Caracterização de isolados de Xanthomonas citri subsp. malvacearum e redução da mancha-angular do algodoeiro mediada pelo silício

Abstract

The bacterial blight caused by Xanthomonas citri subsp. malvacearum (Xcm) is the most important bacterial disease of cotton in the Middle-West and Northeast Regions of Brazil. The variability of a population formed by 100 Xcm strains obtained from commercial fields in the states of Goiás, Mato Grosso, Mato Grosso do Sul and Bahia, Brazil, was analyzed based upon genetic and phenotypic characteristics. The enzymes amylase, lipase, cellulase and the polysaccharide levan were produced by all Xcm isolates while the caseinase was only produced by 20 strains. All strains induced hipersensitivity reaction in tomato leaves. The 100 strains were sensitive to copper oxychloride (1500 mg L-1), copper hidroxide (1614 mg L-1), oxytetracicline (600 mg L-1), oxytetracicline (90 mg L-1) + tribasic copper sulfate (1500 mg L-1) and oxytetracicline (76.6 mg L-1) + streptomycin sulfate (367.2 mg L-1), but resistant to kasugamycin (60 mg L-1). Among the eighteen tested antibiotics the strains presented variable reaction in relation to: nalidixic acid, amoxicillin, azithromycin, bacitracin, eritromicin, kanamycin, pefloxacin, tobramycin e vancomycin. Rep-PCR (REP, ERIC and BOX) showed the formation of five groups at 70% similarity level. Group I contained 98 strains, including the reference strain Xcm IBSBF1733, group II contained only two strains from Bahia and the other three groups included strains of Xanthomonas citri pv. anacardii, X. citri pv. mangiferaeindicae e X. axonopodis pv. spondiae used for comparison. This work also evaluated the effect of silicon (Si) on the bacterial blight of cotton control and the putative mechanisms involved in the resistance potentialized by this element were evaluated. Calcium silicate (CaSiO3) was incorporated into the soil at concentrations of 0.00, 0.25, 0.50, 1.50 and 3.00 g of SiO2 kg-1 soil 25 days before planting. Leaves of 33-day-old-plants were inoculated by infiltration with 0.5 mL suspension of Xcm (108 CFU ml-1). Components of resistance were evaluated every two days until 10 days after inoculation, at which point plant development and the accumulation of Si and Calcium (Ca) were also determined. H2O2 production and the activity of enzymes related to plant defenses were analyzed at 6, 12 and 24 hours after inoculation in plants +/-Si. In vitro inhibition of pathogen growth due to Si was also assessed. No significant difference was seen among the treatments regarding incubation period, disease incidence or inhibition of bacterial growth. However, with the application of 1.50 g of SiO2 kg-1 of soil, a reduction in severity (54.9%) was observed along with a reduction in the area under the disease progression curve (35.76%) and an increase in plant height (7.04%). There was no observable accumulation of Si in the cotton leaves. Levels of soluble proteins and H2O2 and activity of the enzymes SOD, APX, guaiacol peroxidase, PAL and b Glu were altered in the presence of Si (1.80 g SiO2 kg-1). Thus based on the genetic and phenotypic characteristics studied we conclude that the population of 100 Xcm strains presents low variability, and that the reduction of cotton bacterial blight severity mediated by Si is probably associated to the characteristic events of induced resistance observed.