Expression of Magnaporthe grisea Avirulence Gene ACE1 Is Connected to the Initiation of Appressorium-Mediated Penetration

Abstract

Magnaporthe grisea is responsible for a devastating fungal disease of rice called blast. Current control of this disease relies on resistant rice cultivars that recognize M. grisea signals corresponding to specific secreted proteins encoded by avirulence genes. The M. grisea ACE1 avirulence gene differs from others, since it controls the biosynthesis of a secondary metabolite likely recognized by rice cultivars carrying the Pi33 resistance gene. Using a transcriptional fusion between ACE1 promoter and eGFP, we showed that ACE1 is only expressed in appressoria during fungal penetration into rice and barley leaves, onion skin, and cellophane membranes. ACE1 is almost not expressed in appressoria differentiated on Teflon and Mylar artificial membranes. ACE1 expression is not induced by cellophane and plant cell wall components, demonstrating that it does not require typical host plant compounds. Cyclic AMP (cAMP) signaling mutants ⌬cpkA and ⌬mac1 sum1-99 and tetraspanin mutant ⌬pls1::hph differentiate melanized appressoria with normal turgor but are unable to penetrate host plant leaves. ACE1 is normally expressed in these mutants, suggesting that it does not require cAMP signaling or a successful penetration event. ACE1 is not expressed in appressoria of the buf1::hph mutant defective for melanin biosynthesis and appressorial turgor. The addition of hyperosmotic solutes to buf1::hph appressoria restores appressorial development and ACE1 expression. Treatments of young wild-type appressoria with actin and tubulin inhibitors reduce both fungal penetration and ACE1 expression. These experiments suggest that ACE1 appressorium-specific expression does not depend on host plant signals but is connected to the onset of appressorium-mediated penetration.Magnaporthe grisea species complex attacks a wide range of grasses, including wheat, barley, and rice (10,26), and is a model organism for the study of plant fungal interactions (11, 42). The M. grisea infection cycle is characteristic of grass leaf spot diseases. After spore attachment and germination, the fungus differentiates an appressorium through the perception of physical and chemical surface parameters (hydrophobicity, hardness, and cuticle monomers) (21, 42). This differentiation is the result of a complex morphogenetic process that involves cyclic AMP (cAMP), mitogen-activated protein kinases, and calcium signaling pathways (7,45,50). Early stages of appressorium development are associated with the deposition of a melanin layer between the cell wall and plasma membrane (21), migration of lipid bodies from spore to appressorium, mobilization of glycogen, and the formation of a septum sealing the appressorium (5, 43). Maturation of the appressorium is characterized by the degradation of lipid bodies and glycogen (43) and the generation of a high turgor (22). Finally, a reorganization of the cytoskeleton is induced at the point of emergence of the penetration peg that penetrates the host cuticle and cell wall (5,35). Inside the plant, M. grisea differentiates bulbous...

Functional analysis of non-ribosomal peptide synthetases (NRPSs) in Trichoderma virens reveals a polyketide synthase (PKS)/NRPS hybrid enzyme involved in the induced systemic resistance response in maize

“…The only other PKS/NRPS that is known to be involved in plant-fungus interactions is Ace1 of M. grisea. Ace1 putatively encodes an unidentified secondary metabolite that acts as an avirulence factor during interactions with rice cultivars harbouring the Pi33 'R' gene triggering a hypersensitive response in a gene-for-gene fashion (Böhnert et al, 2004;Collemare et al, 2008a, b;Fudal et al, 2007;Khaldi et al, 2008;Vergne et al, 2007). This gene is expressed specifically in the appressoria of M. grisea, and the metabolite produced by this gene cluster has not yet been identified.…”

“…These phytotoxins act as virulence factors (Walton et al, 2004;Hoffmeister & Keller, 2007;Walton, 2006;Johnson et al, 2000;Elliott et al, 2007). Additionally, a PKS/NRPS hybrid enzyme (Ace1) of Magnaporthe grisea acts as an avirulence factor, triggering resistance in selective rice cultivars harbouring the corresponding resistance gene Pi33 (Böhnert et al, 2004;Collemare et al, 2008a, b;Fudal et al, 2007;Khaldi et al, 2008;Vergne et al, 2007). A recent comparative genomics study revealed that the genomes of the two biocontrol species T. virens and Trichoderma atroviride harbour a large number of NRPS and PKS genes, and the genome of T. virens contains more NRPSs than any other filamentous fungi studied so far (Kubicek et al, 2011 …”

Use of Native Promoter-eGFP as a Gene Reporter on Onion Epidermis to Analyze Gene Expression of AVR-Pia, an Avirulence Effector of Rice Blast Pathogen

“…However, further experiments are needed to clarify that hypothesis. ACE1 gene expression has been reported that the expression is connected to the onset of appressorial-mediated penetration using a transcriptional fusion between ACE1 promoter and eGFP [12], therefore, this EGFP reporter method might be applicable for the expression monitoring other AVR genes in M. oryzae.…”

“…To date, ~85 blast R genes have been identified [6]. In Japanese rice cultivars, thirteen dominant genes (Pish, Pik-h, Pia, Pii, Pik, Pik-m, Piz, Pita, Pita-2, Piz-t, Pik-p, Pib, and Pit) have been identified, while twenty -five avirulence genes of M. oryzae have been genetically mapped [7], nine of which were recently isolated, AVRPita [8], AVR-CO39 [9], PWL1 [10], PWL2 [11], ACE1 [12], AVR-Pizt [13], AVR-Pia [14], [15], AVR-Pii, and AVR-Pik/km/kp [16]. The avirulence genes PWL1 and PWL2, isolated by map-based cloning are responsible for the nonpathogenicity of rice pathogenic strains of M. oryzae against Eragrostis curvula, weeping lovegrass [10], [11].…”

Induction of Chlorosis, ROS Generation and Cell Death by a Toxin Isolated fromPyricularia oryzae

“…1) Several avirulence factors have been reported from Pyricularia isolates in rice. [2][3][4] Blast fungi might also produce active factors such as host-specific toxins which some fungal pathogens produce as determinants of pathogenicity. 5) Virulence agents produced by Pyricularia isolates have also been examined.…”