Pathogen Phytotoxins as Agents of Specificity

The fungi generally produce metabolites in their culture medium (Turner, 1971). According to their chemical structures, the metabolites can be classified into several different classes of compounds, such as secondary metabolites. Metabolites

Secondary host of Pyricularia pathotypes showing symptoms in Echinocloa crus-galli A, Festuca arundinacea B. Digitaria sanguinalis C and Stenotaphrum secunda-tum D

FIGURE 6.7 Secondary host of Pyricularia pathotypes showing symptoms in Echinocloa crus-galli A, Festuca arundinacea B. Digitaria sanguinalis C and Stenotaphrum secunda-tum D.

produced by various blast disease isolates from various host plant species were reported (Nukina, 1999; Hu et al., 2011; Sato & Kozaba, 1969). P. oryzae produces toxins made up of different compounds e.g., pyricularin, pyriculariol and pyriculol, phytotoxic metabolites found associated with the production of a dark necrotic spot on rice leaf similar to that of blast (Ogasawara et al., 1957; Nukina et al., 1981; Rao & Suryanarayanan, 1974). Moreover, novel pyriculol-related compounds, epipyriculol, were isolated from cultures of P. oryzae with phytotoxic activity as well as inhibitory activity towards the spore germination of the fungus itself (Kondo et al., 1991). Similarly, Tsurushima et al. (2005) found pyrichalasin H production and the pathogenicity of Digitaria-speciñc isolates of P. grísea. From culture filtrates of 72 isolates of Pyricularia analysed via high-performance liquid chromatography (HPLC), 31 (r9 DNA type) from crabgrass (D. sanguinalis), 1 (r9 DNA type) from Pangolagrass (Digitaria smutsii) and 6 (r8 DNA type) from D. horizontal produced 20-280 pg pyrichalasin H per millilitre of culture and were pathogenic on 5 Digitaria species. Interestingly, two isolates originally isolated from Digitaria plants did not produce pyrichalasin H nor cause blast lesions on Digitaria plants. Other reports also indicate the presence of pyriculol, pyrichalasin H and tenuazonic acid (Iwasaki et al., 1969, 1972a; Nukina, 1987). Tenuazonic acid is also produced by many isolates of Alternaria, but pyriculol and pyrichalasin H are specifically produced by the blast fungus. Tenuazonic acid isolated from Oryza isolates of the blast fungus induced typical brown necrosis on rice leaves (Umetsu et al., 1973; Iwasaki et al., 1969, 1972b). Recently, Nukina and Saito (2014) reported the production of a norvaline-analogue of tenuazonic acid by P. oryzae. Previously, Nukina (1988) reported the production of terrestric acid as a phytotoxic metabolite of Pyricularia isolates. Moreover, its phytotoxicity was demonstrated on rice plants as yellowing of the leaves and growth inhibition of the roots and shoots, indicating that terrestric acid may have had an influence on chlorophyll synthesis. The isolation of picolinic acid as a toxic substance produced by P. oryzae was also reported (Tamari & Kaji, 1954; Ogasawara et al., 1961). Studies on the wheat/P. oryzae system started at CIDEFI by analysing toxins production using the HPLC technique which showed that compounds such as picolinic acid, terrestric acid, benzoic acid and pyricurol were produced by different Argentinian isolates of P. oryzae from wheat in both liquid media tested (Table 6.4). On the other hand, benzoic acid, acetamide, acetic acid, furoic acid, carboxylic acid, pyrichalasin H and pyricularin were only produced in Czapek media. Other substances, e.g. nitrobenzyl, tenuazoic acid

TABLE 6.4

Secondary Metabolites Produced by Different P. oryzae Isolates after 14 Days Grown under Continuous Shaking at 26°C and 3 Liquid Culture Media Detected by HPLC Technique

Czapeck media a-Teralones Terrestric acid Benzoic acid Furoic acid Pyricularin+ Acetamide Acetic acid a Picolinic acid Pyriculol Carboxylic acid Pyrichalasin H+

Oatmeal media Bran wheat media

Nitrobenzyl a-Tetralones Benzoic acid Tenuazoic acid a Picolinic acid Pyriculol Epipyriculol Dihidropyriculol Terrestric acid Pyriculariol+

a Picolinic acid

Pyriculol

Dihidropyriculol

Terrestric acid Tenuazoic acid Pyriculamide+

+ Compounds produced in a particular media and pyriculariol, were produced exclusively in OM media. Interestingly, when a third media was tested, pyriculamide was produced in liquid wheat media (WM) (wheat flakes [30 g] boiled in 500 ml of distilled water for 30 min and filtered through muslin cloth and distilled water to make up a volume of 1000 ml). Currently, from the chemotaxonomical viewpoint, the blast fungi can be divided into several groups based on their metabolite production profiles. This finding indicates the biochemical plasticity of the analysed Argentinian isolates regarding secondary metabolites and the risk of some compounds, e.g. tenuazoic acid, as an important mycotoxin of the Alternaria species group. On the other hand, these results are new findings that contribute to further knowledge of the variability of the isolates analysed. Research on the role of the phytotoxic metabolites of P. oryzae wheat isolates in pathogenesis is underway even though some of them do not show host specificity.

Globally, a complete understanding of the ecological and epidemiological factors that drive wheat blast outbreaks is required. The basic aspects of wheat blast are yet to be well determined including factors that support inoculum buildup and the wide spread of the pathogen.

 
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