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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

Magnaporthe oryzae 70-15 spores treated with EA105 and/or exogenous ABA. (A) Percent germination was quantified at 2 h post treatment and (B) percent appressoria formation was quantified at 6 h post treatment. The experiment was repeated three times with five coverslips per treatment and three images per coverslip. Different letters represent statistical significance based on the Tukey-Kramer test (p < 0.05).
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Figure 4: Magnaporthe oryzae 70-15 spores treated with EA105 and/or exogenous ABA. (A) Percent germination was quantified at 2 h post treatment and (B) percent appressoria formation was quantified at 6 h post treatment. The experiment was repeated three times with five coverslips per treatment and three images per coverslip. Different letters represent statistical significance based on the Tukey-Kramer test (p < 0.05).

Mentions: Pathogenesis of M. oryzae begins with spore germination. The crucial step in virulence is the formation of the appressorium, a specialized infection structure that accumulates high turgor pressure and forms a penetration peg that can enter the rice cuticle. To test whether ABA could enhance pathogenicity in M. oryzae, 70-15 spores were treated with ABA ranging from 10 to 100 μM. By 3 h almost all spores germinated in the untreated controls, and by 24 h most formed appressoria. To see if ABA was accelerating these processes, germination was examined at 2 h and the initiation of appressoria formation was examined at 6 h. Spores that had been exposed to 50 or 100 μM ABA had a higher percent germination than those that were not exposed to ABA at 2 h post treatment (Figure 3). Also, the percent of germinated spores that were forming appressoria at 6 h post treatment was higher in the presence of 50 or 100 μM ABA (Figure 3). In addition to ABA, other plant growth regulators were tested including gibberelic acid (GA), the natural auxin indole-3-acetic acid (IAA), a synthetic auxin indole-3-butryic acid (IBA), and the cytokinin, kinetin. Aside from ABA, only GA was able to increase percent germination at 2 h, but did not increase appressoria formation at 6 h (Supplementary Figure S4). Kinetin had no effect on germination, but increased appressoria formation at 6 h. The natural and synthetic auxins had no effect on M. oryzae 70-15 spore germination or appressoria formation (Supplementary Figure S4). The germination and appressoria formation was also quantified in spores treated with EA105 and ABA together. Previously, we have shown that EA105 had a minimal effect on spore germination at 3 h, but almost completely abolished appressoria formation in M. oryzae at 24 h (Spence et al., 2014a). At the earlier time points, EA105 behaves similarly. The percent germination is not statistically different from the control, while appressoria formation is greatly reduced (Figure 4). When EA105 and ABA are co-treated on spores, the percent germination is about half way between what was seen with ABA or EA105 treatment alone. The percent of spores that formed appressoria decreased from about 84% with ABA treatment alone to about 23% when treated with both EA105 and ABA together (Figure 4).


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)

Magnaporthe oryzae 70-15 spores treated with EA105 and/or exogenous ABA. (A) Percent germination was quantified at 2 h post treatment and (B) percent appressoria formation was quantified at 6 h post treatment. The experiment was repeated three times with five coverslips per treatment and three images per coverslip. Different letters represent statistical significance 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 4: Magnaporthe oryzae 70-15 spores treated with EA105 and/or exogenous ABA. (A) Percent germination was quantified at 2 h post treatment and (B) percent appressoria formation was quantified at 6 h post treatment. The experiment was repeated three times with five coverslips per treatment and three images per coverslip. Different letters represent statistical significance based on the Tukey-Kramer test (p < 0.05).
Mentions: Pathogenesis of M. oryzae begins with spore germination. The crucial step in virulence is the formation of the appressorium, a specialized infection structure that accumulates high turgor pressure and forms a penetration peg that can enter the rice cuticle. To test whether ABA could enhance pathogenicity in M. oryzae, 70-15 spores were treated with ABA ranging from 10 to 100 μM. By 3 h almost all spores germinated in the untreated controls, and by 24 h most formed appressoria. To see if ABA was accelerating these processes, germination was examined at 2 h and the initiation of appressoria formation was examined at 6 h. Spores that had been exposed to 50 or 100 μM ABA had a higher percent germination than those that were not exposed to ABA at 2 h post treatment (Figure 3). Also, the percent of germinated spores that were forming appressoria at 6 h post treatment was higher in the presence of 50 or 100 μM ABA (Figure 3). In addition to ABA, other plant growth regulators were tested including gibberelic acid (GA), the natural auxin indole-3-acetic acid (IAA), a synthetic auxin indole-3-butryic acid (IBA), and the cytokinin, kinetin. Aside from ABA, only GA was able to increase percent germination at 2 h, but did not increase appressoria formation at 6 h (Supplementary Figure S4). Kinetin had no effect on germination, but increased appressoria formation at 6 h. The natural and synthetic auxins had no effect on M. oryzae 70-15 spore germination or appressoria formation (Supplementary Figure S4). The germination and appressoria formation was also quantified in spores treated with EA105 and ABA together. Previously, we have shown that EA105 had a minimal effect on spore germination at 3 h, but almost completely abolished appressoria formation in M. oryzae at 24 h (Spence et al., 2014a). At the earlier time points, EA105 behaves similarly. The percent germination is not statistically different from the control, while appressoria formation is greatly reduced (Figure 4). When EA105 and ABA are co-treated on spores, the percent germination is about half way between what was seen with ABA or EA105 treatment alone. The percent of spores that formed appressoria decreased from about 84% with ABA treatment alone to about 23% when treated with both EA105 and ABA together (Figure 4).

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