Introduction

A major attraction of the application of genetic engineering to microbial inoculant strains is the ability to create new strains with capabilities that may not be easily obtained by a process of natural selection. Such additional features may enhance the performance of an inoculant strain.

The fluorescent Pseudomonas species are an important group of bacteria that can play a beneficial role in the protection of crop plants against deleterious pathogenic micro-organisms. These bacteria produce a wide range of factors antagonistic to pathogens (particularly fungi) they include the yellow-green, fluorescent siderophores that characterize this group.

Our work has concentrated on the role of siderophores and antibiotics of fluorescent Pseudomonas in the biocontrol of ‘damping-off’ disease of sugar-beet seedlings (see Table 14.1). The biosynthesis and excretion of iron-binding siderophores are thought to decrease the pool of iron available to pathogenic fungi and other harmful micro-organisms and so contribute to the plant growth promotion effect observed when plants are inoculated with the appropriate Pseudomonas strain (Kloepper et al 1980). The producing micro-organism can use the resulting Fe3+ (ferric) siderophore complex via specific receptors located in its outer membrane (Magazin et at., 1986).

Outer membrane receptor proteins can be highly specific and only transport the appropriate ferric siderophore into the producing cell. The structures of these fluorescent (yellow-green) siderophores was reviewed by Leong (1986). Fluorescent siderophores are only produced under iron-limiting conditions, as are receptor proteins.