Erradicação de fungos em germoplasma vegetal importado

Resumo: O objetivo deste trabalho foi avaliar os métodos utilizados para a erradicação dos fungos exóticos detectados em germoplasma vegetal importado. Foram utilizadas metodologias convencionais adaptadas na Estação Quarentenária Vegetal Nível 1, da Embrapa Recursos Genéticos e Biotecnologia. No período de 1977 a 2013, foram analisados aproximadamente 300.000 acessos de germoplasma, procedentes de mais de 90 países, pertencentes a mais de 330 produtos diferentes. Dos 37 fungos exóticos detectados, destacam-se quatro espécies quarentenárias que pertencem à Lista de Pragas Quarentenárias Ausentes: Tilletia indica, Phoma exigua var. foveata, Periconia circinata e Dactuliochaeta glycines (Syn. Phoma glycinicola). As medidas fitossanitárias realizadas foram efetivas para a interceptação/erradicação desses patógenos. As metodologias empregadas são fundamentais para a liberação de material genético livre de pragas exóticas e favorecem o enriquecimento genético dos programas de melhoramento do Sistema Nacional de Pesquisa Agropecuária.

Differential Synthesis of Peritoxins and Precursors by Pathogenic Strains of the Fungus Periconia circinata

ABSTRACT Pathogenic strains of the soilborne fungus Periconia circinata produce peritoxins with host-selective toxicity against susceptible genotypes of sorghum. The peritoxins are low-molecular-weight, hybrid molecules consisting of a peptide and a chlorinated polyketide. Culture fluids from pathogenic, toxin-producing (Tox+) and nonpathogenic, non-toxin-producing (Tox−) strains were analyzed directly by gradient high-performance liquid chromatography (HPLC) with photodiode array detection and HPLC-mass spectrometry to detect intermediates and final products of the biosynthetic pathway. This approach allowed us to compare the metabolite profiles of Tox+ and Tox− strains. Peritoxins A and B and the biologically inactive intermediates,N-3-(E-pentenyl)-glutaroyl-aspartate, circinatin, and 7-chlorocircinatin, were detected only in culture fluids of the Tox+ strains. The latter two compounds were produced consistently by Tox+ strains regardless of the amount of peritoxins produced under various culture conditions. In summary, none of the known peritoxin-related metabolites were detected in Tox− strains, which suggests that these strains may lack one or more functional genes required for peritoxin biosynthesis.

Peritoxins and their effects on sorghum

Peritoxins are low molecular weight, chlorinated peptides produced only by pathogenic strains of the sorghum root rot fungus, Periconia circinata. Genetic data relating sensitivity of sorghum (Sorghum bicolor (L.) Moench) to these toxins and susceptibility to the pathogen are consistent with the hypothesis that both traits are controlled by a single semidominant gene (Pc). At low concentrations (5–500 nM), the toxin selectively reproduces disease symptoms, inhibits growth of primary roots, stops mitosis, induces electrolyte leakage, and enhances the synthesis of a group of 16-kDa proteins and the corresponding mRNAs. Enhanced expression of these 16-kDa proteins and disease symptoms are also induced in a nongenotype-specific manner by treatment with mercury, suggesting that the two events are causally related. However, the upregulation of 16-kDa protein expression induced by a fungal elicitor is independent of visible disease symptoms and is, therefore, not a direct cause of damage. Results of experiments with treatments that protect seedlings against the effects of toxin (e.g., proteinase, heat shock, inhibitors of protein or RNA synthesis, inhibitors of protein kinase C activity, and biotinylation of membrane proteins) suggest that disease symptoms result from an interaction of peritoxin with a proteinaceous receptor on or near the cell surface and interference with the normal function of a signal transduction pathway. Key words: peritoxin, sorghum, Periconia circinata, host-selective toxin, milo disease, Periconia root rot.

Periconia circinata. [Distribution map].

A new distribution map is provided for Periconia circinata (Mangin) Sacc. Hosts: Sorghum (Sorghum vulgare[Sorghum bicolor]) etc. Information is given on the geographical distribution in Africa, South Africa, Australasia & Oceania, Australia, Queensland, North America, USA, California, Kansas, New Mexico, Oklahoma, TX.

Morphometric analysis of ultrastructural changes induced by Periconia circinata toxin in the outer root cap of sorghum

Outer root cap cells of sorghum seedlings treated with the host-specific toxin produced by Periconia circinata were analyzed morphometrically to detect changes in the quantities of cytomembranes and numbers of organelles and thus extend our observations of qualitative cytological responses to the toxin. In seedlings susceptible to the pathogen, brief (0.25 h) treatment with the toxin resulted in a marked and permanent decrease in the amounts of secretory vesicle membrane. By 2 h treatment, only secretory vesicle membrane was decreased, but longer treatments led to an increased amount of endoplasmic reticulum (4 h), which later decreased together with the amount of dictyosome membrane, while the amount of tonoplast increased (8 h). In resistant seedlings treated with the toxin, early but transient increases were detected in the quantities of plasma membrane, secretory vesicle membrane, dictyosome membrane, and endoplasmic reticulum and in the number of dictyosomes. Insensitivity to the toxin may involve the ability of resistant genotypes to recover from the toxic effects.

Ultrastructural and developmental alterations induced by Periconia circinata toxin in the root tip of sorghum

Cytological and developmental effects induced by Periconia circinata toxin were examined to better understand the mechanism of action for this toxin. Roots of sorghum seedlings susceptible and resistant to P. circinata were incubated in 500 ng toxin/mL (treated) or water (controls). Root cap cells of resistant seedlings treated with the toxin were cytologically similar to those of controls, although the toxin caused a transient inhibition of mitosis in cells of the primary root tip. In outer root cap cells of susceptible seedlings treated for 0.25 h, hypersecretory activity was lost, secretory vesicles were fewer, and secretory product accumulated between the plasma membrane and cell wall. Also, inner root cap cells showed increased vacuolation. Longer treatments caused increased vacuolation, loss of starch, increased numbers of lipid bodies, pleomorphic amyloplasts, regularly stacked endoplasmic reticulum, apparent changes in the amounts of cytomembranes, dispersion of heterochromatin, and autolysis. Mitochondrial morphology was normal, but lesions in the tonoplast occurred before autolysis. The toxin also inhibited expansion and sloughing off of root cap cells and mitotic activity in the root tip. Stacked endoplasmic reticulum, nonhypersecretory dictyosomes, fewer secretory vesicles, increased vacuolation, reorganization of heterochromatin, and increased secretory product outside the protoplast were induced by P. circinata toxin and by cyanide. These data suggested that a cyanogenic compound is biologically active in cells treated with P. circinata toxin. Our results suggest that the toxin transiently affects resistant seedlings and in susceptible seedlings alters vacuolar expansion, secretory activity, and endomembrane flow, although other processes may also be affected.