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BACKGROUND: Inoculation of plants to enhance yield of crops and performance of other plants is a century old, proven technology for rhizobia and a newer venue for plant growth-promoting bacteria and other plant symbionts. The two main aspects dominating the success of inoculation are the effectiveness of the bacterial isolate and the proper application technology. SCOPE: An assessment of practical aspects of bacterial inoculants for contemporary agriculture and environmental restoration is critically evaluated from the point of view of their current technological status, current applications, and future use. This is done because there are windows of opportunity for new developments in applied research using renewable, non-contaminated natural resources and new venues for research. Special emphasis is given to formulations and polymeric carriers. This review concentrates on practical aspect of inoculation technology dating from 1998 to 2013. Earlier publications are mentioned only for clarification of a specific point. CONCLUSIONS: This review discusses characteristics of a carrier for inoculants, formulations of inoculants including liquid, organic, inorganic, polymeric, and encapsulated formulations. Technical aspects include inoculation techniques (soil and seed application), mass culture production, bulk sterilization, seed coating, shelf-life, and effect of moisture. Future research venues needed are noted.

BACKGROUND: Azospirillum spp. are the most studied plant growth-promoting bacteria (PGPB). The genus represents a common model for plant-bacteria interactions. This genus was initially isolated and tested on cereals and was subsequently commercialized. AIMS: Despite claims of plant specificity, particularly towards cereals, data over the past 40 years does not appear to substantiate claims of such specificity/affinity of Azospirillum species. Consequently an evaluation of the specificity/affinity of the genus Azospirillum across all plants, in general, and cereals, in particular, was undertaken. RESULTS: Although the majority of studies focused on cereals, Azospirillum spp. increase growth of 113 plant species across 35 botanical families, including 14 species of cereals. Amongst Azospirillum spp., several well studied strains have been effective in several plant species, making these organisms potentially valuable for further study. CONCLUSIONS: This review demonstrates that azospirilla are not cereal-specific at the genus and species levels. Azospirillum serves as a general PGPB to every plant species tested so far. Given the paucity of widespread screening, affinity of strains to a plant genotype, cultivar, or plant species cannot be overruled. Definitive conclusions concerning such specificity require molecular and cross-inoculation studies, using various strains of bacteria, and re-isolation after growth of the plants in different plant species. (203 words).

Engineered mutualistic consortia of microalgae and bacteria may be a means of assembling a novel combination of metabolic capabilities with potential biotechnological advantages. Microalgae are promising organisms for the sustainable production of metabolites of commercial interest, such as lipids, carbohydrates, pigments, and proteins. Several studies reveal that microalgae growth and cellular storage of these metabolites can be enhanced significantly by co-cultivation with growth-promoting bacteria. This review summarizes the state of the art of microalgae–bacteria consortia for the production of microalgal metabolites. We discuss the current knowledge on microalgae–bacteria mutualism and the mechanisms of bacteria to enhance microalgae metabolism. Furthermore, the potential routes for a microalgae–bacteria biorefinery are outlined in an attempt to overcome the economic failures and negative energy balances of the existing production processes.

The effect of three different nutritional conditions during the initial 12 h of interaction between the microalgae Chlorella sorokiniana UTEX 2714 and the plant growth–promoting bacterium Azospirillum brasilense Cd on formation of synthetic mutualism was assessed by changes in population growth, production of signal molecules tryptophan and indole-3-acetic acid, starch accumulation, and patterns of cell aggregation. When the interaction was supported by a nutrient-rich medium, production of both signal molecules was detected, but not when this interaction began with nitrogen-free (N-free) or carbon-free (C-free) media. Overall, populations of bacteria and microalgae were larger when co-immobilized. However, the highest starch production was measured in C. sorokiniana immobilized alone and growing continuously in a C-free mineral medium. In this interaction, the initial nutritional condition influenced the time at which the highest accumulation of starch occurred in Chlorella, where the N-free medium induced faster starch production and the richer medium delayed its accumulation. Formation of aggregates made of microalgae and bacteria occurred in all nutritional conditions, with maximum at 83 h in mineral medium, and coincided with declining starch content. This study demonstrates that synthetic mutualism between C. sorokiniana and A. brasilense can be modulated by the initial nutritional condition, mainly by the presence or absence of nitrogen and carbon in the medium in which they are interacting.

Soil restoration is one of the biggest challenges of this century. Besides the negative impacts of climate change, the current increase in food demands has put severe pressure on soil resources, resulting in a significant area of degraded land worldwide. However, beneficial microorganisms, such as microalgae and plant growth-promoting bacteria, have an outstanding ability to restore soil health and fertility. In this mini-review, we summarize state-of-the-art knowledge on these microorganisms as amendments that are used to restore degraded and contaminated soils. Furthermore, the potential of microbial consortia to maximize beneficial effects on soil health and boost the production of plant-growth-promoting compounds within a mutualistic interaction is discussed.

Remote effects (occurring without physical contact) of two plant growth-promoting bacteria (PGPB) Azospirillum brasilense Cd and Bacilus pumilus ES4 on growth of the green microalga Chlorella sorokiniana UTEX 2714 were studied. The two PGPB remotely enhanced the growth of the microalga, up to six-fold, and its cell volume by about three-fold. In addition to phenotypic changes, both bacteria remotely induced increases in the amounts of total lipids, total carbohydrates, and chlorophyll a in the cells of the microalga, indicating an alteration of the microalga’s physiology. The two bacteria produced large amounts of volatile compounds, including CO 2 , and the known plant growth-promoting volatile 2,3-butanediol and acetoin. Several other volatiles having biological functions in other organisms, as well as numerous volatile compounds with undefined biological roles, were detected. Together, these bacteria-derived volatiles can positively affect growth and metabolic parameters in green microalgae without physical attachment of the bacteria to the microalgae. This is a new paradigm on how PGPB promote growth of microalgae which may serve to improve performance of Chlorella spp. for biotechnological applications.

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.Key words: Azospirillum, plant-bacteria interaction, plant-growth-promoting bacteria, PGPB, PGPR, rhizosphere bacteria.

When the freshwater microalga Chlorella sorokiniana and the plant growth-promoting bacterium Azospirillum brasilense were deployed as free suspensions in unsterile, municipal wastewater for tertiary wastewater treatment, their population was significantly lower compared with their populations in sterile wastewater. At the same time, the numbers of natural microfauna and wastewater bacteria increased. Immobilization of C. sorokiniana and A. brasilense in small (2-4 mm in diameter), polymer Ca-alginate beads significantly enhanced their populations when these beads were suspended in normal wastewater. All microbial populations within and on the surface of the beads were evaluated by quantitative fluorescence in situ hybridization combined with scanning electron microscopy and direct measurements. Submerging immobilizing beads in wastewater created the following sequence of events: (a) a biofilm composed of wastewater bacteria and A. brasilense was created on the surface of the beads, (b) the bead inhibited penetration of outside organisms into the beads, (c) the bead inhibited liberation of the immobilized microorganisms into the wastewater, and (d) permitted an uninterrupted reduction of ammonium and phosphorus from the wastewater. This study demonstrated that wastewater microbial populations are responsible for decreasing populations of biological agents used for wastewater treatment and immobilization in alginate beads provided a protective environment for these agents to carry out uninterrupted tertiary wastewater treatment. [PUBLICATION ABSTRACT]

Climate change, due to the altered composition of the global atmosphere from the “greenhouse effect”, is one of the biggest challenges to agricultural production systems [...]

Plant growth-promoting bacteria (PGPB) can improve plant performance in many different ways, operating via a multitude of physiological, molecular, and biochemical pathways. One of the lesser known involvements in these interactions is the role of vitamins. Vitamins can be produced by plants and bacteria and also by PGPB. The main function of vitamins is to (1) act as a cofactor in diverse metabolic pathways, (2) facilitate production of essential compounds for plants and bacteria, (3) induce resistance against pathogens, (4) directly promote plant growth, and (5) participate in energy conversion in the plant from stored compounds. Most of the roles of specific vitamins in PGPB–plant interactions are still little known or completely unknown. This overview presents what is known about vitamins detected in potential PGPB, presents proposals for the potential role of vitamins in PGPB–plant interactions based on the known function of these vitamins in plants and bacteria, and proposes research avenues in this topic that are worth exploring.