditions, shelf life, and application methods should be considered early in the process of selecting BCAs against
.
The mycotoxin deoxynivalenol (DON), produced by several Fusarium spp., acts as a virulence factor and is essential for symptom development after initial wheat infection. Accumulating evidence shows ...that the production of this secondary metabolite can be triggered by diverse environmental and cellular signals, implying that it might have additional roles during the life cycle of the fungus. Here, we review data that position DON in the saprophytic fitness of Fusarium, in defense and in the primary C and N metabolism of the plant and the fungus. We combine the available information in speculative models on the role of DON throughout the interaction with the host, providing working hypotheses that await experimental validation. We also highlight the possible impact of control measures in the field on DON production and summarize the influence of abiotic factors during processing and storage of food and feed matrices. Altogether, we can conclude that DON is a very important compound for Fusarium to cope with a changing environment and to assure its growth, survival, and production of toxic metabolites in diverse situations.
Gibberellins are a class of tetracyclic plant hormones that are well known to promote plant growth by inducing the degradation of a class of nuclear growth-repressing proteins, called DELLAs. In ...recent years, GA and DELLAs are also increasingly implicated in plant responses to pathogen attack, although our understanding of the underlying mechanisms is still limited, especially in monocotyledonous crop plants. Aiming to further decipher the molecular underpinnings of GA- and DELLA-modulated plant immunity, we studied the dynamics and impact of GA and DELLA during infection of the model crop rice (Oryza sativa) with four different pathogens exhibiting distinct lifestyles and infection strategies. Opposite to previous findings in Arabidopsis (Arabidopsis thaliana), our findings reveal a prominent role of the DELLA protein Slender Rice1 (SLR1) in the resistance toward (hemi)biotrophic but not necrotrophic rice pathogens. Moreover, contrary to the differential effect of DELLA on the archetypal defense hormones salicylic acid (SA) and jasmonic acid (JA) in Arabidopsis, we demonstrate that the resistance-promoting effect of SLR1 is due at least in part to its ability to boost both SA- and JA-mediated rice defenses. In a reciprocal manner, we found JA and SA treatment to interfere with GA metabolism and stabilize SLR1. Together, these findings favor a model whereby SLR1 acts as a positive regulator of hemibiotroph resistance in rice by integrating and amplifying SA- and JA-dependent defense signaling. Our results highlight the differences in hormone defense networking between rice and Arabidopsis and underscore the importance of GA and DELLA in molding disease outcomes.
Sucrose non-fermenting-1-related protein kinase-1 (SnRK1) belongs to a family of evolutionary conserved kinases with orthologs in all eukaryotes, ranging from yeasts (SnF1) to mammals (AMP-Activated ...kinase). These kinases sense energy deficits caused by nutrient limitation or stress and coordinate the required adaptations to maintain energy homeostasis and survival. In plants, SnRK1 is a global regulator of plant metabolism and is also involved in abiotic stress responses. Its role in the response to biotic stress, however, is only starting to be uncovered. Here we studied the effect of altered SnRK1a expression on growth and plant defense in rice. OsSnRK1a overexpression interfered with normal growth and development and increased resistance against both (hemi)biotrophic and necrotrophic pathogens, while OsSnRK1a silencing in RNAi lines increased susceptibility. OsSnRK1a overexpression positively affected the salicylic acid pathway and boosted the jasmonate-mediated defense response after inoculation with the blast fungus Pyricularia oryzae. Together these findings strongly suggest OsSnRK1a to be involved in plant basal immunity and favor a model whereby OsSnRK1a acts as a master switch that regulates growth-immunity trade-offs.
Plants are obliged to defend themselves against a wide range of biotic and abiotic stresses. Complex regulatory signaling networks mount an appropriate defense response depending on the type of ...stress that is perceived. In response to abiotic stresses such as drought, cold, and salinity, the function of abscisic acid (ABA) is well documented: elevation of plant ABA levels and activation of ABA-responsive signaling result in regulation of stomatal aperture and expression of stress-responsive genes. In response to pathogens, the role of ABA is more obscure and is a research topic that has long been overlooked. This article aims to evaluate and review the reported modes of ABA action on pathogen defense and highlight recent advances in deciphering the complex role of ABA in plant-pathogen interactions. The proposed mechanisms responsible for positive or negative effects of ABA on pathogen defense are discussed, as well as the regulation of ABA signaling and in planta ABA concentrations by beneficial and pathogenic microorganisms. In addition, the fast-growing number of reports that characterize antagonistic and synergistic interactions between abiotic and biotic stress responses point to ABA as an essential component in integrating and fine-tuning abiotic and biotic stress-response signaling networks.
High-rate electron transfer toward an anode in microbial fuel cells (MFCs) has thus far not been described for bacteria-producing soluble redox mediators. To study the mechanism of electron transfer, ...we used a MFC isolate, Pseudomonas aeruginosa strain KRP1. Bacterial electron transfer toward the MFC anode was enabled through pyocyanin and phenazine-1-carboxamide. The presence of the anode stimulated pyocyanin production. Mutant strains, deficient in the synthesis of pyocyanin and phenazine-1-carboxamide, were unable to achieve substantial electron transfer and reached only 5% of the wild type's power output. Upon pyocyanin addition, the power output was restored to 50%. Pyocyanin was not only used by P. aeruginosa to improve electron transfer but as well enhanced electron transfer by other bacterial species. The finding that one bacterium can produce electron shuttles, which can be used also by other bacteria, to enhance electron-transfer rate and growth, has not been shown before. These findings have considerable implications with respect to the power output attainable in MFCs.
Plant glutamate metabolism (GM) plays a pivotal role in amino acid metabolism and orchestrates crucial metabolic functions, with key roles in plant defense against pathogens. These functions concern ...three major areas: nitrogen transportation via the glutamine synthetase and glutamine-oxoglutarate aminotransferase cycle, cellular redox regulation, and tricarboxylic acid cycle-dependent energy reprogramming. During interactions with pathogens, the host GM is markedly altered, leading to either a metabolic state, termed "endurance", in which cell viability is maintained, or to an opposite metabolic state, termed "evasion", in which the process of cell death is facilitated. It seems that endurance-natured modulations result in resistance to necrotrophic pathogens and susceptibility to biotrophs, whereas evasion-related reconfigurations lead to resistance to biotrophic pathogens but stimulate the infection by necrotrophs. Pathogens, however, have evolved strategies such as toxin secretion, hemibiotrophy, and selective amino acid utilization to exploit the plant GM to their own benefit. Collectively, alterations in the host GM in response to different pathogenic scenarios appear to function in two opposing ways, either backing the ongoing defense strategy to ultimately shape an efficient resistance response or being exploited by the pathogen to promote and facilitate infection.
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