Endophitic infection by toxigenic strain of Fusarium verticillioides on plant development: a comparative study between resistant and susceptible maize hybrids.

ARIAS, SL; THEUMER, MG; RUBINSTEIN HR

Mérida

Congreso; VI Latinamerican Congress of Mycotoxins, II International Symposium on Fungal and Algal Toxins in Industry.; 2010

Background: In Córdoba, Argentina, corn is one of the most important cereal grains produced, being a significant income for regional economy. The fungus Fusarium verticillioides is a pathogen that is associated with disease at all stage of maize (Zea mays L) plant development, including seed rot, damping- off, roots rot, stalk rot and kernel or ear rot and produces fumonisins, food-borne carcinogenic mycotoxin. Consumption maize contaminated with fumonisins has been implicated in a number of animal and human illnesses, including esophageal cancer and neural tube defects. The substantial efforts consist of understand the toxicology of fumonisins as well as the maize-Fusarium pathosistem. Aim: To investigate the fungal distribution and the time course for expression of disease symptoms in maize seedling infected by fumonisin-producing strain of F. verticillioides. Materials and Methods: Two maize hybrids that had exhibited resistance (LT 622 MG) and susceptibility (HX 31P77) to Fusarium ear rot in previous experiments (Presello et al., 2009) were evaluated after seed infection with an inoculum of 1012 CFU/ml of F. verticillioides (2024 strain, UNRC). Three replicates of 10 seed each were planted in 24-cm diameter pots containing autoclaved sand under greenhouse conditions. The seedlings were harvested on days 7, 14 and 21. After harvest, the plants were washed and the roots were separated from the aerial parts and the seeds were removed. Plant height and principal root length were measured on fresh material. Then, the different plant parts were dried in an oven at 60 °C until constant weight for later determination of total biomass, aerial parts and root dry weigh. Data of growth of seedling were compared for significant difference, based on t test, between inoculated and non- inoculated kernel at each sampling date and treatment, as well as for eventual differences between hybrids. The fungal distribution was determined placing surface-disinfected segments from the root, mesocotyl, node, stem and oldest leaf on PDA. A total of 10 segments per plant from each 24 plants were analyzed. Segments from which the fungal grew on PDA were scored as positive and the percentage of positive responses was calculated for each part plant. In order to evaluate the ability to produce fumonisins, a conidial suspension of F. verticillioides 2024 strain was directly inoculated onto soaked, autoclaved, whole maize kernels. The samples were incubated at 25 °C in the dark for 4 weeks. Separation and purification of the toxin were performed in the fermented maize following the methodology of Voss et al. (1996). Samples (100 µl) from the watered extracts were diluted with acetonitrile (100 µl) and they were analyzed by HPLC as described Shepard et al. (1990).   Results: The strain of F. verticillioides showed to be a highly fumonisin producer on maize kernels. Evidence of seedling disease depending on the hybrid was observed. For HX 31P77 hybrid, when considering total biomass, F. verticillioides negatively affected Zea mays L. growth at 14 days and persisted for at least 21 days after planting of inoculated seeds. The reductions in both dry mass of root and arterial parts plant (leaves and stems) of infected seedling were significantly greater than noninfected seedling at 14 (P < 0, 04) and 21 (P < 0,001) days. At 21 days the mean height and principal root length of the seedlings were decreased (P < 0, 05). In contrast, the most resistant hybrid in the 21 day assay there was no significant difference on total biomass, dry mass of root and arterial parts plant between the inoculated and non inoculated treatments. However, at day 14, total biomass and dry weights of aerial parts of infected seedling exceeded that of non infected seedling (P < 0, 001). Height and principal root length by days 14 and 21 were significantly higher for infected than for noninfected kernels (P < 0,001 y P < 0, 04, respectively). F. verticillioides was isolated from both hybrids inoculated at each sampling date, but the highest mean infection was found in susceptible than a resistant hybrid. Mesocotyls was the specific plant parts most affected for the infection in all samples. For the rest of the segments, the percent of infection was similar in the resistant plant. Henceforth, the highest infection was evident in root, mesocotys and nodes and the lower infection was found in leaves and stems, in susceptible hybrids.          Discussion and Conclusion: We found that resistant and susceptible hybrids differed in terms of growth parameters. In fast-growing species as maize, in general, are expected that plants maximize resource acquisition. These species are characterized by a set of leaf traits enabling high carbon acquisition and roots by high specific root length, which is usually, associated with rapid rates of root elongation, high relative grown rate, high nutrient and water uptake capacities and high metabolic activities. Our result showed that the root systems of susceptible hybrid presented reduced development; therefore it is less efficient in terms of the acquisition of mobile resources than the controls and resistant hybrids. For the other hand, morphologic characteristic of infected resistant plant seems beneficial for the host, where the fungus could stimulate growth and development of maize plant, possibly due to the production of plant-growth promoting hormones.   One possible explanation for the symptom/symtomless association with fumonisin-producing strains of F. verticillioides could be the induction of potentials mechanism of action triggered by fumonisins in maize, in this case, the host could be suppress or no fungal pathogen. These issues deserve further study. References:http://www.inta.gov.ar/PERGAMINO/info/documentos/t_maíz/09/artic73.htm Voss KA, Riley RT, Bacon CW, Chamberlain WJ, Corred WP.1996.Subchronic toxic effects of Fusarium verticillioidesand fumonisin B1 in rats and mice. Nat Toxins; 4:16-23.   Shephard GS, Sydenham EW, Thiel PG, Gelderblom WCA. 1990. Quantitative determination of fumonisins B1 and B2 by high- performance liquid chromatography with fluorescence detection. 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