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Crucial Roles of Abscisic Acid Biogenesis in Virulence of Rice Blast Fungus Magnaporthe oryzae.

Spence CA, Lakshmanan V, Donofrio N, Bais HP - Front Plant Sci (2015)

Bottom Line: EA105 may be reducing the virulence of M. oryzae by preventing the pathogen from up-regulating the key ABA biosynthetic gene NCED3 in rice roots, as well as a β-glucosidase likely involved in activating conjugated inactive forms of ABA.EA105, which inhibits appressoria formation, counteracted the virulence-promoting effects of ABA on M. oryzae.ABA is a molecule that is likely implicated in both tactics.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Delaware Newark, DE, USA ; Delaware Biotechnology Institute Newark, DE, USA ; Department of Plant and Soil Sciences, University of Delaware Newark, DE, USA.

ABSTRACT
Rice suffers dramatic yield losses due to blast pathogen Magnaporthe oryzae. Pseudomonas chlororaphis EA105, a bacterium that was isolated from the rice rhizosphere, inhibits M. oryzae. It was shown previously that pre-treatment of rice with EA105 reduced the size of blast lesions through jasmonic acid (JA)- and ethylene (ETH)-mediated ISR. Abscisic acid (ABA) acts antagonistically toward salicylic acid (SA), JA, and ETH signaling, to impede plant defense responses. EA105 may be reducing the virulence of M. oryzae by preventing the pathogen from up-regulating the key ABA biosynthetic gene NCED3 in rice roots, as well as a β-glucosidase likely involved in activating conjugated inactive forms of ABA. However, changes in total ABA concentrations were not apparent, provoking the question of whether ABA concentration is an indicator of ABA signaling and response. In the rice-M. oryzae interaction, ABA plays a dual role in disease severity by increasing plant susceptibility and accelerating pathogenesis in the fungus itself. ABA is biosynthesized by M. oryzae. Further, exogenous ABA increased spore germination and appressoria formation, distinct from other plant growth regulators. EA105, which inhibits appressoria formation, counteracted the virulence-promoting effects of ABA on M. oryzae. The role of endogenous fungal ABA in blast disease was confirmed through the inability of a knockout mutant impaired in ABA biosynthesis to form lesions on rice. Therefore, it appears that EA105 is invoking multiple strategies in its protection of rice from blast including direct mechanisms as well as those mediated through plant signaling. ABA is a molecule that is likely implicated in both tactics.

No MeSH data available.


Related in: MedlinePlus

Expression of ABA biosynthesis gene, NCED3. Roots were inoculated with either (A) EA105, (B) EA106, or (C) EA201, 24 h prior to being exposed to spores. Error bars indicate standard error based on three biological replicates, each including five plants. Different letters represent a statistically significant difference based on the Tukey-Kramer test, p < 0.05.
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Figure 1: Expression of ABA biosynthesis gene, NCED3. Roots were inoculated with either (A) EA105, (B) EA106, or (C) EA201, 24 h prior to being exposed to spores. Error bars indicate standard error based on three biological replicates, each including five plants. Different letters represent a statistically significant difference based on the Tukey-Kramer test, p < 0.05.

Mentions: Previously, we showed that when Pseudomonas chlororaphis EA105 (hereafter EA105) was inoculated onto uninfected rice plants, there was approximately a 10-fold increase in the ETH responsive genes EIL1 and ERF1 as well an approximately threefold increase in the JA responsive genes JAR1 and WRKY30 at 24 h post treatment (Spence et al., 2014b). However, the SA responsive genes PR1 and WRKY77 were minimally affected (Spence et al., 2014b). From that data, it was concluded that isolate EA105 induces systemic resistance in rice against blast in an ETH and JA dependent manner (Spence et al., 2014b). Since elevated ABA levels are associated with increased susceptibility, we examined the expression of NCED3, the rate-limiting enzyme involved in ABA biosynthesis, in roots where NCED3 is most active. 70-15 spores up-regulated NCED3 while EA105 did not affect its expression. Interestingly, 70-15 spores were unable to induce NCED3 expression in plants that were pre-treated with EA105 (Figure 1A). For comparison, two other isolates recovered from the same soil sample as EA105 were also tested (Spence et al., 2014a). Isolate EA201 inhibits fungal diameter in vitro but root treatment does not reduce lesions. Contrastingly, EA106 has no direct antifungal capabilities, but does induce resistance when treated at the root surface. EA105 is distinct from both, because it can directly inhibit fungi and also induce resistance against M. oryzae (Spence et al., 2014a). Pretreatment with rice isolates EA106 and EA201 did not prevent 70-15 from up-regulating NCED3 (Figures 1B,C). To see if NCED3 up-regulation coincided with higher ABA levels, total ABA concentrations in roots and shoots treated with bacteria, fungus, or both were examined. However, there were no significant differences in the ABA content (Figure 2A). In all treatments, there were approximately 2000–2500 picomoles of ABA per gram of plant tissue. ABA content was checked at the same time-point used for expression analysis as well as 24 h later, and still no differences were apparent (Figure 2B). At the second time-point, there was actually a slight increase in ABA levels in plants that were treated with both EA105 and spores (Figure 2B). ABA concentrations were also determined in 70-15 spores and mycelia. We found that ABA is produced by 70-15, with mycelia producing around 200 picomoles per gram and spores producing more than 400 picomoles per gram (Figure 2C).


Crucial Roles of Abscisic Acid Biogenesis in Virulence of Rice Blast Fungus Magnaporthe oryzae.

Spence CA, Lakshmanan V, Donofrio N, Bais HP - Front Plant Sci (2015)

Expression of ABA biosynthesis gene, NCED3. Roots were inoculated with either (A) EA105, (B) EA106, or (C) EA201, 24 h prior to being exposed to spores. Error bars indicate standard error based on three biological replicates, each including five plants. Different letters represent a statistically significant difference based on the Tukey-Kramer test, p < 0.05.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4664623&req=5

Figure 1: Expression of ABA biosynthesis gene, NCED3. Roots were inoculated with either (A) EA105, (B) EA106, or (C) EA201, 24 h prior to being exposed to spores. Error bars indicate standard error based on three biological replicates, each including five plants. Different letters represent a statistically significant difference based on the Tukey-Kramer test, p < 0.05.
Mentions: Previously, we showed that when Pseudomonas chlororaphis EA105 (hereafter EA105) was inoculated onto uninfected rice plants, there was approximately a 10-fold increase in the ETH responsive genes EIL1 and ERF1 as well an approximately threefold increase in the JA responsive genes JAR1 and WRKY30 at 24 h post treatment (Spence et al., 2014b). However, the SA responsive genes PR1 and WRKY77 were minimally affected (Spence et al., 2014b). From that data, it was concluded that isolate EA105 induces systemic resistance in rice against blast in an ETH and JA dependent manner (Spence et al., 2014b). Since elevated ABA levels are associated with increased susceptibility, we examined the expression of NCED3, the rate-limiting enzyme involved in ABA biosynthesis, in roots where NCED3 is most active. 70-15 spores up-regulated NCED3 while EA105 did not affect its expression. Interestingly, 70-15 spores were unable to induce NCED3 expression in plants that were pre-treated with EA105 (Figure 1A). For comparison, two other isolates recovered from the same soil sample as EA105 were also tested (Spence et al., 2014a). Isolate EA201 inhibits fungal diameter in vitro but root treatment does not reduce lesions. Contrastingly, EA106 has no direct antifungal capabilities, but does induce resistance when treated at the root surface. EA105 is distinct from both, because it can directly inhibit fungi and also induce resistance against M. oryzae (Spence et al., 2014a). Pretreatment with rice isolates EA106 and EA201 did not prevent 70-15 from up-regulating NCED3 (Figures 1B,C). To see if NCED3 up-regulation coincided with higher ABA levels, total ABA concentrations in roots and shoots treated with bacteria, fungus, or both were examined. However, there were no significant differences in the ABA content (Figure 2A). In all treatments, there were approximately 2000–2500 picomoles of ABA per gram of plant tissue. ABA content was checked at the same time-point used for expression analysis as well as 24 h later, and still no differences were apparent (Figure 2B). At the second time-point, there was actually a slight increase in ABA levels in plants that were treated with both EA105 and spores (Figure 2B). ABA concentrations were also determined in 70-15 spores and mycelia. We found that ABA is produced by 70-15, with mycelia producing around 200 picomoles per gram and spores producing more than 400 picomoles per gram (Figure 2C).

Bottom Line: EA105 may be reducing the virulence of M. oryzae by preventing the pathogen from up-regulating the key ABA biosynthetic gene NCED3 in rice roots, as well as a β-glucosidase likely involved in activating conjugated inactive forms of ABA.EA105, which inhibits appressoria formation, counteracted the virulence-promoting effects of ABA on M. oryzae.ABA is a molecule that is likely implicated in both tactics.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, University of Delaware Newark, DE, USA ; Delaware Biotechnology Institute Newark, DE, USA ; Department of Plant and Soil Sciences, University of Delaware Newark, DE, USA.

ABSTRACT
Rice suffers dramatic yield losses due to blast pathogen Magnaporthe oryzae. Pseudomonas chlororaphis EA105, a bacterium that was isolated from the rice rhizosphere, inhibits M. oryzae. It was shown previously that pre-treatment of rice with EA105 reduced the size of blast lesions through jasmonic acid (JA)- and ethylene (ETH)-mediated ISR. Abscisic acid (ABA) acts antagonistically toward salicylic acid (SA), JA, and ETH signaling, to impede plant defense responses. EA105 may be reducing the virulence of M. oryzae by preventing the pathogen from up-regulating the key ABA biosynthetic gene NCED3 in rice roots, as well as a β-glucosidase likely involved in activating conjugated inactive forms of ABA. However, changes in total ABA concentrations were not apparent, provoking the question of whether ABA concentration is an indicator of ABA signaling and response. In the rice-M. oryzae interaction, ABA plays a dual role in disease severity by increasing plant susceptibility and accelerating pathogenesis in the fungus itself. ABA is biosynthesized by M. oryzae. Further, exogenous ABA increased spore germination and appressoria formation, distinct from other plant growth regulators. EA105, which inhibits appressoria formation, counteracted the virulence-promoting effects of ABA on M. oryzae. The role of endogenous fungal ABA in blast disease was confirmed through the inability of a knockout mutant impaired in ABA biosynthesis to form lesions on rice. Therefore, it appears that EA105 is invoking multiple strategies in its protection of rice from blast including direct mechanisms as well as those mediated through plant signaling. ABA is a molecule that is likely implicated in both tactics.

No MeSH data available.


Related in: MedlinePlus