Pythium species from rice roots differ in virulence, host colonization and nutritional profile

Abstract

BackgroundProgressive yield decline in Philippine aerobic rice fields has been recently associated with three closely related Pythium spp., P. arrhenomanes, P. graminicola and P. inflatum. To understand their differential virulence towards rice seedlings, we conducted a comparative survey in which three isolates each of P. arrhenomanes, P. graminicola and P. inflatum were selected to investigate host colonization, host responses and carbon utilization profiles using histopathological analyses, phenoarrays, DNA quantifications and gene expression studies.ResultsThe isolate of the most virulent species, P. arrhenomanes, quickly colonized the outer and inner root tissues of rice seedlings, including the xylem, by which it possibly blocked the water transport and induced severe stunting, wilting and seedling death. The lower virulence of the tested P. graminicola and P. inflatum isolates seemed to be reflected in slower colonization processes, limited invasion of the vascular stele and less systemic spread, in which cell wall fortification appeared to play a role. Progressive hyphal invasions triggered the production of reactive oxygen species (ROS) and phenolic compounds, which was the strongest for the P. arrhenomanes isolate and was delayed or much weaker upon inoculation with the P. inflatum isolate. The necrosis marker OsJamyb seemed faster and stronger induced by the most virulent isolates. Although the isolate of P. inflatum was nutritionally the most versatile, the most virulent Pythium isolate appeared physiologically more adapted to its host, evidenced by its broad amino acid utilization profile, including D-amino acids, L-threonine and hydroxyl-L-proline. The latter two compounds have been implicated in plant defense and their use by P. arrhenomanes could therefore represent a part of its virulence strategy.ConclusionsThis study illustrates that the differential virulence of rice-pathogenic P. arrhenomanes, P. graminicola and P. inflatum isolates is related to their root colonization capacity, the intensity of induced root responses and their ability to utilize amino acids in their colonization niche. Accordingly, this paper presents important knowledge concerning rice root infections by oomycetes, which could be helpful to further disentangle virulence tactics of soil-borne pathogens.

Exogenous Trehalose Treatment Enhances the Activities of Defense-Related Enzymes and Triggers Resistance against Downy Mildew Disease of Pearl Millet

“…The auto-fluorescence is associated with papilla deposition, resulting in slowing down or even blocking fungal penetration (Maor and Shirasu, 2005; de Leon and Montesano, 2013). Enhanced activities of these enzymes also result in enhancement of signal transduction, preventing further pathogen spread (Kowalcyzk et al, 2004; Van Buyten and Höfte, 2013; Falcón-Rodríguez et al, 2014). PAL and POX are important components of phenylpropanoid pathway, and enhanced activities of these enzymes will lead to increased production of lignin and secondary metabolites, such as flavonoids and phenolic compounds, which have anti-microbial and anti-oxidant potential (Fraser and Chapple, 2011; Mahmoudi et al, 2016).…”

“…Therefore, our finding on the early increase of enzyme activities between 3 and 24 h after pathogen infection supports this hypothesis. This state of enhanced physiological and biochemical readiness could be used by the host as a potential platform for further enhancement of oxidative and/or anti-oxidative defense in case of pathogen attack (Kawano, 2003; Van Buyten and Höfte, 2013). …”

The role of thionins in rice defence against root pathogens

“…() and Van Buyten and Höfte (). Pythium graminicola strain PB912 132 (Van Buyten and Höfte, ), isolated from a diseased aerobic rice field in Los Banos, Philippines, was cultivated on potato dextrose agar (PDA; Difco Laboratories, Bierbeek, Belgium) at 28 °C. The husks of rice seeds (wild‐type, empty vector and OX22) were removed and the seed surfaces were sterilized with 6% sodium hypochlorite for 20 min, and subsequently rinsed three times with sterile water.…”

“…Pythium graminicola strain PB912 132 culture, inoculation and disease monitoring were performed according to De Vleesschauwer et al . () and Van Buyten and Höfte (). Pythium graminicola strain PB912 132 (Van Buyten and Höfte, ), isolated from a diseased aerobic rice field in Los Banos, Philippines, was cultivated on potato dextrose agar (PDA; Difco Laboratories, Bierbeek, Belgium) at 28 °C.…”

“…After rapid colonization of rice roots and the rice seed surface, Pythium spp. may kill the seed embryo prior to germination or penetrate seedling roots directly and spread intra‐ and intercellularly as hemi‐biotrophic pathogens (Chun and Schneider, ; De Vleesschauwer et al ., ; Van Buyten and Höfte, ). Strong root necrosis and nutrient depletion stimulate the end of the pathogen life cycle, which is paralleled by oospore production inside the tissue.…”