Yaacov Okon

Plant Growth-Promoting Effects of Diazotrophs in the Rhizosphere

Because of their ability to transform atmospheric N2 into ammonia that can be used by the plant, researchers were originally very optimistic about the potential of associative diazotrophic bacteria to promote the growth of many cereals and grasses. However, multiple inoculation experiments during recent decades failed to show a substantial contribution of Biological Nitrogen Fixation (BNF) to plant growth in most cases. It is now clear that associative diazotrophs exert their positive effects on plant growth directly or indirectly through (a combination of) different mechanisms. Apart from fixing N2, diazotrophs can affect plant growth directly by the synthesis of phytohormones and vitamins, inhibition of plant ethylene synthesis, improved nutrient uptake, enhanced stress resistance, solubilization of inorganic phosphate and mineralization of organic phosphate. Indirectly, diazotrophs are able to decrease or prevent the deleterious effects of pathogenic microorganisms, mostly through the synthesis of antibiotics and/or fungicidal compounds, through competition for nutrients (for instance, by siderophore production) or by the induction of systemic resistance to pathogens. In addition, they can affect the plant indirectly by interacting with other beneficial microorganisms, for example, Azospirillum increasing nodulation of legumes by rhizobia. The further elucidation of the different mechanisms involved will help to make associative diazotrophs a valuable partner in future agriculture.

Agronomic applications of azospirillum: An evaluation of 20 years worldwide field inoculation

By evaluating worldwide data accumulated over the past 20 years on field inoculation experiments with Azospirillum, it can be concluded that these bacteria are capable of promoting the yield of agriculturally-important crops in different soils and climatic regions. Various strains of A. brasilense and A. lipoferum have been used to inoculate cultivars of different species of plants. It is however difficult to accurately estimate the percentage of success due to Azospirillum inoculation. The data indicates 60–70% occurrence of success with statistically significant increases in yield of the order of 5–30%. Successful inoculation experiments appear to be those in which the researchers have paid special attention to the optimal number of cells of Azospirillum in the inoculant, using inoculation methods where the optimal number of cells remained viable and available to colonize the roots. Furthermore, experiments taking into consideration the potentialities and limitations of this technology have been better able to explain successes and failures. The different formulations (analogous to those of rhizobia) of the genus Azospirillum, irrespective of their form of application and their mode of action on the plant, are indeed inoculants. The term biofertilizer is not appropriate as it does not replace fertilizer but improves their utilization. We very strongly suggest the implementation by regulatory authorities of quality control on commercial Azospirillum inoculants.

Responses of agronomically important crops to inoculation with Azospirillum

Azospirilla are free-living rhizobacteria that are able to promote plant growth and increase yields in many crops of agronomic importance. It is assumed that the bacteria affect plant growth mainly by the production of plant growth promoting substances, which leads to an improvement in root development and an increase in the rate of water and mineral uptake. In the present review, we discuss the physiological responses of the plant roots to inoculation with Azospirillum, and report on field and greenhouse experiments carried out with these bacteria during 1994-2001 in Belgium, Uruguay, Mexico and Israel.

Azospirillum as a potential inoculant for agriculture

Abstract Azospirillum sp. contribute to increased yields of cereal and forage grasses by improving root development in properly colonized roots, increasing the rate of water and mineral uptake from the soil, and by biological nitrogen fixation. A better understanding of the basic biology of the Azospirillum —root interaction, aided by the application of genetic engineering techniques, may lead to greater efficiency in its use as a biofertilizer.

Development and function of Azospirillum-inoculated roots

The surface distribution of Azospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs. Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains of Azospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO 3 - , K+ and H2PO 4 - uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher in Azospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.

A Vesicular Arbuscular Mycorrhizal Fungus (Glomus intraradix) Induces a Defense Response in Alfalfa Roots.

Flavonoid accumulation and activities of phenylalanine ammonia-lyase (PAL), chalcone isomerase (CHI), and chitinase were followed during early colonization of alfalfa roots (Medicago sativa L. cv Gilboa) by vesicular arbuscular (VA) fungi (Glomus intraradix). Formononetin was the only flavonoid detected that showed a consistent increase in the inoculated roots. This increase depended only on the presence of the fungus in the plant rhizosphere; no colonization of the root tissue was required. CHI and chitinase activities increased in inoculated roots prior to colonization, whereas the increase in PAL activity coincided with colonization. After reaching a maximum, activities of all enzymes declined to below those of uninoculated roots. PAL inactivation was not caused by a soluble inhibitor. Our results indicate that VA fungi initiate a host defense response in alfalfa roots, which is subsequently suppressed.

Development and function ofAzospirillum-inoculated roots

SummaryThe surface distribution ofAzospirillum on inoculated roots of maize and wheat is generally similar to that of other members of the rhizoplane microflora. During the first three days, colonization takes place mainly on the root elongation zone, on the base of root hairs and, to a lesser extent, on the surface of young root hairs.Azospirillum has been found in cortical tissues, in regions of lateral root emergence, along the inner cortex, inside xylem vessels and between pith cells. Inoculation of several cultivars of wheat, corn, sorghum and setaria with several strains ofAzospirillum caused morphological changes in root starting immediately after germination. Root length and surface area were differentially affected according to bacterial age and inoculum level. During the first three weeks after germination, the number of root hairs, root hair branches and lateral roots was increased by inoculation, but there was no change in root weight. Root biomass increased at later stages. Cross-sections of inoculated corn and wheat root showed an irregular arrangement of cells in the outer layers of the cortex. These effects on plant morphology may be due to the production of plant growth-promoting substances by the colonizing bacteria or by the plant as a reaction to colonization. Pectic enzymes may also be involved. Morphological changes had a physiological effect on inoculated roots. Specific activities of oxidative enzymes, and lipid and suberin content, were lower in extracts of inoculated roots than in uninoculated controls. This suggests that inoculated roots have a larger proportion of younger roots. The rate of NO−3, K+ and H2PO−4 uptake was greater in inoculated seedlinds. In the field, dry matter, N, P and K accumulated at faster rates, and water content was higher inAzospirillum-inoculated corn, sorghum, wheat and setaria. The above improvements in root development and function lead in many cases to higher crop yield.

Ecological and agricultural significance of bacterial polyhydroxyalkanoates.

Abstract Polyhydroxyalkanoates (PHAs) are a group of carbon and energy storage compounds that are accumulated during suboptimal growth by many bacteria, and intracellularly deposited in the form of inclusion bodies. Accumulation of PHAs is thought to be used by bacteria to increase survival and stress tolerance in changing environments, and in competitive settings where carbon and energy sources may be limited, such as those encountered in the soil and the rhizosphere. Understanding the role that PHAs play as internal storage polymers is of fundamental importance in microbial ecology, and holds great potential for the improvement of bacterial inoculants for plants and soils. This review summarizes the current knowledge on the ecological function of PHAs, and their strategic role as survival factors in microorganisms under varying environmental stress is emphasized. It also explores the phylogeny of the PHA cycle enzymes, PHA synthase, and PHA depolymerase, suggesting that PHA accumulation was earlier acquired and maintained during evolution, thus contributing to microbial survival in the environment.

Involvement of the Reserve Material Poly-β-Hydroxybutyrate in Azospirillum brasilense Stress Endurance and Root Colonization

ABSTRACT When grown under suboptimal conditions, rhizobacteria of the genus Azospirillum produce high levels of poly-β-hydroxybutyrate (PHB). Azospirillum brasilense strain Sp7 and a phbC (PHB synthase) mutant strain in which PHB production is impaired were evaluated for metabolic versatility, for the ability to endure various stress conditions, for survival in soil inoculants, and for the potential to promote plant growth. The carbon source utilization data were similar for the wild-type and mutant strains, but the generation time of the wild-type strain was shorter than that of the mutant strain with all carbon sources tested. The ability of the wild type to endure UV irradiation, heat, osmotic pressure, osmotic shock, and desiccation and to grow in the presence of hydrogen peroxide was greater than that of the mutant strain. The motility and cell aggregation of the mutant strain were greater than the motility and cell aggregation of the wild type. However, the wild type exhibited greater chemotactic responses towards attractants than the mutant strain exhibited. The wild-type strain exhibited better survival than the mutant strain in carrier materials used for soil inoculants, but no difference in the ability to promote plant growth was detected between the strains. In soil, the two strains colonized roots to the same extent. It appears that synthesis and utilization of PHB as a carbon and energy source by A. brasilense under stress conditions favor establishment of this bacterium and its survival in competitive environments. However, in A. brasilense, PHB production does not seem to provide an advantage in root colonization under the conditions tested.

Improvement of the water status and yield of field-grown grain sorghum {Sorghum bicolor) by inoculation with Azospirillum brasilense

SUMMARY The effect of inoculation with Azospirillum brasilense on growth, water status and yield of dryland sorghum (cv. RS 610 and cv. H-226) growing on stored soil moisture was examined in three field experiments conducted during the years 1983-5. Plants were sampled at regular intervals, and the following characteristics were measured: dry-matter accumulation, leaf area, grain yield, percentage nitrogen and phosphorus in leaves, leaf water potential, canopy temperature, transpiration, stomatal conductance and soil water depletion. Inoculation led to an average increase of 19% in total stover dry-matter yield, as a result of higher rates of dry-matter accumulation during the early stages of growth. Azospirillum inoculation caused a 15-18% increase in grain yield in all three experiments. This increase was associated with a greater number of seeds per panicle. The water regime of sorghum plants was improved by inoculation, as seen in their higher leaf water potential, lower canopy temperatures and greater stomatal conductance and transpiration. Total extraction of soil moisture by inoculated plants was greater (by about 15%) and occurred from deeper soil layers, compared with non-inoculated controls. These findings indicate that inoculation with Azospirillum can lead to yield increases in dryland grain sorghum, primarily through improved utilization of soil moisture.