Screening, diversity and partial sequence comparison of vegetative insecticidal protein (vip3A) genes in the local isolates of Bacillus thuringiensis Berliner

Unraveling the abundance of vip3-type genes in Indian Bacillus thuringiensis across the agroclimatic landscape and impact of amino acid substitutions for safer agriculture

Vegetative insecticidal protein (vip) genes of Bacillus thuringiensis (Bt) are candidates for gene pyramiding in the resistance management of pests. The prevalence of vip genes in Bt isolates is relatively under-explored. Bt isolates recovered from 29 diverse sources in nine agro-climatic zones of India were screened for the presence of vip3-type genes by PCR with 4 sets of oligonucleotide primers. Out of 155 Bt isolates, 70.32 % (109) and 44.52 % (69) isolates were positive for amplification of partial vip3-type genes with primer sets 1 and 4, respectively. The primer set-2 was found to be more efficient for amplifying full-length genes (29.03 % /45 isolates) as compared with primer set-3 (3.23 %/ 5 isolates), also corroborated in the amplification of full-length vip3 genes in ten Bt BGSC strains used as reference. Frequency analysis revealed presence of vip3 genes in Bt isolates across all agro-climatic zones. Thus, Indian Bt isolates from diverse sources have a rich repertoire of vip3-type genes. Our study reports the highest number (45) of full-length vip3-type genes detected in a native Bt isolates collection, demonstrating enrichment of Indian Bt isolates for vip3 genes. Twelve of these genes have been cloned, sequenced, and out of these, six were found to be effective against Helicoverpa armigera in our laboratory previously. Comparison of substitutions in deduced amino acids sequence of these genes and expression of Vip3 proteins in SDS-PAGE analysis of selected native Bt isolates positive for full-length vip3-type genes indicated their biopesticidal potential.

Vegetative Insecticidal Protein (Vip): A Potential Contender From Bacillus thuringiensis for Efficient Management of Various Detrimental Agricultural Pests

Bacillus thuringiensis (Bt) bacterium is found in various ecological habitats, and has natural entomo-pesticidal properties, due to the production of crystalline and soluble proteins during different growth phases. In addition to Cry and Cyt proteins, this bacterium also produces Vegetative insecticidal protein (Vip) during its vegetative growth phase, which is considered an excellent toxic candidate because of the difference in sequence homology and receptor sites from Cry proteins. Vip proteins are referred as second-generation insecticidal proteins, which can be used either alone or in complementarity with Cry proteins for the management of various detrimental pests. Among these Vip proteins, Vip1 and Vip2 act as binary toxins and have toxicity toward pests belonging to Hemiptera and Coleoptera orders, whereas the most important Vip3 proteins have insecticidal activity against Lepidopteran pests. These Vip3 proteins are similar to Cry proteins in terms of toxicity potential against susceptible insects. They are reported to be toxic toward pests, which can’t be controlled with Cry proteins. The Vip3 proteins have been successfully pyramided along with Cry proteins in transgenic rice, corn, and cotton to combat resistant pest populations. This review provides detailed information about the history and importance of Vip proteins, their types, structure, newly identified specific receptors, and action mechanism of this specific class of proteins. Various studies conducted on Vip proteins all over the world and the current status have been discussed. This review will give insights into the significance of Vip proteins as alternative promising candidate toxic proteins from Bt for the management of pests in most sustainable manner.

Bacterial Vegetative Insecticidal Proteins (Vip) from Entomopathogenic Bacteria

Entomopathogenic bacteria produce insecticidal proteins that accumulate in inclusion bodies or parasporal crystals (such as the Cry and Cyt proteins) as well as insecticidal proteins that are secreted into the culture medium. Among the latter are the Vip proteins, which are divided into four families according to their amino acid identity. The Vip1 and Vip2 proteins act as binary toxins and are toxic to some members of the Coleoptera and Hemiptera. The Vip1 component is thought to bind to receptors in the membrane of the insect midgut, and the Vip2 component enters the cell, where it displays its ADP-ribosyltransferase activity against actin, preventing microfilament formation. Vip3 has no sequence similarity to Vip1 or Vip2 and is toxic to a wide variety of members of the Lepidoptera. Its mode of action has been shown to resemble that of the Cry proteins in terms of proteolytic activation, binding to the midgut epithelial membrane, and pore formation, although Vip3A proteins do not share binding sites with Cry proteins. The latter property makes them good candidates to be combined with Cry proteins in transgenic plants (Bacillus thuringiensis-treated crops [Bt crops]) to prevent or delay insect resistance and to broaden the insecticidal spectrum. There are commercially grown varieties of Bt cotton and Bt maize that express the Vip3Aa protein in combination with Cry proteins. For the most recently reported Vip4 family, no target insects have been found yet.

Bacterial Vegetative Insecticidal Proteins (Vip) from Entomopathogenic Bacteria

Entomopathogenic bacteria produce insecticidal proteins that accumulate in inclusion bodies or parasporal crystals (such as the Cry and Cyt proteins) as well as insecticidal proteins that are secreted into the culture medium. Among the latter are the Vip proteins, which are divided into four families according to their amino acid identity. The Vip1 and Vip2 proteins act as binary toxins and are toxic to some members of the Coleoptera and Hemiptera. The Vip1 component is thought to bind to receptors in the membrane of the insect midgut, and the Vip2 component enters the cell, where it displays its ADP-ribosyltransferase activity against actin, preventing microfilament formation. Vip3 has no sequence similarity to Vip1 or Vip2 and is toxic to a wide variety of members of the Lepidoptera. Its mode of action has been shown to resemble that of the Cry proteins in terms of proteolytic activation, binding to the midgut epithelial membrane, and pore formation, although Vip3A proteins do not share binding sites with Cry proteins. The latter property makes them good candidates to be combined with Cry proteins in transgenic plants (Bacillus thuringiensis-treated crops [Bt crops]) to prevent or delay insect resistance and to broaden the insecticidal spectrum. There are commercially grown varieties of Bt cotton and Bt maize that express the Vip3Aa protein in combination with Cry proteins. For the most recently reported Vip4 family, no target insects have been found yet.

A deletion mutant ndv200 of the Bacillus thuringiensis vip3BR insecticidal toxin gene is a prospective candidate for the next generation of genetically modified crop plants resistant to lepidopteran insect damage

MAIN CONCLUSION

Ectopic expression of a deletion mutant ( ndv200 ) of Bacillus thuringiensis vip3BR gene in tobacco plant provided almost complete protection against major crop pests cotton boll worm ( Helicoverpa armigera ), black cut worm ( Agrotis ipsilon ) and cotton leaf worm ( Spodoptera littoralis ). Whereas vip3BR transgenic tobacco plant failed to protect themselves from these insects and showed resistance towards cotton leaf worm only. An analogous form of the Bacillus thuringiensis vip3Aa insecticidal toxin gene, named vip3BR, was identified and characterized, and exhibited similar attributes to the well-known Vip3Aa toxin. Vip3BR possessed broad-spectrum lepidopteran-specific insecticidal properties effective against most major crop pests of the Indian subcontinent. A Vip3BR toxin protein N-terminal deletion mutant, Ndv200, showed increased insecticidal potency relative to the native toxin, which conferred efficacy against four major crop pests, including cotton boll worm (Helicoverpa armigera), black cut worm (Agrotis ipsilon), cotton leaf worm (Spodoptera littoralis), and rice yellow stem borer (Scirpophaga incertulas). Ligand blot analysis indicated the Ndv200 toxin recognized the same larval midgut receptors as the native Vip3BR toxin, but differed from receptors recognized by Cry1A toxins. In the present study, we tested the prospect of the vip3BR and ndv200 toxin gene as candidate in development of insect-resistant genetically engineered crop plants by generating transgenic tobacco plant. The study revealed that the ndv200 mutant of vip3BR insecticidal toxin gene is a strong and prospective candidate for the next generation of genetically modified crop plants resistant to lepidopteran insects.

A screening of five Bacillus thuringiensis Vip3A proteins for their activity against lepidopteran pests

Five Bacillus thuringiensis Vip3A proteins (Vip3Aa, Vip3Ab, Vip3Ad, Vip3Ae and Vip3Af) and their corresponding trypsin-activated toxins were tested for their toxicity against eight lepidopteran pests: Agrotis ipsilon, Helicoverpa armigera, Mamestra brassicae, Spodoptera exigua, Spodoptera frugiperda, Spodoptera littoralis, Ostrinia nubilalis and Lobesia botrana. Toxicity was first tested at a high dose at 7 and 10 days. No major differences were found when comparing protoxins vs. trypsin-activated toxins. The proteins that were active against most of the insect species were Vip3Aa, Vip3Ae and Vip3Af, followed by Vip3Ab. Vip3Ad was non-toxic to any of the species tested. Considering the results by insect species, A. ipsilon, S. frugiperda and S. littoralis were susceptible to Vip3Aa, Vip3Ab, Vip3Ae and Vip3Af; S. exigua was susceptible to Vip3Aa and Vip3Ae, and moderately susceptible to Vip3Ab; M. brassicae and L. botrana were susceptible to Vip3Aa, Vip3Ae and Vip3Af; H. armigera was moderately susceptible to Vip3Aa, Vip3Ae and Vip3Af, and O. nubilalis was tolerant to all Vip3 proteins tested, although it showed some susceptibility to Vip3Af. The results obtained will help to design new combinations of insecticidal protein genes in transgenic crops or in recombinant bacteria for the control of insect pests.

R, R. A, Mahmood R, N.K. KK. Isolation and identification of culturable bacteria from honeydew of whitefly, Bemisia tabaci (G.) (Hemiptera: Aleyrodidae)

Bemisia tabaci (G.) is an important pest and a vector of Gemini viruses infecting plants. During the process of feeding B. tabaci excretes honeydew which is rich in nutrients, and an excellent medium for microbial growth. Recent report proved that volatile emitted by the honeydew associated bacteria of aphid, Acyrthosiphon pisum Harris was involved in natural enemy calling. Thus understanding the honeydew associated bacteria is of paramount importance from the non-chemical method of insect pest management. In this perspective, very less information is available on bacteria associated with the honeydew excreted by B. tabaci. Therefore, in the present study we have isolated and characterized three culturable bacteria from the honeydew of B. tabaci viz. Bacillus endophyticus, Bacillus niacini and Roseomonas species by employing 16Sr DNA BLASTx analyses which revealed that both B. endophyticus and B. niacini had high similarity (> 99%) to the respective species, while Roseomonas sp. showed only 95% similarity to the existing Roseomonas sp. specificity of honeydew association of Roseomonas sp. was confirmed by developing specific primers as this genus is reported from immunocompromised persons and recently from ticks and mites. The present study also indicated the possible host-plant origin of these honeydew associated bacteria.

Purification and characterization of Bacillus thuringiensis vegetative insecticidal toxin protein(s)

Bacillus thuringiensis subsp. aegypti C18 is an Egyptian isolate, obtained from dead pink bollworm larvae. Insecticidal active proteins against different insect were purified from BtaC18 strain during vegetative states. Both the bacterial pellet and cell-free supernatant obtained during vegetative growth had insecticidal activity against black cutworm (BCW). Bioassays revealed that the pellet after 48 h of growth is more potent and toxic against BCW. The toxin in the pellet was active at very high temperatures but lost toxicity after boiling or autoclaving. Proteins extracted from the BtaC18 pellet were further purified by ammonium sulfate precipitation, and the 40% fraction was then subjected to fast protein liquid chromatography (FPLC). Seven major protein peaks were detected after FPLC (Pi- a, b, c, d, e, f and g). Pic protein fraction was active against BCW with an estimated LC50  = 26 ng cm(-2) , Pid protein killed 50% of European corn borer (ECB) at 46 ng cm(-2) , and Pif showed insecticidal activity against western corn root worm (WCRW) with estimated LC50 was 94 ng cm(-2) . Based on the significant and high toxicity of Pic against BCW and Pif against WCRW, the 88- and 44-kDa proteins were further characterized by N-terminal amino acid sequencing.

SIGNIFICANCE AND IMPACT OF THE STUDY

Insecticidal activity of Bacillus thuringiensis subsp. aegypti was determined, and its vegetative insecticidal protein was subjected to FPLC for protein purification. This work contributes to improve understanding the different toxins secreted during vegetative growth of Bt. Moreover, the N-terminal amino acid sequences of 88-kDa protein was only 92% identical to that of vip3A, and for 44 kDa was 92% identical with Cry35a, suggesting that we might have identified a new genes. Finally, we have proven these proteins to be novel insecticidal agents that may complement the use of known insecticidal proteins derived from Bacillus.