Emerging strategies for enhancing crop resistance to microbial pathogens.

There are marked differences in the pattern of host gene expression in incompatible plant:microbial pathogen interactions compared with compatible interactions, associated with the elaboration of inducible defenses. Constitutive expression of genes encoding a chitinase or a ribosome-inactivating protein in transgenic plants confers partial protection against fungal attack, and a large repertoire of such antimicrobial genes has been identified for further manipulation. In addition, strategies are emerging for the manipulation of multigenic defenses such as lignin deposition and synthesis of phytoalexin antibiotics by overexpression of genes encoding rate determining steps, modification of transcription factors or other regulatory genes, and engineering production of novel phytoalexins by interspecies transfer of biosynthetic genes. The imminent cloning of disease resistance genes, further molecular dissection of stress signal perception and transduction mechanisms, and identification of genes that affect symptom development will provide attractive new opportunities for enhancing crop protection. Combinatorial integration of these novel strategies into ongoing breeding programs should make an important contribution to effective, durable field resistance.