In this study we cloned a chitinase gene (SmchiC), from Serratia marcescens isolated from the corpse of a Diatraea magnifactella lepidopteran, which is an important sugarcane pest. The chitinase gene SmchiC amplified from the S. marcescens genome was cloned into the transformation vector p2X35SChiC and used to transform tobacco (Nicotiana tabacum L. cv Petit Havana SR1). The resistance of these transgenic plants to the necrotrophic fungus Botrytis cinerea and to the pest Spodoptera frugiperda was evaluated: both the activity of chitinase as well as the resistance against B. cinerea and S. frugiperda was significantly higher in transgenic plants compared to the wild-type.
The objective of this paper was to analyze information related to entomopathogenic-associated Diatraea spp. Gaining a better understanding of the effects of these microorganisms will help in the development of successful microbial control strategies against stem borers that attack sugarcane plants.
The fall armyworm, Spodoptera frugiperda, is an invasive maize pest that has spread from the Americas into Africa and Asia causing severe crop damages worldwide. Most populations of S. frugiperda show low susceptibility to Bacillus thuringiensis (Bt) Cry1Ab or Cry1Ac toxins, which have been proved to be effective against several other lepidopteran pests. In addition, S. frugiperda has evolved resistance to transgenic maize expressing Cry1Fa toxin. Cry toxins specificity and toxicity is determined by their binding to different larvae midgut proteins such as aminopeptidase-N (APN), alkaline phosphatase (ALP) and cadherin (CAD), among other proteins, by means of exposed domain II loop regions and also by domain III β16-β22 region. Here we analyzed different Cry1Ab mutants in domain III β22 region. The alanine scanning mutagenesis of this region revealed that all mutants showed increased toxicity against a non-susceptible Cry1Ab S. frugiperda population. Further analysis of the mutant toxin Cry1AbS587A revealed that it showed significant increased toxicity, compared to Cry1Ab, to other three different S. frugiperda populations from Mexico but retained similar toxicity to Manduca sexta larvae. The Cry1AbS587A bound to BBMV and its higher toxicity correlated with a higher binding affinity to APN, ALP and CAD recombinant proteins. Furthermore, silencing the expression of APN1 and CAD receptors in S. frugiperda larvae by RNAi showed that Cry1AbS587A toxicity relied on CAD expression in contrast to Cry1Ab. These data support that the increased toxicity of Cry1AbS587A to S. frugiperda is in part due to an improved binding interaction with CAD receptor.
IMPORTANCE Spodoptera frugiperda is an important worldwide pest of maize and rice crops that has evolved resistance to Cry1Fa-maize in different countries. Therefore, identification of additional toxins with different mode of action is needed to provide alternative tools to control this insect pest. Bacillus thuringiensis (Bt) Cry1Ab and Cry1Ac toxins are highly active against several important lepidopteran pests but show variable and low toxicity against different S. frugiperda populations. Thus, identification of Cry1A mutants that gain toxicity to S. frugiperda and retain toxicity to other pests could be of great value to produce transgenic crops that resist a broader spectrum of lepidopteran pests. Here we characterized Cry1Ab domain III β22 mutants, and we found that Cry1AbS587A mutant display increased toxicity against different S. frugiperda populations. Thus, Cry1AbS587A could be a good toxin candidate to produce transgenic maize with broader efficacy against this important insect pests in the field.
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