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

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.

Suppression of abscisic acid biosynthesis at the early infection stage of Verticillium longisporum in oilseed rape (Brassica napus)

“…Since ABA is often reported to play a role in attenuating defence reactions towards biotic stress (Cao et al , ; Clay et al , ; Mialoundama et al , ), attempts to elevate ABA levels might be a genuine strategy for pathogens to manipulate their hosts. This can occur by producing ABA themselves (Cao et al , ; Morrison et al , ; Siewers et al , ; Spence et al , ; de Torres‐Zabala et al ., ) or by activation of the plant ABA biosynthesis, for example via enhancing NCED3 gene expression as observed in the X. oryzae –rice interaction, resulting in increased susceptibility (Xu et al , ). Similar observations have been made for Pseudomonas syringae DC3000, which deploys type III effectors to induce the Arabidopsis NCED3 gene, resulting in elevation of ABA levels (de Torres‐Zabala et al , , ).…”

Plant hormones: a fungal point of view

“…Although exogenous ABA triggered a faster development of necrotic lesions, the role of fungal ABA in virulence was not described until recently. Knocking out one gene homologous to the B. cinerea ABA4 gene responsible for ABA biosynthesis reduced, by two‐fold, ABA levels in M. oryzae (Siewers et al ., ; Spence et al ., ). Appressorium formation in vitro was severely reduced in the M. oryzae Δaba4 mutant, a phenotype that could be reversed by the exogenous application of ABA.…”

Phytohormones: the chemical language in Magnaporthe oryzae-rice pathosystem

“…Knowledge about ABA biosynthesis is limited in M. oryzae . Three ABA gene homologs ( MoABA1, MoABA2 and MoABA4 ) and an ABA G-protein couple receptor were identified in M. oryzae (Spence et al 2015). Deletion of MoABA4 gene resulted in loss of pathogenicity, indicating that ABA production may be crucial for M. oryzae pathogenicity (Spence et al 2015).…”

“…Three ABA gene homologs ( MoABA1, MoABA2 and MoABA4 ) and an ABA G-protein couple receptor were identified in M. oryzae (Spence et al 2015). Deletion of MoABA4 gene resulted in loss of pathogenicity, indicating that ABA production may be crucial for M. oryzae pathogenicity (Spence et al 2015). M. oryzae was able to up-regulate the rice NCED3 gene (for rice ABA biosynthesis) expression, suggesting that it may stimulate ABA synthesis in rice to facilitate its own pathogenicity and subvert host resistance (Spence et al 2015).…”