Contribution of Peroxisomes to Secondary Metabolism and Pathogenicity in the Fungal Plant Pathogen Alternaria alternata

Abstract

The filamentous fungus Alternaria alternata includes seven pathogenic variants (pathotypes) which produce different host-selective toxins and cause diseases on different plants. The Japanese pear pathotype produces the host-selective AK-toxin, an epoxy-decatrienoic acid ester, and causes black spot of Japanese pear. Previously, we identified four genes, AKT1, AKT2, AKT3, and AKTR, involved in AK toxin biosynthesis. AKT1, AKT2, and AKT3 encode enzyme proteins with peroxisomal targeting signal type 1 (PTS1)-like tripeptides, SKI, SKL, and PKL, respectively, at the C-terminal ends. In this study, we verified the peroxisome localization of Akt1, Akt2, and Akt3 by using strains expressing N-terminal green fluorescent protein (GFP)-tagged versions of the proteins. To assess the role of peroxisome function in AK-toxin production, we isolated AaPEX6, which encodes a peroxin protein essential for peroxisome biogenesis, from the Japanese pear pathotype and made AaPEX6 disruption-containing transformants from a GFP-Akt1-expressing strain. The ⌬AaPEX6 mutant strains did not grow on fatty acid media because of a defect in fatty acid ␤ oxidation. The import of GFP-Akt1 into peroxisomes was impaired in the ⌬AaPEX6 mutant strains. These strains completely lost AK toxin production and pathogenicity on susceptible pear leaves. These data show that peroxisomes are essential for AK-toxin biosynthesis. The ⌬AaPEX6 mutant strains showed a marked reduction in the ability to cause lesions on leaves of a resistant pear cultivar with defense responses compromised by heat shock. This result suggests that peroxisome function is also required for plant invasion and tissue colonization in A. alternata. We also observed that mutation of AaPEX6 caused a marked reduction of conidiation.Peroxisomes are single-membrane-bound organelles and have a wide range of metabolic functions, including ␤ oxidation of fatty acids, peroxide detoxification, and glyoxylate metabolism (60,62

Defects in Mitochondrial and Peroxisomal β-Oxidation Influence Virulence in the Maize Pathogen Ustilago maydis

“…A fox2/mfe2 mutant of M. grisea was also defective in appressorium formation and plant penetration (64). In A. alternata, a pex6 mutant was unable to grow on fatty acids, it was nonpathogenic, and it did not produce AK toxin (25). Finally, the expression of a peroxisomal 3-ketoacyl-CoA thiolase was also important for virulence in L. maculans (12).…”

“…Defects in ␤-oxidation are known to influence virulence in other phytopathogenic fungi. For example, overexpression of a ketoacyl-CoA thiolase reduces the size of lesions caused by Leptosphaeria maculans on Brassica napus cotyledons and defects in ␤-oxidation and peroxisome biogenesis block appressorium-based invasion in Magnaporthe grisea, Colletotrichum species, and Alternaria alternata (1,3,12,14,25,26,46,64). Therefore, ␤-oxidation represents an attractive target for the development of control strategies.…”

Role of the Alternaria alternata Blue-Light Receptor LreA (White-Collar 1) in Spore Formation and Secondary Metabolism

“…Despite the wide distribution and the great economic impact, the number of molecular studies is rather limited. Only some genes have been analyzed by gene deletion; e.g., the role of peroxisomes in toxin formation has recently been addressed by deletion of a gene encoding a Pex6 homologue (31). In addition to methods for gene deletion by homologous replacement, high-throughput gene disruption methods have been established (20).…”

An improved Akt reporter reveals intra- and inter-cellular heterogeneity and oscillations in signal transduction

“…The extent of Akt membrane recruitment in these studies was highly variable. Furthermore, none of these studies appear to have compared the activity and/or function of these constructs to the endogenous protein (Asano et al, 2008;Calleja et al, 2003;Carpten et al, 2007;Cenni et al, 2003;Currie et al, 1999;Du et al, 2014;Feng et al, 2014;Gonzalez and McGraw, 2009;Huang et al, 2011;Imazaki et al, 2010;Kontos et al, 1998;Lasserre et al, 2008;Parikh et al, 2012;Rodríguez-Escudero et al, 2005;Servant et al, 2000;Terashima et al, 2005;Watton and Downward, 1999;Zhang et al, 2009). While GFP remains the most commonly used fluorescent protein, there are a growing number of publications that report fusion protein dysfunction (Goto et al, 2003;Huang and Shusta, 2006;Kalatskaya et al, 2006;Liu et al, 1999;Yantsevich et al, 2009) and mislocalisation (Skube et al, 2010;Zhu et al, 2013), which one group has attributed to the affinity of eGFP for the nucleus (Seibel et al, 2007).…”