Evolution of Protein Molecules

The rhizosphere is a complex environment where roots interact with physical, chemical and biological properties of soil. Structural and functional characteristics of roots contribute to rhizosphere processes and both have significant influence on the capacity of roots to acquire nutrients. Roots also interact extensively with soil microorganisms which further impact on plant nutrition either directly, by influencing nutrient availability and uptake, or indirectly through plant (root) growth promotion. In this paper, features of the rhizosphere that are important for nutrient acquisition from soil are reviewed, with specific emphasis on the characteristics of roots that influence the availability and uptake of phosphorus and nitrogen. The interaction of roots with soil microorganisms, in particular with mycorrhizal fungi and non-symbiotic plant growth promoting rhizobacteria, is also considered in relation to nutrient availability and through the mechanisms that are associated with plant growth promotion.

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

The rhizosphere supports the development and activity of a huge and diversified microbial community, including microorganisms capable to promote plant growth. Among the latter, plant growth-promoting rhizobacteria (PGPR) colonize roots of monocots and dicots, and enhance plant growth by direct and indirect mechanisms. Modification of root system architecture by PGPR implicates the production of phytohormones and other signals that lead, mostly, to enhanced lateral root branching and development of root hairs. PGPR also modify root functioning, improve plant nutrition and influence the physiology of the whole plant. Recent results provided first clues as to how PGPR signals could trigger these plant responses. Whether local and/or systemic, the plant molecular pathways involved remain often unknown. From an ecological point of view, it emerged that PGPR form coherent functional groups, whose rhizosphere ecology is influenced by a myriad of abiotic and biotic factors in natural and agricultural soils, and these factors can in turn modulate PGPR effects on roots. In this paper, we address novel knowledge and gaps on PGPR modes of action and signals, and highlight recent progress on the links between plant morphological and physiological effects induced by PGPR. We also show the importance of taking into account the size, diversity, and gene expression patterns of PGPR assemblages in the rhizosphere to better understand their impact on plant growth and functioning. Integrating mechanistic and ecological knowledge on PGPR populations in soil will be a prerequisite to develop novel management strategies for sustainable agriculture.

/pdf/plant-growth-promoting-rhizobacteria-and-root-system-3ygxbzsvj6.pdf

Plant growth-promoting rhizobacteria and root system functioning.

This review presents a critical and comprehensive documentation and analysis of the developments in agricultural, environmental, molecular, and physiological studies related to Azospirillum cells, and to Azospirillum interactions with plants, based solely on information published between 1997 and 2003. It was designed as an update of previous reviews (Bashan and Levanony 1990; Bashan and Holguin 1997a), with a similar scope of interest. Apart from an update and critical analysis of the current knowledge, this review focuses on the central issues of Azospirillum research today, such as, (i) physiological and molecular studies as a general model for rhizosphere bacteria; (ii) co-inoculation with other microorganisms; (iii) hormonal studies and re-consideration of the nitrogen contribution by the bacteria under specific environmental conditions; (iv) proposed Azospirillum as a non-specific plant-growth-promoting bacterium; (v) re-introduction of the "Additive Hypothesis," which suggests involvement of multip...

/pdf/azospirillum-plant-relationships-physiological-molecular-4uxixnij1g.pdf

Azospirillum-plant relationships: physiological, molecular, agricultural, and environmental advances (1997-2003)

During the last 35 years of studies of Azospirillum–plant interaction, over 20 proposals were suggested for the mechanism of action by which Azospirillum spp., the most intensively studied plant growth-promoting bacteria, enhances plant growth. The proposals include a single phytohormone activity, multiple phytohormones, nitrogen fixation, assortments of small-sized molecules and enzymes, enhanced membrane activity, proliferation of the root system, enhanced water and mineral uptake, mobilization of minerals, mitigation of environmental stressors of plants, and direct and indirect biological control of numerous phytopathogens. By volume, the largest number of published information involves hormonal activities, nitrogen fixation, and root proliferation. After analyzing the accumulated knowledge, it was concluded that this versatile genus possesses a large array of potential mechanisms by which it can effect plant growth. Consequently, this review proposes the “Multiple Mechanisms Theory,” based on the assumption that there is no single mechanism involved in promotion of plant growth by Azospirillum, but a combination of a few or many mechanisms in each case of inoculation. These may vary according to the plant species, the Azospirillum strain, and environmental conditions when the interaction occurred. The effect can be cumulative, an “additive hypothesis” (proposed before), where the effects of small mechanisms operating at the same time or consecutively create a larger final effect on plant. Additionally, the observed effect on plant growth can be the result of a tandem or a cascade of mechanisms in which one mechanism stimulates another, yielding enhanced plant growth, such as the plausible relations among phytohormones, nitric oxide, membrane activities, and proliferation of roots. Finally, the growth promotion can also be a combination of unrelated mechanisms that operate under environmental or agricultural conditions needed by the crop at particular locations, such as mitigating stress (salt, drought, toxic compounds, adverse environment), and the need for biological control of or reducing pathogenic microflora.

/pdf/how-the-plant-growth-promoting-bacterium-azospirillum-jkw2p8pxwd.pdf

How the Plant Growth-Promoting Bacterium Azospirillum Promotes Plant Growth—A Critical Assessment

D.S. T HULER, E .I.S. F LOH, W . H ANDRO A ND H .R. B ARBOSA. 2003. Aims: To investigate the ability of Azospirillum sp., a facultative endophitic diazotrophic bacterium, to release plant growth regulators (PGR) such as polyamines, ethylene, indoleacetic acid and amino acids in both combined-N and N-free cultures. Methods and Results: The presence of those substances was analysed by HPLC. Azospirillum sp. is capable of releasing PGR and amino acids into the culture medium. Conclusions: The type and quantity of the released substances varied, depending on the presence of combined-N in the medium. Significance and Impact of the Study: A better knowledge of PGR produced by Azospirillum sp. has been gained.

/pdf/plant-growth-regulators-and-amino-acids-released-by-33n1lk5ady.pdf

Plant growth regulators and amino acids released by Azospirillum sp in chemically defined media.

D . S . T H U L E R , E . I . S . F L O H , W . H A N D R O A N D H . R . B A R B O S A . 2003. Aims: This study aims at evaluating the ability of Beijerinckia derxii, a free-living nitrogen (N)-fixing bacterium frequently isolated from tropical soils, to release certain plant growth regulators [indoleacetic acid (IAA), ethylene, polyamines] and amino acids into the growth medium. Methods and Results: The production of those substances was compared using both cultures in which nitrogenase was active (N-free medium) and cultures in which nitrogenase was repressed (combined-N cultures). Those cultures were grown under agitation and in absence of agitation. Total IAA production was higher in agitated, N-free cultures but specific production was greater in combined-N cultures under agitation. Putrescine and spermidine were detected under all conditions tested. Ethylene was produced in both N-free and combined-N cultures. A greatest diversity of amino acids was released in N-free cultures. Conclusions: There was no inhibition of the production of the analysed substances under conditions where nitrogenase was inactive. Significance and Impact of the Study: Beijerinckia derxii is potentially a producer of plant-active substances; its presence in the natural environment suggests that this bacterium may contribute to the development of other living organisms.

/pdf/beijerinckia-derxii-releases-plant-growth-regulators-and-1uea5veyms.pdf

Beijerinckia derxii releases plant growth regulators and amino acids in synthetic media independent of nitrogenase activity.

http://www.icb.usp.br/bmm/hrbarbos/arquivos/microbiological_research_158_2003.pdf

During stationary phase, Beijerinckia derxii shows nitrogenase activity concomitant with the release and accumulation of nitrogenated substances.

http://www.scielo.br/pdf/bjm/v33n3/v33n3a07.pdf

Nitrogenase activity of Beijerinckia derxii is preserved under adverse conditions for its growth

http://www.scielo.br/pdf/bjm/v31n3/v31n3a04.pdf

Daniela Strauss Thuler

Papers

Plant growth regulators and amino acids released by Azospirillum sp in chemically defined media.

Beijerinckia derxii releases plant growth regulators and amino acids in synthetic media independent of nitrogenase activity.

During stationary phase, Beijerinckia derxii shows nitrogenase activity concomitant with the release and accumulation of nitrogenated substances.

Nitrogenase activity of Beijerinckia derxii is preserved under adverse conditions for its growth

Beijerinckia derxii stimulates the viability of non-N2-fixing bacteria in nitrogen-free media