A review on Pyricularia oryzae; biological and taxonomical finding in Iran

Taxonomic history of the pathogen

In 1880, Sacardo created the generic name Pyricularia based on the asexual form, P. grisea, isolated from the Digitaria sanguinalis. Isolates from rice were introduced as P. oryzae by Cavara in 1892, now known as the rice blast causal agent. Afterward, two species, P. oryzae and P. grisea, considered synonymous (Rosman et al. 1990), because there were not enough morphological criteria to separate the two species, and the cross of isolates from rice and non-rice hosts leads to the production of sexual form, which indicates a genetic relationship between the two groups of isolates. The survey of type samples in herbarium was confirmed the morphological similarity of the two species. Therefore, P. grisea (Cooke) Sacc. was introduced as the correct name for the asexual stage of both groups of isolates (rice and non-rice isolates) due to the name's priority. In 2002, phylogenetic analysis based on three gene regions, including Actin, Beta-tubulin, and Calmodulin, resulted in separating P. oryzae from P. grisea as new species (Couch & Kohn 2002).

In Iran, based on the study on morphology criteria of 150 isolates from weeds and crop plants, P. oryzae differ from P. grisea in morphology and host plant (Pordel et al. 2016). Conidiophores size are 100-250 µm in P. oryzae and 70-175 µm in P. grisea, and conidia size are 16-25 × 7-10 µm in P. oryzae; and 26-31 × 6-8 µm in P. grisea (Fig 3) (Pordel et al. 2016). Phylogenetic analyses have been done by partial sequences of the internal described spacer regions with the 5.8S nuclear ribosomal RNA gene (ITS), the large subunit of the nuclear ribosomal RNA gene cluster (LSU), and several protein-encoding genes, including the partial RNA polymerase II largest subunit (RPB1), actin (ACT), calmodulin (CAL), and DNA replication licensing factor (MCM7), proved the name of Pyricularia oryzae for isolates on maize (Zea mays), rice (Oryza sativa), Indian goosegrass (Eleusine indica), barnyard grass (Echinochloa crus-galli), bermuda grass (Cynodon dactylon), knotgrass (Paspalum distichum), wild foxtail millet (Setaria sp.), foxtail millet (Setaria italica) and banana (Musa acuminata) and the P. grisea name is approved only for isolates on Digitaria sp. (Pordel et al. 2015, 2016).

Host-specificity of the pathogen

Multilocus sequence typing and whole-genome sequence showed P. oryzae is subdivided into multiple clades; each could infect the limited number of host species (Couch et al. 2005; Gladieux et al. 2018). Host specificity is, therefore, a probable driver of genetic divergence (Couch et al. 2005; Gladieux et al. 2018). Mostly, pathogenicity test was confirmed this species' host-specificity, except for barley, which seems to be susceptible to strains from all clades. Strains from rice have a limited host range and were reported to infect only oat and Lolium in addition to rice (Tosa & Chuma 2014; Klaubauf et al. 2014). Pyricularia oryzae strains from other host plants such as Eleusine, Setaria, and Triticum are also host-specific and unable to infect rice (Kato et al. 2000; Couch et al. 2005; Murata et al. 2014; Tosa & Chuma 2014; Castroagudin et al. 2016).

Host specificity of strains from different hosts have been assessed by spray inoculation of spore suspensions on important crops and weeds susceptible to P. oryzae (Pordel et al. 2019; Pordel et al. 2021). Strains from rice, maize, barnyard grass, wild foxtail millet, and foxtail millet showed severe symptoms on their isolated host (Table 1). Rice strains do not produce symptoms on five tested hosts and made different types of lesions on Iranian rice varieties (Javan-Nikkhah et al. 2003). Eight strains from barnyard grass produced susceptible-type lesions on maize and two maize strains out of ten produced susceptible-type lesions on barnyard grass (Table 1). The strains of wild foxtail millet produced severe lesions on foxtail millet, and the strains of foxtail millet made severe lesions on wild foxtail millet plants (Table 1) (Pordel et al. 2016, 2019, 2021).

To survey the reaction of the different cultivars of rice to P. oryzae, fifty isolates were checked by Javan-Nikkhah et al. (2003). They studied differential varieties from international standard cultivars set, CO39 and five near-isogenic lines (NILs) and twelve selected Iranian cultivars. In this study, the differentiating ability of NILs was evaluated by comparing those international differentials and Iranian cultivars. The set of five NILs was able to differentiate 50 isolates into two pathotypes (races) in which one isolate represented pathotype ″A″ and 49 were grouped into pathotype ″B″. Fifty isolates were classified into seven pathogenicity groups by their reaction to twelve Iranian cultivars. Their results showed the P. oryzae isolates had high specificity on different cultivars (Javan-Nikkhah et al. 2003). Many studies confirmed that P. oryzae has various types of lesions on the different rice cultivars (Moradi et al. 2010, Mousanejad et al. 2010). 

Population structure and genetic diversity

DNA fingerprinting has been successfully used to assess genetic diversity and population genetics of pathogens by inter-simple sequence repeat (ISSR), random amplified polymorphic DNA (RAPD), and amplified fragment length polymorphism (AFLP) techniques (Dubina et al. 2020). Genetic analysis of P. oryzae population structure was performed for the first time by Javan Nikkhah et al. in Iran (2004). His studies based on DNA fingerprinting using a molecular rep-PCR technique using Pot2 primer on 221 monoconidial isolates divided the isolates into six clonal lineages. The results showed the low genetic diversity among the populations, and a specific relationship was observed between rice cultivars. DNA fingerprinting using RAPD-PCR markers on P. oryzae population isolated from weeds of Echinochloa sp., Digitaria sp., Setaria sp., unknown weed and rice, showed that isolates from wild foxtail millet and rice are similar to each other and separate from Digitaria sp. isolates (Bargnil et al. 2012). In recent years, interesting studies have been conducted in genetic diversity, adaptive vegetative groups, and mating types in Iran (Musanejad et al. 2005; Salimi 2018).

Fig. 3. Morphological comparison of the Pyricularia oryzae (a-c) with P. grisea (d-f).

Sexual status and mating type information

Mousanejad et al. (2005) studied the fertility status and distribution of causal agents of the blast disease in Iran. They investigated 158 rice blast pathogen isolates, including 94 isolates from different regions of Guilan province and 64 isolates from the experimental fields of the Rice Research Institute of Iran., in Amol, Mazandaran province. The isolates crossed with standard hermaphrodite and fertile isolates (KA3 and TH12 (representatives of Mat1-1 mating type) and KA9 and TH16 (representatives of Mat1-2 mating type). These crosses' results showed that 62.76% of the isolates from different parts of Guilan province, and 32.81% of the isolates of the Rice Research Institute's experimental field were fertile. The rest of the isolates were infertile in both cases. All fertile isolates were male fertile and of mating-type Mat1-1. Salimi et al. (2018) examined 142 isolates of P. oryzae obtained from leaf and neck blast to identify idiomorphs of mating-type using Multiplex PCR. In this study, 555 bp fragment was amplified for all isolates, so the mating type of all isolates was determined to be Mat 1-1. Pordel et al. (2019) identified idiomorphs of mating-type 141 isolates from rice, barnyard grass, maize, wilt foxtail millet, and foxtail millet. The results showed Mat 1-1 mating type is the dominant type in isolates from rice and foxtail millet, and Mat 1-2 mating type is the predominant type in isolates from barnyard grass, maize, and wild foxtail millet. They crossed the weeds and crops isolates with tester strains from CIRAD institute, France. The sexual state's induction between isolates from rice, wild foxtail millet, and foxtail millet crossed with tester isolates was successful under controlled laboratory conditions. Long typical ascocarps of Pyricularia sp. were observed over the media after 21 days incubation. Typical ascospore having four cells, spindle-shaped, 17-25 × 4-8 µm. All strains from weeds and crops did not produce perithecia when crossed with each other (Pordel et al. 2019).

Table 1. Pathogenicity results of Pyricularia oryzae strains on maize, barnyard grass, rice, foxtail millet, and wild foxtail millet in Iran (Pordel et al., 2016, 2019, 2021).

Lesion types are as follows: -, not tested; 1, no signs of infection; 2, small brown lesions; 3, small lesions with yellow centers and brown margins; 6, large diamond-shaped lesions (Silue, et al., 1992). * ND, no data.