and investigations should address the behavior of the pests in situ, not just in the laboratory.

One role of signaling between different organisms within communities can be seen in the response of pests to signals from potential host plants. For example, acetosyringone released from wounds of certain plant species induces the transcription of the vir genes of Ti plasmid-containing strains of Agrobacterium tumefaciens, initiating the process of crown gall formation (Gelvin, 1992). It is reasonable to assume that similar signaling occurs between many of the coevolved components of rhizosphere and phylloplane communities.

One of the most useful applications of chemical signaling has been the use of pheromones for trapping and quantifying populations of specific arthropods. Although there has as yet been no comparable practical applications in the control of plant pathogens, pheromones are known to control fungal sexual cycles (Spellig et al., 1994). With better understanding, fungal pheromones can be used to control fungal behavior.

Using Natural Antagonism, Toxicity, and Antibiosis to Control Pests

During the past decade, compelling evidence of the ecological roles of natural antibiotics produced in situ by microorganisms has been reported (Weller and Thomashow, 1993). Whereas many scientists formerly believed that antibiotic compounds were produced as artifacts of laboratory culture conditions, it is now known that such compounds can be and are produced by microorganisms—e.g., B. thuringiensis and A. radiobacter—inhabiting natural substrates and in concentrations adequate to inhibit pests. Microorganisms that produce antibiotic compounds are among the most successful biological-control agents. Future research could lead to

  • production of novel antibiotics through domain switching, as has been done for polyketide antibiotics;

  • understanding when genes encoding antibiotic biosynthetic enzymes are expressed in situ and what environmental cues induce antibiotic biosynthesis;

  • optimizing in situ antibiotic production by placing biosynthetic operons under regulatory control of constitutive promoters or by modifying the biological environment to enhance biosynthetic gene expression; and

  • evaluation of pest resistance to antibiotics or toxins produced by biological-control agents.

To date, manipulation of genes encoding the -endotoxin (Bt toxin) of B. thuringiensis has received the most attention for practical application as a biological insecticide. B. thuringiensis produces massive amounts of these crystalline insecticidal proteins, which differ in shape, size, and host-specificity. Bt-toxin genes are being introduced into plants as resistance genes; into endophytic bacteria that reside in the xylem of plants; and into bacteria that, after heat-

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