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Phytohormones mediated mechanisms of plant growth promotion induced by micro-organisms

One of the most visible effects on plants after inoculation with PGPB is the huge development - and sometimes changes in the architecture - of the root of the plant. This general improvement of root growth, including root hairs development, is one of the characteristic phenotypes of the interaction plant-PGPB.

It is likely that water and mineral uptake is consequently improved because of the increase in the root system, although the specific mechanism is not completely clear. Changes in hormone balance, enhancement of proton efflux activity extrusion and modification in a wide range of related enzymatic activities would be part of the mechanisms behind this phenotype (Bashan and de-Bashan, 2010; Cohen et al., 2009). Most of the existing data is, however, descriptive.

Auxins

This general root improvement phenotype can be reproduced by replacing phytohormones with PGPB, and phytohormones-like substances have been detected in bacterial culture supernatants so it is likely that this phenotype is mediated by phytohormones synthesised by the bacteria (Costacurta and Vanderleyden, 1995). Auxin- related substances, such as indole acetic acid (IAA), appear to be involved in one of the most important mechanisms regarding this general root development improvement. Nevertheless, bacterial production of IAA in planta has not yet been demonstrated. There are no IAA completely deficient mutants, but IAA attenuated mutants were ineffective as PGPB, compared to parental strains (Bashan and de-Bashan, 2010).

Gibberellins

Gibbelleric acids (GAs) are produced by some PGPB species in vitro and have also been shown in planta since those PGPB strains capable of producing GAs in vitro were able to complement GA-deficient mutant dwarf rice by inoculation (Bottini et al., 2004). PGPB producing GAs were also active in improving seed germination. In vitro results support the hypothesis that PGPB effect would be a combination of GA production and GA-glucoside/glucosyl ester deconjugation by the PGPB.

Cytokinins

The adenine-type cytokinins represented by kinetin, zeatin and 6-benzylaminopurine which occur in plants have also been produced in a defined culture medium by many PGPB (Strzelczyk et al., 1994). The role of citokinins in the promotion of root development is not clear, but cytokinin-producing PGPB stimulate nodulation in legumes when co-inoculated with rhizobia, and it was recently demonstrated that there is a Nod factor independent mechanism for infection and nodulation (Giraud et al., 2007), probably mediated by rhizobial cytokinin (see helper bacteria, below). This particular area deserves more attention in the future.

Ethylene/ACC deaminase

Ethylene is a plant hormone related to general plant responses when a stress condition appears, even if it is a very low stress situation (Glick, 2004). When this happens, the plant synthesises ethylene and stops its growth temporarily because of the regulatory effects of ethylene on different cell functions. 1-aminocyclopropane-1-carboxylate (ACC) is a precursor of ethylene synthesis. The enzyme ACC deaminase is present in some bacteria which can even use ACC as C (carbon) and N sources. When ACC deaminase is expressed by a rhizospheric bacteria root growth and development is enhanced, it is probably because of the elimination of the inhibitory concentrations of ethylene produced by the plant (Glick, 2004). This enzyme is not ubiquity present in bacteria and its activity is codified by a single gen acdS. The introduction of this gene from Pseudomona putida into other bacteria species confers plant growth-promoting functions to the recipient bacteria that were absent in the parental strain (Glick et al., 2007). This represents a potential biotechnological tool to improve micro-organisms to be used as biofertilizers.

Nitric oxide

Nitric oxide (NO), a plant regulator volatile phytohormone, is also produced by some PGPB as Azospirillum spp. (Molina-Favero et al., 2008). Bacterial NO is an intermediary in IAA-induced root development. NO can also mediate plant growth-promoting activity in Azospirillum brasilense Sp245 inducing morphological changes in tomato roots regardless of the full bacterial capacity for IAA synthesis.

Polyamines

Azospirillum spp. can produce different ployamines in culture (Perrig et al., 2007, Cassan et al., 2009). Cadaverine is synthesised by these bacteria from lysine mitigated osmotic stress in rice seedlings, based on improved water status and decreased production of ABA in inoculated seedlings (Cassan et al., 2009).

 
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