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ScienceDirect Role of plant growth regulators as chemical signals in plant–microbe interactions: a double edged sword Carla Spence1,2 and Harsh Bais1,2 Growth regulators act not only as chemicals that modulate plant growth but they also act as signal molecules under various biotic and abiotic stresses. Of all growth regulators, abscisic acid (ABA) is long known for its role in modulating plants response against both biotic and abiotic stress. Although the genetic information for ABA biosynthesis in plants is well documented, the knowledge about ABA biosynthesis in other organisms is still in its infancy. It is known that various microbes including bacteria produce and secrete ABA, but the overall functional significance of why ABA is synthesized by microbes is not known. Here we discuss the functional involvement of ABA biosynthesis by a pathogenic fungus. Furthermore, we propose that ABA biosynthesis in plant pathogenic fungi could be targeted for novel fungicidal discovery. Addresses 1 Delaware Biotechnology Institute, Newark, DE 19711, United States 2 Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716, United States Corresponding author: Bais, Harsh ([email protected])
Current Opinion in Plant Biology 2015, 27:52–58 This review comes from a themed issue on Cell signalling and gene regulation Edited by Xiaofeng Cao and Blake C Meyers
http://dx.doi.org/10.1016/j.pbi.2015.05.028 1369-5266/# 2015 Elsevier Ltd. All rights reserved.
Introduction Plant growth regulators mediate important chemical dialogues between plants and microbes. The outcome of this important communication shapes the nature of the interaction, which has the potential to play a crucial role in the survival and fitness of the plant. Plant association with microbes has a drastic impact on plant growth as well as disease resistance. However, the type of interaction depends on the identity of both plant and microbe as well as the surrounding conditions. Taking those factors into account, signaling growth regulators such as salicylic acid (SA), jasmonic acid (JA), ethylene (ETH), abscisic acid (ABA), as well as many others, play a large role in the communications that underlie interactions between plants and microbes. Current Opinion in Plant Biology 2015, 27:52–58
Chemical signals in the rhizosphere and phyllosphere As stationary organisms, plants rely on chemical signaling to perceive and modulate their environment. Plants send and receive signals above and belowground, and they maintain microbial associations on aerial and root structures. Above ground, the area surrounding the plant is called the phyllosphere. This is a place where plants can send and receive chemical messages with microbes as well as neighboring plants. However, microbial density and diversity is low in the phyllosphere due to frequent environmental fluctuations, high levels of ultraviolet radiation, a scarcity of water at times, and minimal nutrient availability [1]. Contrastingly, the rhizospheric environment, which directly surrounds plant roots, is rich in nutrients, protected from harsh environmental fluctuations, and generally a prime location for microbial growth. Therefore, the rhizosphere also fosters competition between microbes for a spot in this prime location [2]. More interestingly, plants modulate their root secretions to manipulate rhizospheric communications and interactions (Figure 1). For example, Arabidopsis thaliana secretes malate from the roots, which in turn attracts its beneficial partner Bacillus subtilis [3]. The fascinating aspect of the microbial recruitment is that plants are able to selectively entice specific microbes. Related to this, there is consistency in the microbial composition of the rhizosphere across growing seasons [4] and there are associations between a specific plant and microbe pair which are found frequently, even in geographic locations that are completely separated [5]. Many studies have examined specific two-way communications between a specific plant and microbe. This is a great starting point to understand the nature of the relationship between the two, but communications in the natural environment are much more complex, as a multitude of organisms are all communicating at once [6]. Of particular interest is how communications between plants and microbes change in the presence of a pathogen. It is well established that plants use chemical signals to communicate, but to fully understand this communication it is necessary to look at the specific chemical signals and their impact.
Hormones as mediators of plant–microbe interactions Plant hormones were first discovered for their roles in plant development and normal functioning within the plant, but they are also crucial to communications outside www.sciencedirect.com
Chemical signaling in plant–microbe interactions Spence and Bais 53
Figure 1
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Rhizospheric microbiome and plant pathogenic fungi interactions. (a) Root colonization by a rice isolate on rice and Arabidopsis roots. Green fluorescence in both the panels represents bacterial colonization. (b) M. oryzae mutants impaired in biosynthesizing and sensing ABA (DABA4 and DGPCR). (c) Appresoria formation in wild type M. oryzae and DABA4 mutant. (d) Supplementation of ABA exogenously revives appresoria formation in DABA mutant.
the plant which can affect growth and disease resistance. Classically, SA, JA, and ETH are known for their roles in plant defense by stimulating systemic acquired resistance (SAR) and induced systemic resistance (ISR). SAR is a response to pathogens while ISR is triggered by beneficial bacteria and stimulates a less harmful defense response to prepare the plant for potentially impending pathogen attack. There have been many studies of tritrophic interactions where a beneficial soil microbe stimulates and ISR www.sciencedirect.com
response in plants, making them more resistant to a foliar pathogen [7]. More recently, plant hormones which were originally studied for their roles in other plant processes are being discovered to play a role in plant defense. Moreover, some of these signals may even affect other organisms, including associated bacteria and fungi. ABA has a multifaceted role in plant survival and fitness. It was named for its role in abscission, a process during Current Opinion in Plant Biology 2015, 27:52–58
54 Cell signalling and gene regulation
which plants shed leaves, flowers, fruit, or seeds [8]. It also plays a role in seed development by promoting biosynthesis of storage compounds and antagonizing gibberellic acid (GA) to maintain seed dormancy and prevent precocious germination [9,10]. In addition to being involved in normal developmental processes, ABA also modulates stress responses in plants. ABA has been perhaps most well studied in terms of its function in plant drought tolerance, though ABA also plays a role in response to extreme temperatures and high salinity, promoting accumulation of compatible solutes and synthesis of dehydrins to prevent water loss and plasmolysis [11]. ABA slows photosynthesis which leads to stunted growth during long-term stress, but can effectively protect against short term abiotic stresses. Under cold stress, ABA is biosynthesized directly in the shoots [12,13] while heat, drought, and high salinity typically induce root ABA biosynthesis followed by translocation to the shoots [14]. ABA also plays a crucial role in response to biotic stress, interfering with a plant’s ability to mount an effective defense response (Figure 2). Various lines of work have shown that ABA modulation in plants directly interferes with SAR and ISR by antagonizing defense signaling
hormones SA, JA, and ETH, leading to increased susceptibility in plants against various bacterial and fungal pathogens [15]. Many specific examples have shown that elevated ABA content in plants is associated with increased susceptibility while decreased ABA content resulted in increased resistance [16–18,19]. By contrast, it is also shown that ABA may promote resistance in some plant–pathogen interactions [20]. For example, ABA triggers stomatal closure, which reduces pathogen entry through the stomata [15]. ABA may also promote deposition of callose at the cell wall for fortification, though there are conflicting reports on this subject [15,21,22]. Interestingly, the ABA response in plants may also be modulated by the infection stage of the invading pathogen, which was demonstrated in the case of sugar beet infection by Cercospora beticola [23]. Typically, the modulation of ABA in plants is important during the initial stages of infection [18] but in the case of the hemibiotroph C. beticola, ABA levels also show a dramatic increase fifteen days post infection, at the onset of necrosis [23]. The crosstalk between ABA signaling and other hormone defense signaling is complex, and the outcome depends on the plant as well as the lifestyle and infection strategy of the pathogen [24].
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A Simplified model depicting the functional role of fungal ABA in virulence in rice blast pathogen (M. oryzae). The model explains the complexity in plant and microbial signaling under a tritrophic regime. EA105 is a rice isolate that induces ISR in rice plants. M. oryzae induces SAR in plants but also suppresses SAR through ABA biogenesis. Under tritrophic interactions rice isolate (EA105) suppresses ABA biosynthesis in fungus and induces resistance in plants. Red and green lines in the figure depict susceptibility and resistance against a fungal pathogen. Current Opinion in Plant Biology 2015, 27:52–58
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Chemical signaling in plant–microbe interactions Spence and Bais 55
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56 Cell signalling and gene regulation
The regulation of ABA biosynthesis in plants is still unclear, but the pathway by which the molecule is synthesized has been fully elucidated. ABA biosynthesis in plants occurs through a carotenoid catabolism pathway which was first identified in maize [25,26]. The rate limiting and key regulatory step in plant ABA biosynthesis is a cleavage by a 9-cis-epoxycarotenoid dioxygenase (NCED) [27] though the expression patterns of this ABA biosynthesis gene often do not correlate with ABA concentrations [28]. ABA is a crucial hormone with many distinct functions in plants, and therefore ABA signaling is tightly regulated at multiple steps, and the signaling cascade can differ based on the circumstance which triggered the signaling (drought stress, pathogen stress, developmental cues, etc.) [14,29,30,31].
Why do pathogenic fungi biosynthesize ABA? ABA is an ancient signaling molecule that was probably present in early unicellular ancestors of eukaryotes [32] and the signaling pathways have since diverged. This supports its presence across a wide diversity of organisms and also could explain its differing functionality in different organisms. While ABA biosynthesis differs between plants and fungi, it is likely that perception of ABA is similar. One GPCR which recognizes ABA is highly conserved across kingdoms (Figure 3). ABA production has been shown in several phytopathogens including Botrytis cinerea [33–35] and multiple Cercospora species [36–40] as well as rice blast pathogen Magnaporthe oryzae [17,41]. There is variation in the intermediate compounds involved in fungal ABA biosynthesis, however, the biosynthetic pathway is entirely different than what is observed in plants. In B. cinerea, ABA biosynthesis is catalyzed by four enzymes encoded by bcABA1-4 [33]. The pathway is a more direct pathway than the plant process using carotenoid precursors. Fungi are able to directly use mevalonic acid as a precursor, subsequently forming a farnesyl diphosphate intermediate before ultimately forming ABA [33,42]. Catabolism of ABA also differs. In plants, ABA is hydroxylated at carbon 8 and then converted into phaseic acid and subsequently to dihydrophaseic acid [43]. Fungal ABA catabolism has not been elucidated, but it is not converted to phaseic or dihydrophaseic acid [44]. It has been speculated that endogenous fungal ABA content may be controlled through regulating secretion of ABA [44]. In Cercospora cruenta, it was shown that fungal cells were highly permeable to uptake and release of ABA anions, with ABA secretion being about 10 times higher in the fungus in comparison to barley roots [44]. Aside from one study which showed weak promotion of mycelial growth of Ceratocystis fimbriata with the addition of exogenous
ABA [45], there has not been any evidence for the role of ABA in fungal processes such as development, growth, or pathogenesis. Recently, it was shown that exogenous ABA could accelerate spore germination and appressoria formation in rice blast pathogen Magnaporthe orzyae [41]. Additionally, knocking out one of the ABA biosynthesis genes orthologous to bcABA4 reduced ABA content by half in M. oryzae and rendered the pathogen unable to infect its host. This clearly shows a crucial role for ABA in fungal pathogenicity. It is also important to note that the levels of ABA in rice were not altered, and are typically 10 fold higher than fungal ABA levels. Still, the M. oryzae ABA4 mutant was not able to infect, underlining a role for endogenous fungal ABA (Figure 1) [41].
Could ABA biosynthesis be a target for novel fungicide discovery? ABA is a crucial plant hormone, required for normal growth and metabolism. Although ABA may be responsible for virulence in fungal pathogens, this molecule itself is not a feasible target for fungicides due to its crucial roles in plants. We have shown that fungal ABA biosynthesis is required for infection in the case of M. oryzae, and may have similar roles in other phytopathogenic fungi. Fortunately, ABA biosynthesis in fungi proceeds through a pathway entirely different than the plant ABA biosynthetic pathway, making the enzymes and intermediates in fungal ABA biosynthesis perfect targets for novel fungicides. In M. oryzae, potential mutants in genes orthologous to bcABA1 and 2 were extremely small and slowgrowing, suggesting that ABA may have multiple roles in the fungus, paralleling what is seen in plants. Knocking out a M. oryzae ortholog to bcABA4 resulted in growth defects, but more importantly a loss of virulence. Therefore, targeting any of these genes is likely to have a considerable impact on growth, development, and virulence in the case of M. oryzae and possibly in other phytopathogens as well.
Conclusions and future prospects The complexity and vastness of chemical signals in the rhizosphere make it a daunting task to study the multitrophic communications that are occurring. However, it illustrates a beautifully intricate system which has evolved between plants and their microbial neighbors, with each trying to manipulate the situation to maximize their own fitness. As with the case of ABA, it is not only the signal that matters, but also the timing and regulation of signaling which impacts the outcome. This chemical messenger, which plays multiple roles within rice, also is intrinsically important to fungal pathogen M. oryzae. The difficulty in studying the vast and complicated multitrophic
( Figure 3 Legend ) Phylogenetic trees depicting two key ABA genes. Trees were constructed using amino acid sequences of a fungal ABA biosynthesis gene, ABA4 (a), and ABA GPCR (b), a receptor for ABA. While biosynthesis of ABA differs across plants and fungi, the GPCR is highly conserved. Bootstrap values are shown at branch-points. Current Opinion in Plant Biology 2015, 27:52–58
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Chemical signaling in plant–microbe interactions Spence and Bais 57
communications is outweighed by the potential of being able to manipulate these signals and interactions to maximize plant fitness, therefore enhancing food security.
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ScienceDirect Role of plant growth regulators as chemical signals in plant–microbe interactions: a double edged sword Carla Spence1,2 and Harsh Bais1,2 Growth regulators act not only as chemicals that modulate plant growth but they also act as signal molecules under various biotic and abiotic stresses. Of all growth regulators, abscisic acid (ABA) is long known for its role in modulating plants response against both biotic and abiotic stress. Although the genetic information for ABA biosynthesis in plants is well documented, the knowledge about ABA biosynthesis in other organisms is still in its infancy. It is known that various microbes including bacteria produce and secrete ABA, but the overall functional significance of why ABA is synthesized by microbes is not known. Here we discuss the functional involvement of ABA biosynthesis by a pathogenic fungus. Furthermore, we propose that ABA biosynthesis in plant pathogenic fungi could be targeted for novel fungicidal discovery. Addresses 1 Delaware Biotechnology Institute, Newark, DE 19711, United States 2 Department of Plant and Soil Sciences, University of Delaware, Newark, DE 19716, United States Corresponding author: Bais, Harsh ([email protected])
Current Opinion in Plant Biology 2015, 27:52–58 This review comes from a themed issue on Cell signalling and gene regulation Edited by Xiaofeng Cao and Blake C Meyers
http://dx.doi.org/10.1016/j.pbi.2015.05.028 1369-5266/# 2015 Elsevier Ltd. All rights reserved.
Introduction Plant growth regulators mediate important chemical dialogues between plants and microbes. The outcome of this important communication shapes the nature of the interaction, which has the potential to play a crucial role in the survival and fitness of the plant. Plant association with microbes has a drastic impact on plant growth as well as disease resistance. However, the type of interaction depends on the identity of both plant and microbe as well as the surrounding conditions. Taking those factors into account, signaling growth regulators such as salicylic acid (SA), jasmonic acid (JA), ethylene (ETH), abscisic acid (ABA), as well as many others, play a large role in the communications that underlie interactions between plants and microbes. Current Opinion in Plant Biology 2015, 27:52–58
Chemical signals in the rhizosphere and phyllosphere As stationary organisms, plants rely on chemical signaling to perceive and modulate their environment. Plants send and receive signals above and belowground, and they maintain microbial associations on aerial and root structures. Above ground, the area surrounding the plant is called the phyllosphere. This is a place where plants can send and receive chemical messages with microbes as well as neighboring plants. However, microbial density and diversity is low in the phyllosphere due to frequent environmental fluctuations, high levels of ultraviolet radiation, a scarcity of water at times, and minimal nutrient availability [1]. Contrastingly, the rhizospheric environment, which directly surrounds plant roots, is rich in nutrients, protected from harsh environmental fluctuations, and generally a prime location for microbial growth. Therefore, the rhizosphere also fosters competition between microbes for a spot in this prime location [2]. More interestingly, plants modulate their root secretions to manipulate rhizospheric communications and interactions (Figure 1). For example, Arabidopsis thaliana secretes malate from the roots, which in turn attracts its beneficial partner Bacillus subtilis [3]. The fascinating aspect of the microbial recruitment is that plants are able to selectively entice specific microbes. Related to this, there is consistency in the microbial composition of the rhizosphere across growing seasons [4] and there are associations between a specific plant and microbe pair which are found frequently, even in geographic locations that are completely separated [5]. Many studies have examined specific two-way communications between a specific plant and microbe. This is a great starting point to understand the nature of the relationship between the two, but communications in the natural environment are much more complex, as a multitude of organisms are all communicating at once [6]. Of particular interest is how communications between plants and microbes change in the presence of a pathogen. It is well established that plants use chemical signals to communicate, but to fully understand this communication it is necessary to look at the specific chemical signals and their impact.
Hormones as mediators of plant–microbe interactions Plant hormones were first discovered for their roles in plant development and normal functioning within the plant, but they are also crucial to communications outside www.sciencedirect.com
Chemical signaling in plant–microbe interactions Spence and Bais 53
Figure 1
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Rhizospheric microbiome and plant pathogenic fungi interactions. (a) Root colonization by a rice isolate on rice and Arabidopsis roots. Green fluorescence in both the panels represents bacterial colonization. (b) M. oryzae mutants impaired in biosynthesizing and sensing ABA (DABA4 and DGPCR). (c) Appresoria formation in wild type M. oryzae and DABA4 mutant. (d) Supplementation of ABA exogenously revives appresoria formation in DABA mutant.
the plant which can affect growth and disease resistance. Classically, SA, JA, and ETH are known for their roles in plant defense by stimulating systemic acquired resistance (SAR) and induced systemic resistance (ISR). SAR is a response to pathogens while ISR is triggered by beneficial bacteria and stimulates a less harmful defense response to prepare the plant for potentially impending pathogen attack. There have been many studies of tritrophic interactions where a beneficial soil microbe stimulates and ISR www.sciencedirect.com
response in plants, making them more resistant to a foliar pathogen [7]. More recently, plant hormones which were originally studied for their roles in other plant processes are being discovered to play a role in plant defense. Moreover, some of these signals may even affect other organisms, including associated bacteria and fungi. ABA has a multifaceted role in plant survival and fitness. It was named for its role in abscission, a process during Current Opinion in Plant Biology 2015, 27:52–58
54 Cell signalling and gene regulation
which plants shed leaves, flowers, fruit, or seeds [8]. It also plays a role in seed development by promoting biosynthesis of storage compounds and antagonizing gibberellic acid (GA) to maintain seed dormancy and prevent precocious germination [9,10]. In addition to being involved in normal developmental processes, ABA also modulates stress responses in plants. ABA has been perhaps most well studied in terms of its function in plant drought tolerance, though ABA also plays a role in response to extreme temperatures and high salinity, promoting accumulation of compatible solutes and synthesis of dehydrins to prevent water loss and plasmolysis [11]. ABA slows photosynthesis which leads to stunted growth during long-term stress, but can effectively protect against short term abiotic stresses. Under cold stress, ABA is biosynthesized directly in the shoots [12,13] while heat, drought, and high salinity typically induce root ABA biosynthesis followed by translocation to the shoots [14]. ABA also plays a crucial role in response to biotic stress, interfering with a plant’s ability to mount an effective defense response (Figure 2). Various lines of work have shown that ABA modulation in plants directly interferes with SAR and ISR by antagonizing defense signaling
hormones SA, JA, and ETH, leading to increased susceptibility in plants against various bacterial and fungal pathogens [15]. Many specific examples have shown that elevated ABA content in plants is associated with increased susceptibility while decreased ABA content resulted in increased resistance [16–18,19]. By contrast, it is also shown that ABA may promote resistance in some plant–pathogen interactions [20]. For example, ABA triggers stomatal closure, which reduces pathogen entry through the stomata [15]. ABA may also promote deposition of callose at the cell wall for fortification, though there are conflicting reports on this subject [15,21,22]. Interestingly, the ABA response in plants may also be modulated by the infection stage of the invading pathogen, which was demonstrated in the case of sugar beet infection by Cercospora beticola [23]. Typically, the modulation of ABA in plants is important during the initial stages of infection [18] but in the case of the hemibiotroph C. beticola, ABA levels also show a dramatic increase fifteen days post infection, at the onset of necrosis [23]. The crosstalk between ABA signaling and other hormone defense signaling is complex, and the outcome depends on the plant as well as the lifestyle and infection strategy of the pathogen [24].
Figure 2
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A Simplified model depicting the functional role of fungal ABA in virulence in rice blast pathogen (M. oryzae). The model explains the complexity in plant and microbial signaling under a tritrophic regime. EA105 is a rice isolate that induces ISR in rice plants. M. oryzae induces SAR in plants but also suppresses SAR through ABA biogenesis. Under tritrophic interactions rice isolate (EA105) suppresses ABA biosynthesis in fungus and induces resistance in plants. Red and green lines in the figure depict susceptibility and resistance against a fungal pathogen. Current Opinion in Plant Biology 2015, 27:52–58
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Chemical signaling in plant–microbe interactions Spence and Bais 55
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56 Cell signalling and gene regulation
The regulation of ABA biosynthesis in plants is still unclear, but the pathway by which the molecule is synthesized has been fully elucidated. ABA biosynthesis in plants occurs through a carotenoid catabolism pathway which was first identified in maize [25,26]. The rate limiting and key regulatory step in plant ABA biosynthesis is a cleavage by a 9-cis-epoxycarotenoid dioxygenase (NCED) [27] though the expression patterns of this ABA biosynthesis gene often do not correlate with ABA concentrations [28]. ABA is a crucial hormone with many distinct functions in plants, and therefore ABA signaling is tightly regulated at multiple steps, and the signaling cascade can differ based on the circumstance which triggered the signaling (drought stress, pathogen stress, developmental cues, etc.) [14,29,30,31].
Why do pathogenic fungi biosynthesize ABA? ABA is an ancient signaling molecule that was probably present in early unicellular ancestors of eukaryotes [32] and the signaling pathways have since diverged. This supports its presence across a wide diversity of organisms and also could explain its differing functionality in different organisms. While ABA biosynthesis differs between plants and fungi, it is likely that perception of ABA is similar. One GPCR which recognizes ABA is highly conserved across kingdoms (Figure 3). ABA production has been shown in several phytopathogens including Botrytis cinerea [33–35] and multiple Cercospora species [36–40] as well as rice blast pathogen Magnaporthe oryzae [17,41]. There is variation in the intermediate compounds involved in fungal ABA biosynthesis, however, the biosynthetic pathway is entirely different than what is observed in plants. In B. cinerea, ABA biosynthesis is catalyzed by four enzymes encoded by bcABA1-4 [33]. The pathway is a more direct pathway than the plant process using carotenoid precursors. Fungi are able to directly use mevalonic acid as a precursor, subsequently forming a farnesyl diphosphate intermediate before ultimately forming ABA [33,42]. Catabolism of ABA also differs. In plants, ABA is hydroxylated at carbon 8 and then converted into phaseic acid and subsequently to dihydrophaseic acid [43]. Fungal ABA catabolism has not been elucidated, but it is not converted to phaseic or dihydrophaseic acid [44]. It has been speculated that endogenous fungal ABA content may be controlled through regulating secretion of ABA [44]. In Cercospora cruenta, it was shown that fungal cells were highly permeable to uptake and release of ABA anions, with ABA secretion being about 10 times higher in the fungus in comparison to barley roots [44]. Aside from one study which showed weak promotion of mycelial growth of Ceratocystis fimbriata with the addition of exogenous
ABA [45], there has not been any evidence for the role of ABA in fungal processes such as development, growth, or pathogenesis. Recently, it was shown that exogenous ABA could accelerate spore germination and appressoria formation in rice blast pathogen Magnaporthe orzyae [41]. Additionally, knocking out one of the ABA biosynthesis genes orthologous to bcABA4 reduced ABA content by half in M. oryzae and rendered the pathogen unable to infect its host. This clearly shows a crucial role for ABA in fungal pathogenicity. It is also important to note that the levels of ABA in rice were not altered, and are typically 10 fold higher than fungal ABA levels. Still, the M. oryzae ABA4 mutant was not able to infect, underlining a role for endogenous fungal ABA (Figure 1) [41].
Could ABA biosynthesis be a target for novel fungicide discovery? ABA is a crucial plant hormone, required for normal growth and metabolism. Although ABA may be responsible for virulence in fungal pathogens, this molecule itself is not a feasible target for fungicides due to its crucial roles in plants. We have shown that fungal ABA biosynthesis is required for infection in the case of M. oryzae, and may have similar roles in other phytopathogenic fungi. Fortunately, ABA biosynthesis in fungi proceeds through a pathway entirely different than the plant ABA biosynthetic pathway, making the enzymes and intermediates in fungal ABA biosynthesis perfect targets for novel fungicides. In M. oryzae, potential mutants in genes orthologous to bcABA1 and 2 were extremely small and slowgrowing, suggesting that ABA may have multiple roles in the fungus, paralleling what is seen in plants. Knocking out a M. oryzae ortholog to bcABA4 resulted in growth defects, but more importantly a loss of virulence. Therefore, targeting any of these genes is likely to have a considerable impact on growth, development, and virulence in the case of M. oryzae and possibly in other phytopathogens as well.
Conclusions and future prospects The complexity and vastness of chemical signals in the rhizosphere make it a daunting task to study the multitrophic communications that are occurring. However, it illustrates a beautifully intricate system which has evolved between plants and their microbial neighbors, with each trying to manipulate the situation to maximize their own fitness. As with the case of ABA, it is not only the signal that matters, but also the timing and regulation of signaling which impacts the outcome. This chemical messenger, which plays multiple roles within rice, also is intrinsically important to fungal pathogen M. oryzae. The difficulty in studying the vast and complicated multitrophic
( Figure 3 Legend ) Phylogenetic trees depicting two key ABA genes. Trees were constructed using amino acid sequences of a fungal ABA biosynthesis gene, ABA4 (a), and ABA GPCR (b), a receptor for ABA. While biosynthesis of ABA differs across plants and fungi, the GPCR is highly conserved. Bootstrap values are shown at branch-points. Current Opinion in Plant Biology 2015, 27:52–58
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Chemical signaling in plant–microbe interactions Spence and Bais 57
communications is outweighed by the potential of being able to manipulate these signals and interactions to maximize plant fitness, therefore enhancing food security.
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Current Opinion in Plant Biology 2015, 27:52–58
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