The interactive effects of simultaneous biotic and abiotic stresses on plants: Mechanistic understanding from drought and pathogen combination

Plants synthesize jasmonates (JAs) in response to developmental cues or environmental stresses, in order to coordinate plant growth, development or defense against pathogens and herbivores. Perception of pathogen or herbivore attack promotes synthesis of jasmonoyl-L-isoleucine (JA-Ile), which binds to the COI1-JAZ receptor, triggering the degradation of JAZ repressors and induction of transcriptional reprogramming associated with plant defense. Interestingly, some virulent pathogens have evolved various strategies to manipulate JA signaling to facilitate their exploitation of plant hosts. In this review, we focus on recent advances in understanding the mechanism underlying the enigmatic switch between transcriptional repression and hormone-dependent transcriptional activation of JA signaling. We also discuss various strategies used by pathogens and insects to manipulate JA signaling and how interfering with this could be used as a novel means of disease control.

Jasmonate signaling and manipulation by pathogens and insects.

The most conspicuous function of plastids is the oxygenic photosynthesis of chloroplasts, yet plastids are super-factories that produce a plethora of compounds that are indispensable for proper plant physiology and development. Given their origins as free-living prokaryotes, it is not surprising that plastids possess their own genomes whose expression is essential to plastid function. This semi-autonomous character of plastids requires the existence of sophisticated regulatory mechanisms that provide reliable communication between them and other cellular compartments. Such intracellular signaling is necessary for coordinating whole-cell responses to constantly varying environmental cues and cellular metabolic needs. This is achieved by plastids acting as receivers and transmitters of specific signals that coordinate expression of the nuclear and plastid genomes according to particular needs. In this review we will consider the so-called retrograde signaling occurring between plastids and nuclei, and between plastids and other organelles. Another important role of the plastid we will discuss is the involvement of plastid signaling in biotic and abiotic stress that, in addition to influencing retrograde signaling, has direct effects on several cellular compartments including the cell wall. We will also review recent evidence pointing to an intriguing function of chloroplasts in regulating intercellular symplasmic transport. Finally, we consider an intriguing yet less widely known aspect of plant biology, chloroplast signaling from the perspective of the entire plant. Thus, accumulating evidence highlights that chloroplasts, with their complex signaling pathways, provide a mechanism for exquisite regulation of plant development, metabolism and responses to the environment. As chloroplast processes are targeted for engineering for improved productivity the effect of such modifications on chloroplast signaling will have to be carefully considered in order to avoid unintended consequences on plant growth and development.

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Chloroplast signaling within, between and beyond cells.

Biotic factors affect plant immune responses and plant resistance to pathogen infections. Despite the considerable progress made over the past two decades in manipulating genes, proteins and their levels from diverse sources, no complete genetic tolerance to environmental stresses has been developed so far in any crops. Plant defense response to pathogens, including Botrytis cinerea, is a complex biological process involving various changes at the biochemical, molecular (i.e. transcriptional) and physiological levels. Once a pathogen is detected, effective plant resistance activates signaling networks through the generation of small signaling molecules and the balance of hormonal signaling pathways to initiate defense mechanisms to the particular pathogen. Recently, studies using Arabidopsis thaliana and crop plants have shown that many genes are involved in plant responses to B. cinerea infection. In this article, we will review our current understanding of mechanisms regulating plant responses to B. cinerea with a particular interest on hormonal regulatory networks involving phytohormones salicylic acid (SA), jasmonic acid (JA), ethylene (ET) and abscisic acid (ABA). We will also highlight some potential gene targets that are promising for improving crop resistance to B. cinerea through genetic engineering and breeding programs. Finally, the role of biological control as a complementary and alternative disease management will be overviewed.

Mechanisms and strategies of plant defense against Botrytis cinerea

STRESSing the role of the plant circadian clock

Jasmonic acid (JA) is a natural hormone regulator involved in development, responses against wounding and pathogen attack. Upon perception of pathogens, JA is synthesized and mediates a signaling cascade initiating various defense responses. Traditionally, necrotrophic fungi have been shown to be the primary activators of JAdependent defenses through the JA-receptor, COI1. Conversely, plants infected with biotrophic fungi have classically been associated with suppressing JA-mediated responses. However, recent evidence has shown that certain biotrophic fungal species also trigger activation of JA-mediated responses and mutants deficient in JA signaling show an increase in susceptibility to certain biotrophic fungal pathogens. These findings suggest a new role for JA in defense against fungal biotrophs. This review will focus on recent research advancing our knowledge of JA-dependant responses involved in defense against both biotrophic and necrotrophic fungi.

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Insights into the role of jasmonic acid-mediated defenses against necrotrophic and biotrophic fungal pathogens

Plants are continually exposed to a variety of pathogenic organisms, including bacteria, fungi and viruses. In response to these assaults, plants have developed various defense pathways to protect themselves from pathogen invasion. An understanding of the expression and regulation of genes involved in defense signaling is essential to controlling plant disease. ATL9, an Arabidopsis RING zinc finger protein, is an E3 ubiquitin ligase that is induced by chitin and involved in basal resistance to the biotrophic fungal pathogen, Golovinomyces cichoracearum (G. cichoracearum). To better understand the expression and regulation of ATL9, we studied its expression pattern and the functions of its different protein domains. Using pATL9:GUS transgenic Arabidopsis lines we found that ATL9 is expressed in numerous tissues at various developmental stages and that GUS activity was induced rapidly upon wounding. Using a GFP control protein, we showed that ATL9 is a short-lived protein within plant cells and it is degraded via the ubiquitin-proteasome pathway. ATL9 contains two transmembrane domains (TM), a RING zinc-finger domain, and a PEST domain. Using a series of deletion mutants, we found that the PEST domain and the RING domain have effects on ATL9 degradation. Further infection assays with G. cichoracearum showed that both the RING domain and the TM domains are important for ATL9's resistance phenotype. Interestingly, the PEST domain was also shown to be significant for resistance to fungal pathogens. This study demonstrates that the PEST domain is directly coupled to plant defense regulation and the importance of protein degradation in plant immunity.

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Christopher J. Antico

Papers

Insights into the role of jasmonic acid-mediated defenses against necrotrophic and biotrophic fungal pathogens

Expression and regulation of ATL9, an E3 ubiquitin ligase involved in plant defense.