Integration of Insecticidal Protein Vip3Aa1 into Beauveria bassiana Enhances Fungal Virulence to Spodoptera litura Larvae by Cuticle and Per Os Infection

Abstract

The entomopathogenic fungus Beauveria bassiana acts slowly on insect pests through cuticle infection. Vegetative insecticidal proteins (Vip3A) of Bacillus thuringiensis kill lepidopteran pests rapidly, via per os infection, but their use for pest control is restricted to integration into transgenic plants. A transgenic B. bassiana strain (BbV28) expressing Vip3Aa1 (a Vip3A toxin) was thus created to infect the larvae of the oriental leafworm moth Spodoptera litura through conidial ingestion and cuticle adhesion. Vip3Aa1 (ϳ88 kDa) was highly expressed in the conidial cytoplasm of BbV28 and was detected as a digested form (ϳ62 kDa) in the larval midgut 18 and 36 h after conidial ingestion. The median lethal concentration (LC 50 ) of BbV28 against the second-instar larvae feeding on cabbage leaves sprayed with conidial suspensions was 26.2-fold lower than that of the wild-type strain on day 3 and 1.1-fold lower on day 7. The same sprays applied to both larvae and leaves for their feeding reduced the LC 50 of the transformant 17.2-and 1.3-fold on days 3 and 7, respectively. Median lethal times (LT 50 s) of BbV28 were shortened by 23 to 35%, declining with conidial concentrations. The larvae infected by ingestion of BbV28 conidia showed typical symptoms of Vip3A action, i.e., shrinkage and palsy. However, neither LC 50 nor LT 50 trends differed between BbV28 and its parental strain if the infection occurred through the cuticle only. Our findings indicate that fungal conidia can be used as vectors for spreading the highly insecticidal Vip3A protein for control of foliage feeders such as S. litura.

Improving mycoinsecticides for insect biological control

“…The grampositive bacterium, Bacillus thuringiensis, is well known for producing a variety of insecticidal toxins including delta endo-or crystalline toxins (CRY, also referred to as Bt-toxins) and vegetative insecticidal proteins (VIPs). Expression of Bt-Vip3A in B. bassiana increased fungal virulence to oriental leafworm moth (Spodoptera litura) larvae, decreasing LD 50 values between 15-to 26-fold and LT 50 values by 23-35 % (Qin et al 2010). The Vip3A-expressing B. bassiana strain represents one of the few examples of actual field trials using a genetically engineering entomopathogenic fungus (Liu et al 2013).…”

Analysis of Whitefly Transcriptional Responses to Beauveria bassiana Infection Reveals New Insights into Insect-Fungus Interactions

“…These disadvantages limit the wide usage of B. bassiana [16]. In order to overcome these difficulties, it is critical to determine crucial virulent factors of B. bassiana , as well as to elucidate how insects respond to fungal infections [24], [25]. A thorough understanding of the basic principles of whitefly-fungus interactions may enable the genetic modification of the fungus to enhance its virulence to the whitefly as achieved in previous studies [26], [27].…”

Strain improvement of fungal insecticides for controlling insect pests and vector-borne diseases

“…It is also possible to mix and match virulence genes from insect pathogenic viruses and bacteria with those from fungi to create novel combinations of insect specificity and virulence. Expressing a gut active toxin (Vip3A) from Bacillus thuringiensis (Bt) did not increase the infectivity of B. bassiana spores, but in contrast to the wild type, the transformants were also lethal following ingestion [19]. Bt toxins expressed by B. bassiana may also be more environmentally stable as, for example, when expressed by Pseudomonas and Rhizobium bacteria [20,21].…”