In the studies of plant microbe interaction which induced some kind of plant growth promotion, there are other cases that do not fit into the previous definitions but which can be considered as another kind of biofertilizer. That is the case of bacteria which improve a plant-microbe interaction as a third partner in the interaction. An example can be found in rhizospheric actinomycetes isolated from legumes or actinorhizal nitrogen-fixing nodules (Solans, 2007) which are able to stimulate nodulation, consequently nitrogen fixation in the plant, and finally plant growth (Solans et al., 2009). This tripartite plant-microbe interaction is not well known yet in terms of mechanisms, but clearly shows that biofertilizers can be improved by the use of more than one micro-organism at a time.
Although not all the different bacterial mechanisms that have been claimed to be responsible for the plant growth promotion phenomenon are present in a single strain, it is also true that each single strain usually shows more than one characteristic activity related to plant growth promotion. Thus, it has been almost impossible to prove with certainty the relevance of each and every mechanism described as plant growth-promoting activities in selected micro-organisms. This is especially true when the plant growth-promoting activity is tested in field conditions. Despite this uncertainty, the positive results are reproducible and no harmful effects have appeared. Thus, practical application of biofertilizers is increasing worldwide.
The nature of multiple mechanisms discovered for PGPR actions and the possibility of genetic modification of a particular strain to enhance its PGPR activity, suggest that the use of genetically modified organisms is not needed to implement this technology but could be a way to improve what can be found in nature.
In addition to all these descriptions which try to give an overview of the current state of the art in biofertilizers, it must be pointed out that nowadays the picture of soil microbial ecology is completely different from what it was when biofertilizers were discovered and began to be studied. Microbial soil ecology appears as a very complex and mostly unknown scenario where all these PGPR-plant interactions take place. The study of the soil microbial ecology and its dynamics will certainly improve the development of new and better biofertilizer technology for the future of agriculture. Since the same plant growth-promotion function or mechanism could be driven by many different bacteria or micro-organisms, this functional redundancy in soil microbial diversity may be managed in favour of plant development.
As this chapter has shown, the mechanisms that are at the basis of plant growth promotion by micro-organisms are beginning to be unravelled at the molecular level. This knowledge is already used for strain improvement by genetic modification, and there are several areas, e.g. introducing an ACC deaminase gene in PGPB strains which lack this particular activity (Glick et al., 2007), creating overproducing IAA strains (Bashan and de-Bashan 2010), genetically modified strains which release the fixed ammonium (Van Dommelen et al., 2009), where important improvements of the potential for plant growth stimulation of bacterial strains may be achieved. Environmental risk assessment of the use of such strains will require a solid knowledge about the mechanisms behind plant growth stimulation. For instance, horizontal gene transfer of ACC deaminase genes in rhizospheric bacteria has been suggested (Hontzeas et al., 2005). It is clear that quite some new insights and knowledge have become available in this area since the previous OECD publication (OECD, 1995).