## 6 Genetic and Genomic Diversity of DAPG-Producing _Pseudomonas_ spp.  
An important diversity of DAPG-producing _Pseudomonas_ spp. has been isolated across the world from the rhizosphere of a myriad of plants [4; 122; 123]. The genetic diversity of DAPG-producing _Pseudomonas_ spp. has been studied by various methods, including genomic fingerprinting using BOX/ERIC-PCR [97; 124; 125], restriction fragment length polymorphism of _phlD_[97; 123; 125; 126; 127; 128] and phylogenetic analysis of _phlD_[127; 129]. This led to the description of 22 genotypes (genotypes A-Q, R, S, T, PFY, PfZ) of DAPG-producing _Pseudomonas_ spp. [4]. It is noteworthy that several genotypes are often present in the same field [93; 97; 98; 124; 125]. As an example, 5 distinct genotypes (B, C, D, E and F) were isolated from the rhizosphere of wheat grown in the Quincy soils (Washington State, USA) [124]. The phylogeny of DAPG-producing _Pseudomonas_ spp. was also studied using multi-locus sequence analysis [35], leading to their classification into six distinct phylogenetic groups, with groups from A to E corresponding to lineages from the _P. corrugata_ subgroup and group F corresponding to lineages from the _P. protegens_ subgroup [18; 35; 42]. Several studies indicate that DAPG production also occurs outside of these six phylogenetic groups [18; 130].  
The genomes of numerous DAPG-producing _Pseudomonas_ strains have been sequenced and, as of today, 151 genomes harbouring the _phl_ BCG can be found in the _Pseudomonas_ Genome Database [131]. This includes the genomes of well-known strains Pf-5, F113, Q2-87, Q8r1-96 and CHA0 [132; 133; 134; 135]. The analysis of these genomes has enabled the discovery of several unknown phytochemical traits. For example, three orphan gene clusters were found in the genome of _P. protegens_ Pf-5 [132]. This led to the characterization of two novel antibiotics, namely rhizoxin [136] and oramide [137].  
DAPG-producing _Pseudomonas_ strains harbour numerous BCG involved in the production of secondary metabolites. Most DAPG-producing strains harbour the _hcnABC_ cluster [42; 138], which is responsible for the production of hydrogen cyanide (HCN) [139]. HCN was shown to contribute to the suppression of tobacco black root rot when produced by _P. protegens_ CHA0, as a mutant impaired in HCN production protected tobacco plants less effectively than the wild type did [140]. Likewise, DAPG and HCN were both shown to contribute to the suppression of bacterial cancer of tomato, a disease caused by _Clavibacter michiganensis_ subsp. _michiganensis_[100]. Several strains from the _P. protegens_ subgroup harbour numerous BCG involved in the production of secondary metabolites. For example, _P. protegens_ Pf-5 can produce six metabolites toxic to various oomycetes, fungi, and bacteria: DAPG, hydrogen cyanide, pyoluterorin, pyrrolnitin, rhizxin and oramide [132; 137]. The diversity of phytochemicalatik traits present in the different DAPG-producing strains may act in synergy to control various plant pathogens or it could expand the biocontrol range by controlling plant pathogens less susceptible to DAPG-mediated inhibition.  
The diversity of DAPG-producing _Pseudomonas_ spp. is also important, as different genotypes colonize the rhizosphere of various plants with different abilities. For example, strains from genotype D (which includes strain Q8r1-96) are aggressive colonizers of the wheat root surface and rhizosphere, and represent the most common genotype isolated from the rhizosphere of plants grown in Washington State soils that are suppressive to take-all [141]. Genotype K (which includes _P. kiloensis_ F113) also colonizes the rhizosphere of wheat to a similar extent, while genotype P was not as efficient [142]. However, genotype P outcompetes genotype D and K in the pea rhizosphere [142]. This diversity could be used to select biocontrol agents that are adapted to colonize the rhizosphere of a specific plant species.