ABSTRACT

Having established the basic requirements for bacterial entry into plants, and some of the factors that determine pathogenicity, we can now consider hostspecific determinants, and address why plants are able to defend themselves against some bacteria but not others. The same basic gene-for-gene concept applies to many bacterial pathogens as for fungal pathogens (see Section 4.1). Bacteria, particularly those that use stealth as their means of attack, possess avirulence genes (avr), expression of which culminates in the production of elicitors, and plants possess resistance genes, the products of which are able to recognise these elicitors and respond. In bacteria, it has been found that avr gene function is generally dependent on the function of a further group of genes, the hrp genes (hypersensitive response and pathogenicity) that are responsible for their export by a type III secretion mechanism. It is the nature of the avirulence genes and these secretion mechanisms, along with the genetics of bacterial plant pathogens that we focus on in this chapter.