Entomotoxic effects of fungal lectin from Rhizoctonia solani towards Spodoptera littoralis.

The effects of the Rhizoctonia solani lectin (RSA) on the growth, development and survival of an economically important caterpillar in agriculture and horticulture, the cotton leafworm, Spodoptera littoralis were studied. The high lectin concentration present in the sclerotes of the soil pathogen R. solani allowed the purification of large amounts of the pure lectin for feeding experiments with cotton leafworm. Rearing of insects on a diet containing different concentrations of RSA exerted a strong effect on the larval weight gain. This effect was visible at the lowest concentration of 0.1 % RSA at day 8 and day 11. Interestingly with 1 % RSA, there was a dramatic reduction in larval weight of 89 % at the end of L6 which was followed by a high mortality rate of 82 % in the treated larvae. Furthermore, the other developmental stages of pupation and adult formation were also affected. In addition, the data demonstrated that the combination of RSA with Bt toxin yielded synergistic effects. For instance, 0.03 % RSA+0.005 % Bt toxin caused reduced growth rate and higher mortalities. These findings suggest that RSA is an interesting tool that can be used for bioengineering insect resistance in important agronomical crops.

Analysis of lectin concentrations in different Rhizoctonia solani strains.

Lectins are carbohydrate-binding proteins that contain at least one carbohydrate binding domain which can bind to a specific mono- or oligosaccharide. These proteins are widely distributed in plants. However, over the last decade evidence is accumulating that lectins occur also in numerous fungi belonging to both the Ascomycota and Basiodiomycota. Rhizoctonia solani is known to be an important pathogen to a wide range of host plants. In this study, isolates of R. solani from different anastomosis groups have been screened for the presence of lectin using agglutination assays to detect and quantitate lectin activity. The evaluation included determination of the lectin content in mycelium as well as in sclerotia. The amount of lectin in the sclerotia was higher than in the mycelium of the same strains. The R. solani strains with the highest amounts of lectin have been selected for cultivation, extraction and purification of the lectin.