Page 2 Plant-Bacteria Interactions Edited by Iqbal Ahmad, John ...
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Beijerinckia and Derxia were to be included in Azotobacteraceae, but Beijerinckia<br />
remained in the family Beijerinckiaceae of Alphaproteobacteria and Derxia has been<br />
placed in the family Alcaligenaceae of Betaproteobacteria [87].<br />
4.4.3<br />
Firmicutes. Genera Bacillus and Paenibacillus<br />
4.4 Asymbiotic <strong>Plant</strong> Growth Promoting <strong>Bacteria</strong>j73<br />
A review on the applications and systematics of Bacillus and related genera presented<br />
at a meeting held in Brugges (Belgium) in August 2000 ( Bacillus 2000 Meeting) was<br />
published in 2002 [88].<br />
4.4.3.1 Bacillus<br />
Numerous Bacillus strains have been reported to be PGPR [3,5,11,15,18,24,89] and<br />
these employ the widest range of plant growth promoting mechanisms found for<br />
any genera as revealed in Table 4.1 and reviewed <strong>by</strong> Chanway [90].<br />
The historyofthegenusBacillusoriginatedearlyinthe historyofbacteriology,whenit<br />
was proposed <strong>by</strong> Cohn in 1872 and subsequently experienced great fluctuations in the<br />
number of valid Bacillus species recognized in Bergey s Manual of Bacteriology,ranging<br />
froma peak of 146 species in the fifth edition [91] to the lowest number(22)in the eighth<br />
edition [92]. The establishment of the phylogenetic relationships among the different<br />
type strains of Bacillus species and the use of polyphasic taxonomy applied to the genus,<br />
made possible the splitting of Bacillus sensu lato into 11 genera [93]. However, far from<br />
solving its taxonomy, the application of phylogenetic studies to the genus made evident<br />
that groupings used in the traditional phenotypically based schemes for Bacillus [94,95]<br />
did not always correlate with current, phylogenetically led classifications [96] and taxonomic<br />
progress has not yet revealed readily determinable features characteristic of each<br />
genus. Many species described recently represent genomic groups disclosed <strong>by</strong> DNA–<br />
DNApairingexperiments,androutinephenotypiccharacteristicstodistinguishsomeof<br />
these species are very few and of unproven value [97]. In relation to the taxonomy of<br />
Bacillus strains describedasPGPR, itisimportant toemphasizethepossibilityofstrains<br />
identified as B. circulans or B. firmus as having been incorrectly classified given the<br />
difficult taxonomic position of members in these two species, whose taxonomy is under<br />
revision at present (Dr Rodríguez-Díaz, personal communication).<br />
4.4.3.2 Paenibacillus<br />
ThegenusPaenibacilluswasdescribed<strong>by</strong>Ashetal.[98]followingthestudyof16SrRNA<br />
gene sequences of the type strains of many Bacillus species. Since then, the genus was<br />
reassessed <strong>by</strong> Heyndrickx et al. [99] to accommodate former Bacillus species and the<br />
transfer of Bacillus species to Paenibacillus seems to have no end. For example, numerousstrainsofBacilluscirculanssensulatohavebeenfoundtobelongtoninePaenibacillus<br />
species [98–101]. Examples of PGPR described as Bacillus species indeed being membersof<br />
thegenusPaenibacillusarefound intheearlystudiesof plantgrowth promotion<br />
<strong>by</strong> bacteria, mainly with regard to nitrogen fixation. Nonsymbiotic nitrogen fixation <strong>by</strong><br />
a strain of the genus Bacillus was first reported <strong>by</strong> Bredemann in 1908, but this claim<br />
was later discredited due to uncertainties in the determination of the fixed nitrogen
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