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

Abstract

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 multiple mechanisms employed by the bacteria to affect plant growth; (vi) comment on the less researched areas, such as inoculant and pesticide research; and (vii) proposes possible avenues for the exploitation of this bacterium in environmental areas other than agriculture.

Inoculation of Diazotrophic Bacteria and Nitrogen Fertilization in Topdressing in Irrigated Corn

“…It is worth noting the effect of inoculation with Azospirillum spp., which according to the literature on studies conducted in Brazil, can be correlated with increased absorption of not only nitrogen, but also other nutrients, a result also reported in other countries (BASHAN; HOLGUIN, 1997;STEENHOUDT;VANDERLEYDEN, 2000;DE-BASHAN, 2004), and consistent with the results of the present study.…”

“…This result corroborates those described by Bashan, Holguin and De-Bashan (2004) that evaluated Azospirillum bacteria and the plant-Azospirillum interrelationship, and found an increase in various photosynthetic pigments (chlorophyll a and b) and photoprotective pigments (carotenoids and xanthophylls) in plants. Barassi et al (2008) reported improvements in photosynthetic parameters of the leaves, including chlorophyll content and stomatal conductance, with a higher biomass production and increased plant height of corn inoculated with Azospirillum.…”

“…These nutrients are of great importance in tropical soils, where they are naturally scarce. This fact has also been reported in other countries (BASHAN; HOLGUIN, 1997;STEENHOUDT;VANDERLEYDEN, 2000;DE-BASHAN, 2004), where improvements were found in root volume and soil exploitation.…”

“…(Table 2) showed higher concentrations of N, P, K, S, and reduced levels of Ca in the inoculated corn compared to the without inoculation. This effect on the Ca can be attributed to competitive inhibition by increased absorption of K (MALAVOLTA et al, 1997;DE-BASHAN, 2004). The increased nutritional content in corn leaves (N, P, K, S) is due to the main action of diazotrophic bacteria of the genus Azospirillum that are associated with the rhizosphere of plants.…”

Inoculation with Bradyrhizobium sp. and Azospirillum brasilense Associated with Application of Cobalt and Molybdenum on Nutrition and Soybean Yield

“…The leaf area of soybean was greater in treatment with the application of A. brasilense, differing significantly from the application of Co and Mo in the seed ( Table 3). This bacterium can influence the plant growth by producing auxins, gibberellins, and cytokinins, which provide improved root growth [34] and consequently greater absorption of water and nutrients [22], resulting in more vigorous and productive plant [35,36].…”