During the past decade the pesticidal bacterium Bacillus thuringiensis has been the subject of intensive research. These efforts have yielded considerable data about the complex relationships between the structure, mechanism of action, and genetics of the organism’s pesticidal crystal proteins, and a coherent picture of these relationships is beginning to emerge. Other studies have focused on the ecological role of the B. thuringiensis crystal proteins, their performance in agricultural and other natural settings, and the evolution of resistance mechanisms in target pests. Armed with this knowledge base and with the tools of modern biotechnology, researchers are now reporting promising results in engineering more-useful toxins and formulations, in creating transgenic plants that express pesticidal activity, and in constructing integrated management strategies to insure that these products are utilized with maximum efficiency and benefit.

Bacillus thuringiensis and Its Pesticidal Crystal Proteins

In recent years, the diamondback moth, Plutella xylostella (Lepidoptera: Yponomeutidae), has become the most destructive insect of cruciferous plants throughout the world, and the annual cost for managing it is estimated to be US $I billion (168) Members of the plant family Cruciferae occur temperate and tropical climates and represent a diverse, widespread, and important plant group that includes cabbage, broccoli, cauliflower, collards, rapeseed, mustard, and Chinese cabbage, the most important vegetable crop grown in China (90), the most populous country in the world Although the diamondback moth is believed to have originated in the Mediterranean area (64), the source of some of our most important crucifers (185), diamondback moths now occur wherever cmcifers are grown, and this insect is believed to be the most universally distributed of all Lepidoptera (107) Absence of effective natural enemies, especially parasitoids, is believed to be a major cause of the diamondback moth’s pest status in most parts of the world (92) Lack of parasitoids in a particular area may have occurred because the diamondback moth is better able than its natural-enemy complex to become established in newly planted cmcifers Reports on the ability of diamondback moths to migrate long distances are numerous (19, 40, 54, 58, 108, 120,

http://web.entomology.cornell.edu/shelton/publications/pdf/Talekar-Shelton_1993_Ann_Rev_Ent_DBM.pdf

Biology, Ecology, and Management of the Diamondback Moth

The crystal proteins of Bacillus thuringiensis have been extensively studied because of their pesticidal properties and their high natural levels of production. The increasingly rapid characterization of new crystal protein genes, triggered by an effort to discover proteins with new pesticidal properties, has resulted in a variety of sequences and activities that no longer fit the original nomenclature system proposed in 1989. Bacillus thuringiensis pesticidal crystal protein (Cry and Cyt) nomenclature was initially based on insecticidal activity for the primary ranking criterion. Many exceptions to this systematic arrangement have become apparent, however, making the nomenclature system inconsistent. Additionally, the original nomenclature, with four activity-based primary ranks for 13 genes, did not anticipate the current 73 holotype sequences that form many more than the original four subgroups. A new nomenclature, based on hierarchical clustering using amino acid sequence identity, is proposed. Roman numerals have been exchanged for Arabic numerals in the primary rank (e.g., Cry1Aa) to better accommodate the large number of expected new sequences. In this proposal, 133 crystal proteins comprising 24 primary ranks are systematically arranged.

/pdf/revision-of-the-nomenclature-for-the-bacillus-thuringiensis-2buw579rwz.pdf

Revision of the Nomenclature for the Bacillus thuringiensis Pesticidal Crystal Proteins

We introduced a synthetic gene encoding a truncated version of the CryIA(b) protein derived from Bacillus thuringiensis into immature embryos of an elite line of maize using microprojectile bombardment. This gene was expressed using either the CaMV 35S promoter or a combination of two tissue specific promoters derived from maize. High levels of CryIA(b) protein were obtained using both promoter configurations. Hybrid maize plants resulting from crosses of transgenic elite inbred plants with commercial inbred lines were evaluated for resistance to European corn borer under field conditions. Plants expressing high levels of the insecticidal protein exhibited excellent resistance to repeated heavy infestations of this pest.

Field Performance of Elite Transgenic Maize Plants Expressing an Insecticidal Protein Derived from Bacillus thuringiensis

The use of synthetic organic insecticides developed during the last half of this century may pose risks to human health and can cause environmental prob­ lems. Consequently, interest has developed in using alternative strategies for insect pest management. One contemporary approach that has received atten­ tion is the development of Bacillus thuringiensis (Bt) toxins as insecticides. B. thuringiensis, a gram-positive bacteria, produces a proteinaceous paraspor­ al crystalline inclusion during sporulation. Upon ingestion by insects, this crystalline inclusion is solubilized in the midgut, releasing proteins called S-endotoxins. These proteins (protoxins) are activated by midgut proteases, and the activated toxins interact with the larval midgut epithelium causing a disruption in membrane integrity and ultimately leading to insect death. In spite of significant efforts directed towards the study of Bt, its use in pest control is restricted, in part because of the selectivity of Bt and in part because of its moderate efficacy. Attempts to improve these two qualities have met with only moderate success because inadequate effort has been devoted to understanding the molecular basis of selectivity and insecticidal properties of these toxins.

The mode of action of Bacillus thuringiensis endotoxins.

characterized by its ability to produce crystalline inclusions during spor-ulation. TIhese inclusions consist of proteins exhibiting a highly specific insecticidal activity (reviewed in references 4 and 97). Many B. thuriuigiensis strains with different insect host spectra have been identified (9). TIhey ar-e classified into different serotypes or subspecies based on their flagellar antigens. Most strains are active against larvae of certain members of the Lepidoptera, but some show toxicity against dipterian (reviewed in refer-ence 22) or coleopteran (53) species. For several cryst.al-producing strains.

H.R. Whiteley

Papers

Insecticidal crystal proteins of Bacilllus thuringiensis