Effect of linoleic acid on reproduction and yeast–mycelium dimorphism in the Dutch elm disease pathogens

Erika Sayuri Naruzawa, Fabienne Malagnac, Louis Bernier
2016 Botany  
Keyword: fatty acid, Ophiostoma, yeast, hypha, perithecium https://mc06.manuscriptcentral.com/botany-pubs Botany Abstract 1 2 Elm populations from North America and Europe were devastated by Dutch elm disease 3 (DED), a vascular disease caused by fungi from the genus Ophiostoma (Ascomycota). 4 These pathogens feature a yeast-mycelium dimorphism that may be related to virulence 5 by facilitating colonization of the host xylem. Cyclooxygenases (Cox) have been 6 proposed to modulate yeast-mycelium
more » ... late yeast-mycelium dimorphism of DED fungi, and homologs of Cox-7 encoding genes have been found in the nuclear genome of O. novo-ulmi subsp. novo-8 ulmi. Linoleic acid, a substrate for Cox, was reported to stimulate the formation of 9 asexual and sexual reproduction structures in DED strains grown on complex media. 10 We hypothesized that linoleic acid also induced mycelium production in liquid shake 11 culture conditions. Linoleic acid was found to enhance production of reproductive 12 structures in sexual crosses conducted on complex medium (elm sapwood agar) but 13 was not sufficient for these structures to form on minimal medium. In liquid shake 14 cultures grown in minimal medium, addition of linoleic acid stimulated mycelial formation. 15 Our results suggest that linoleic acid plays a role in two complex pathways, reproduction 16 and dimorphism, in the DED pathogens. 17 18 Résumé 23 24 Les populations d'ormes américains et européens ont été dévastées par la maladie 25 hollandaise de l'orme (MHO), une maladie vasculaire causée par des champignons 26 ophiostomatoïdes (Ascomycota). Ces agents pathogènes affichent un dimorphisme 27 levure-mycélium probablement lié à la virulence en facilitant la colonisation du xylème 28 de l'hôte. On a proposé antérieurement que des cyclooxygénases (Cox) modulent ce 29 dimorphisme. Des homologues de gènes cox sont présents dans le génome nucléaire 30 d'O. novo-ulmi subsp. novo-ulmi. L'acide linoléique, un substrat pour Cox, semble 31 stimuler la formation de structures de reproduction sexuée et asexuée chez les 32 ophiostomatoïdes sur milieux complexes. Nous avons émis l'hypothèse selon laquelle 33 l'acide linoléique induit la production de mycélium en milieux agités liquides. L'acide 34 linoléique a augmenté la production de structures de reproduction dans des 35 croisements sexués sur milieu complexe (milieu au bois d'orme), mais pas sur milieu 36 minimum. En milieu minimum liquide agité, l'acide linoléique a stimulé la formation de 37 mycélium. L'acide linoléique semble jouer un rôle dans les voies de reproduction et du 38 dimorphisme chez ces champignons. 39 40 Mot-clés: Acide gras, Ophiostoma, levure, hyphe, perithèce, transition morphologique 41 42 43 44 Page 3 of 34 https://mc06.manuscriptcentral.com/botany-pubs Botany 57 58 Several stimuli are known to control yeast to mycelium transition in shake liquid cultures 59 of DED fungi. They include nitrogen source (Kulkarni and Nickerson 1981, Naruzawa 60 and Bernier 2014), yeast cell concentration akin to quorum sensing (Kulkarni and 61 Nickerson 1981, Hornby et al. 2004; Berrocal et al. 2012) and calcium-calmodulin 62 interaction (Muthukumar and Nickerson 1984; Brunton and Gadd 1989; Gadd and 63 Brunton 1992). According to Jensen et al. (1992), inhibitors of lipoxygenases (Lox) also 64 interfere with yeast-mycelium transition and favor the production of yeast cells over 65 mycelium. This was recently confirmed by Naruzawa and Bernier (2014) who further 66 reported that salicylic acid, an inhibitor of cyclooxygenases (Cox), had a similar effect. 67 Page 4 of 34 https://mc06.manuscriptcentral.com/botany-pubs Botany 68 incorporating oxygen atoms into polyunsaturated fatty acids (PUFAs) (Bugg 2003). 69 According to results by Jensen et al. (1992) and Naruzawa and Bernier (2014), oxylipins 70 could be related to the yeast-mycelium transition of DED pathogens. 71 72 Oxylipins function as hormone signals to modulate timing and balance between sexual 73 and asexual development and toxin production in fungi. According to Tstsigiannis et al. 74 (2004a), an Aspergillus nidulans mutant in which the dioxygenase gene ppoC had been 75 deleted exhibited increased sexual spore production and decreased asexual spore 76 formation. The opposite response occurred in the case of an A. nidulans ∆ppoA mutant 77 (Tsistigiannis et al 2004b). An A. nidulans ∆ppoA, ∆ppoB and ∆ppoC triple mutant was 78 unable to produce sterigmatocystin, a precursor of aflatoxin, both in vivo and in planta. 79 On the other hand, this mutant overproduced penicillin (Tsitsigiannis and Keller 2006). 80 Also, the impaired mutant could no longer colonize peanut seeds. The latter phenotype 81 could be explained by decreased conidiation and lipase enzyme production 82 (Tsitsigiannis and Keller 2006). In contrast, an A. fumigatus ∆ppoA, ∆ppoB and ∆ppoC 83 triple mutant was hypervirulent in the invasive pulmonary aspergillosis murine model 84 system and had tolerance to hydrogen peroxide, which implicates resistance to reactive 85 oxygen species defense of host (Tsitsigiannis et al. 2005). Therefore, these enzymes 86 and oxylipins are involved in the regulation of fungal metabolism. Interestingly, 87 homologs of ppo genes are found in the Ophiostoma novo-ulmi genome (Comeau et al., 88 2015; Naruzawa et al. unpublished). 89 90 Page 5 of 34 https://mc06.manuscriptcentral.com/botany-pubs Botany 103 104 Based on the above results, we speculated that linoleic acid was involved in the 105 reproduction and yeast-mycelium dimorphism of DED pathogens. Our specific 106 objectives were to verify whether linoleic acid stimulated the formation of asexual and 107 sexual reproductive structures on defined solid growth media and enhanced the 108 production of mycelium in liquid shake cultures.
doi:10.1139/cjb-2015-0156 fatcat:y2lixi3jdve77gluqastwxu2pa