Symbiotic relationships established between plants and bacteria such as rhizobium and some actinomycetes provide most of the nitrogen available to legumes and actinorhizal plant species. A number of studies attempted to develop new association between non legume plants and N fixing bacteria, however, these attempts were mostly unsuccessful . The discovery of endophytic diazotrophs that provided reduced N to the non legume plant species represented an alternative way of exploiting plant microbe interaction for N nutrition. Brazilian sugarcane plants harboring N fixing endophytes were grown for many years with low fertilizer inputs, and showed no symptoms of N deficiencies . Other non leguminosae plant species were also shown to benefit from association with diazotrophic endophytes, such as wheat and rice . Iniguez et al. generated a nifH mutant from Klebsiella pneumoniae and compared this to the wild type strain in order to demonstrate that the wild type K. pneumoniae strain provided N to the wheat plant thus alleviating N deficiency. The wheat plants were supplemented with stable N isotope labeled nutrients.
Mass spectrometry was used to show that less 15N was present in wheat plants inoculated with the wild type K. pneumoniae strain. Since these plants obtained a higher percentage of their N from nonisotopically labeled atmospheric N2,vertical home farming the majority of the plant nitrogen was derived from the wild type strain. In a different study, the transcript abundance of the nifH gene was measured by RT PCR in endophytic Herbaspirillum sp. during colonization of internal regions of wild rice plants . Interestingly, they showed higher nifH transcript abundance during the light period compared to the dark period. Given the tight relationship between N fixing activity and nifH gene transcription, the high level of nifH mRNA during the light period indicated that the endophytes produced higher level of reduced N at a time when they could benefit most from photosynthate production. This study described a promising avenue for non legume plants to obtain N from the reduction of atmospheric N2. It remains to be seen that similar results are achievable with soil grown plants or in the field where plants may be less limited for growth than in sand filled pots. In addition, competition from other bacteria may prevent colonization by endophytic diazotroph. Plant genetic contribution to this symbiosis is an important area of research leading to selection of agriculturally important plant genotypes with increased capacity to establish symbiosis for nutrient acquisition. Several studies have begun to pave the way in this area.
The identification of genetic loci underlying complex traits in Lotus spp and Medicago led to the sequencing of specific genes essential for the establishment of root symbiosis. Kistner et al. characterized seven mutants of Lotus japonicus that were impaired in nodulation and were also defective in arbuscular mycorrhiza formation. Kanamori et al. used a F2 mapping population established by crossing the Lotus japonicus mutant and wild type for positional cloning of a gene essential for symbiosis that encoded a plant nucleoporin gene required for Ca2+ spiking occurring after contact between Nod factor molecules and root hair cells. The combination of genetic and genomic analysis resulted in a deeper understanding of the plant genetic contribution to the establishment of symbiosis with rhizosphere bacteria. One drawback of this approach is that it may take many years, from the identification of the genetic basis for complex traits to the development of new cultivars with enhanced capacity for establishing and maintaining symbiosis. Rhizosphere bacteria and phyllosphere colonizing epiphytes have been shown to produce a range of plant growth stimulating phytohormones. A recent study by Boiero et al. evaluated the phytohormone synthesis of three commercially available strains of Bradyrhizobium japonicum grown in pure culture. They showed that the three strains have differential capability to produce the five major phytohormones: auxins, cytokinins, gibberellic acid , abscisic acid and ethylene. This is important to consider as each individual and combinations of phytohormones may have different impact on plant growth.
The auxin compound indole 3 acetic acid can be generated in bacteria through different biosynthetic pathways . Saravanan et al. reviewed the roles in plant growth promotion of two N fixing organisms, Gluconacetobacter diazotrophicus and an Acetobacteraceae strain. In particular they discussed additional roles for G. diazotrophicus, including the production of plant hormones such as IAA, and gibberellins. Idris et al. showed for the first time that the gram positive bacterium Bacillus amyloliquefaciens produced and secreted significant amounts of IAA. They also showed the positive effect of IAA on the growth of Lemna minor. Patten and Glick demonstrated the direct positive effect of IAA produced by Pseudomonas putida through the indolepyruvic acid pathway, on root development. Roots from Canola seeds treated with the wild type P. putida were longer in length than that of seeds treated with an IAA deficient mutant. However, the bacterial production of IAA may not always be beneficial for plant as it is involved in pathogenesis and that high concentration of IAA can also inhibit root cell growth . Ethylene has many physiological effects on plant growth, development and modulation of responses to biotic and abiotic stresses. Bacteria such as Pseudomonas spp., Burkholderia caryophylli, Achromobacter piechaudii were shown to lower the endogenous ethylene level in planta by producing a degradative enzyme 1 aminocyclopropane 1 carboxylic acid deaminase . The effects of ACC deaminase producing rhizobacteria on plants included increased root growth,vertical growers and improved tolerance of salt and water stress . These effects were noted in axenic conditions and more recently in field conditions. Inoculation of wheat plants with Pseudomonas spp. and B. caryophylli improved grain and straw yield by as much as 43% and 44%, respectively . However no measurements of ethylene concentration were carried out on plants in pots or in the field to show direct correlation of ACC deaminase production and ethylene degradation. It is therefore possible that other bacteria associated effects might have improved wheat plant growth. Cytokinins constitute a group of plant hormones that promote cell division in conjunction with auxin and are known to induce stomata opening. Rhizosphere bacteria and fungi associated with plants were shown to produce cytokinins. Arkhipova et al. studied the production of different cytokinins by Bacillus subtilis and showed improved growth for lettuce plants after inoculation with cytokin in producing bacteria. This was also the case when plants were grown under water stress . The authors mainly considered the production of cytokinins by B. subtilis as mechanism of plant production.
It would be interesting to investigate possible alternative mechanism by which this species is able to promote plant growth. Lettuce plants were grown in sand and the study did not address whether similar results could be obtained in the field where competition from other rhizosphere bacteria might prevent B. subtilis growth and where bacterial production of other phytohormones might interfere with the effect of cytokinins. The modulation of phytohormone by bacteria that lead to disease resistance is addressed in the Biocontrol section. Plant growth promoting bacteria have been shown to positively impact plant performance through different mechanisms. For future development of commercial inoculant, it is therefore important to consider all potential metabolic activity of the phytohormone producing bacteria. In addition, components of the inoculant growth media could be transformed into metabolites that could impact plant growth in the early stages of plant development . For successful application of plant growth promotion using bacterial inoculant, many aspects of the plant environment have to be considered in the field. In particular, it is important that the inoculant be beneficial to the crop, and do not improve the growth of the weedy species and do not render the plants more susceptible to biotic and abiotic stresses.Further development of plant growth promoting bacteria could benefit from selection, through breeding programs of plant genotypes that respond better to the plant growth promoting bacteria. The efficiency of bio control agents for plant disease has been demonstrated and some are commercially available . General mechanisms of action for plant pathogen control include competition for nutrients and space at the infection site, antibiosis, parasitism, production of cell wall degrading enzymes, induced resistance in the plant, and manipulation of bacterial signaling molecules. It is likely that several mechanisms of action are at work in many bio control agents. Compant et al. reviews the mechanisms of plant disease bio control by plant growth promoting bacteria. Here, we discuss two mechanisms of disease control: induced resistance and manipulation of signaling molecules. Interactions with bacteria can induce two types of plant defense responses that help protect against further infection. Systemic acquired resistance is a specific response that triggers both a local increase in phytohormone accumulation and the formation of phloem mobile signal. Non pathogenic free living rhizosphere bacteria and endophytes can trigger the second type of plant defense called induced systemic resistance . A major distinction between SAR and ISR is the involvement of salicylic acid , with ISR being activated via a SA independent pathway. One example of plant disease control is the use of Pseudomonas strains with biocontrol activity to induce resistance in appleagainst the pathogenic fungus Venturia inaequalis , which causes apple scab. The movement of Pseudomonas on apple leaves was studied in order to understand its antagonistic interactions against V. inaequalis . Using confocal laser scanning microscopy , it was demonstrated that P. fluorescens Bk3 localized near stomatal openings. The P. fluorescens Bk3 traversed the cuticle through secretion of cutinases, and acquired nutrient from fluid isolated from the apoplastic space. Also, isolated leaf cuticles stimulated bacterial extracellular proteins . In order to understand the interaction between the V. inaequalis and apple tree, cDNA libraries using suppression subtractive hybridization were constructed for the resistant and susceptible cultivars . Many plant defense related transcripts such as those encoding for β 1,3 glucanase, cystein protease inhibitor and metallothioneins were at higher levels in the resistant cultivar. On the other hand, more RuBisCo transcripts were found in the susceptible cultivar than in the resistant one. Similar proteins were expressed in apple trees treated with the bio control strain P. fluorescens Bk3 and V. inaequalis . The presence of P. fluorescens Bk3 was demonstrated to elevate the defense mechanisms in the apple trees and could serve as effective bio control strategy. The authors implied that low levels of RuBisCo and high levels of metallothioneins resembled that of “old leaves” and were “unattractive” to V. inaequalis . The authors did not investigate whether P. fluorescens Bk3 induced SAR or ISR type of plant resistance. Some bacteria rely on signaling molecules for the development of pathogenesis. Other microbes living in the same environment may degrade these signaling molecules. Recombinant bio control strains producing carAB, genes required for degradation of a fatty acid signaling molecule, were shown to reduce virulence caused by Xanthomonas sp. and Xylella fastidiosa . Secondary metabolites produced by plants were shown to mimic or inhibit quorum sensing molecules. M. truncatula was shown to produce more than a dozen compounds that stimulated or inhibited QS . Plants have been manipulated to produce molecules that mimic or block QS signal, and enzymes that degrade QS molecules or quorum quenching strategy. Scott et al. engineered tobacco plants to synthesize acyl homoserine lactones in the chloroplast and demonstrated that AHL was transported and secreted on the phyllosphere and in the rhizosphere. Pre treatment of potato slices with Bacillus thuriensis resulted in reduced maceration from Erwinia carotovora virulence . The authors showed that the decreased pathogenesis involved the disruption of E. carotovora quorum sensing by deterioration of QS molecules with AHL lactonase. Pre treatment of potato slices with B. thuriensis lacking the ability to produce AHLlactonase did not reduce maceration. Manipulating plants to express AHL lactonase has shown positive effect in protecting plants against pathogens . Recently, a gene called qsdA from Rhodococcus erythropolis, encoding a new type of AHL lactonase has been characterized. The gene was able to confer quorum quenching capacity to P. fluorescens that led to enhanced protection of potato tuber against the soft rot pathogen P. carotovorum . Elucidating the mechanism by which bacteria elicit ISR is important for the development of commercial bio control agent.