Soil Chemical Analysis

The worldwide increases in both environmental damage and human population pressure have the unfortunate consequence that global food production may soon become insufficient to feed all of the world's people. It is therefore essential that agricultural productivity be significantly increased within the next few decades. To this end, agricultural practice is moving toward a more sustainable and environmentally friendly approach. This includes both the increasing use of transgenic plants and plant growth-promoting bacteria as a part of mainstream agricultural practice. Here, a number of the mechanisms utilized by plant growth-promoting bacteria are discussed and considered. It is envisioned that in the not too distant future, plant growth-promoting bacteria (PGPB) will begin to replace the use of chemicals in agriculture, horticulture, silviculture, and environmental cleanup strategies. While there may not be one simple strategy that can effectively promote the growth of all plants under all conditions, some of the strategies that are discussed already show great promise.

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Plant Growth-Promoting Bacteria: Mechanisms and Applications

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A User’s Guide

Soil salinity: A serious environmental issue and plant growth promoting bacteria as one of the tools for its alleviation.

Mechanisms and applications of plant growth promoting rhizobacteria: Current perspective

Phytoremediation: Environmentally sustainable way for reclamation of heavy metal polluted soils.

A series of experiments were conducted to assess the effectiveness of rhizobacteria containing 1-aminocyclopropane- 1-carboxylate (ACC) deaminase for growth promotion of peas under drought conditions. Ten rhizobacteria isolated from the rhizosphere of different crops (peas, wheat, and maize) were screened for their growth promoting ability in peas under axenic condition. Three rhizobacterial isolates, Pseudomonas fluorescens biotype G (ACC-5), P. fluorescens (ACC-14), and P. putida biotype A (Q-7), were selected for pot trial on the basis of their source, ACC deaminase activity, root colonization, and growth promoting activity under axenic conditions. Inoculated and uninoculated (control) seeds of pea cultivar 2000 were sown in pots (4 seeds/pot) at different soil moisture levels (25, 50, 75, and 100% of field capacity). Results revealed that decreasing the soil moisture levels from 100 to 25% of field capacity significantly decreased the growth of peas. However, inoculation of peas with rhizobacteria containing ACC deaminase significantly decreased the "drought stress imposed effects" on growth of peas, although with variable efficacy at different moisture levels. At the lowest soil moisture level (25% field capacity), rhizobacterial isolate Pseudomonas fluorescens biotype G (ACC-5) was found to be more promising compared with the other isolates, as it caused maximum increases in fresh weight, dry weight, root length, shoot length, number of leaves per plant, and water use efficiency on fresh and dry weight basis (45, 150, 92, 45, 140, 46, and 147%, respectively) compared with respective uninoculated controls. It is highly likely that rhizobacteria containing ACC deaminase might have decreased the drought-stress induced ethylene in inoculated plants, which resulted in better growth of plants even at low moisture levels. Therefore, inoculation with rhizobacteria containing ACC deaminase could be helpful in eliminating the inhibitory effects of drought stress on the growth of peas.

https://www.koreascience.or.kr:443/article/JAKO200822179194535.pdf

Effectiveness of rhizobacteria containing ACC deaminase for growth promotion of peas (Pisum sativum) under drought conditions.

Rhizobacteria were isolated from the rhizosphere of different Brassica species and assayed for their ability to produce auxins in vitro. The isolates varied greatly in their potential for auxin production (ranging from 0.33 to 11.40 µg ml–1). L-Tryptophan (an auxin precursor) addition to the media increased the auxin production by several fold. Based upon in vitro auxin production and growth promotion of B. juncea seedlings caused by various isolates under gnotobiotic conditions, promising isolates were selected and tested in pot trial to observe their effects on growth, yield and oil content of the same Brassica species. Results showed that seed inoculation with different isolates of rhizobacteria significantly increased plant height (up to 56.5%), stem diameter (up to 11.0%), number of branches (up to 35.7%), number of pods per plant (up to 26.7%), 1,000-grain weight (up to 33.9%), grain yield (up to 45.4%) and oil content (up to 5.6%) over the uninoculated control. Isolate S54 gave the most promising and consistent results. Highly significant correlations between L-TRP-derived auxin production by plant growth-promoting rhizobacteria (PGPR) in vitro and grain yield (r =0.77**), number of pods (r =0.78**) and number of branches per plant (r =0.77**) of B. juncea were found. It was hypothesized that these PGPR may influence the growth and yield of inoculated plants by production of auxins in the rhizosphere of inoculated plants from the L-TRP present in the root exudates, although other mechanisms of action might have also contributed.

Relationship between in vitro production of auxins by rhizobacteria and their growth-promoting activities in Brassica juncea L.

Ethylene synthesis is accelerated in response to various environmental stresses like salinity. Ten rhizobacterial strains isolated from wheat rhizosphere taken from different salt affected areas were screened for growth promotion of wheat under axenic conditions at 1, 5, 10 and 15 dS m−1. Three strains, i.e., Pseudomonas putida (N21), Pseudomonas aeruginosa (N39) and Serratia proteamaculans (M35) showing promising performance under axenic conditions were selected for a pot trial at 1.63 (original), 5, 10 and 15 dS m−1. Results showed that inoculation was effective even in the presence of higher salinity levels. P. putida was the most efficient strain compared to the other strains and significantly increased the plant height, root length, grain yield, 100-grain weight and straw yield up to 52, 60, 76, 19 and 67%, respectively, over uninoculated control at 15 dS m−1. Similarly, chlorophyll content and K+/Na+ of leaves also increased by P. putida over control. It is highly likely that under salinity stress, 1-aminocyclopropane-1-carboxylic acid-deaminase activity of these microbial strains might have caused reduction in the synthesis of stress (salt)-induced inhibitory levels of ethylene. The results suggested that these strains could be employed for salinity tolerance in wheat; however, P. putida may have better prospects in stress alleviation/reduction.

Comparative effectiveness of Pseudomonas and Serratia sp. containing ACC-deaminase for improving growth and yield of wheat (Triticum aestivum L.) under salt-stressed conditions.

Rhizobacteria may promote plant growth by cleaving plant-produced 1-aminocyclopropane-1-carboxylic acid (ACC) through ACC-deaminase activity. The effectiveness of plant-growth-promoting rhizobacteria (PGPR) for improving the growth and yield of wheat (Triticum aestivum L.) was evaluated under salinity stress. The effect of high ethylene concentrations on plant growth and performance of these strains for reducing this negative impact was also evaluated by conducting a classical triple-response bioassay. Eighteen rhizobacterial strains containing ACC-deaminase were screened by conducting a jar experiment under axenic conditions at 1 (original), 5,10, and 15 dS m -1 . The four most effective strains were further evaluated in a pot trial at salinity levels of 1.46 (original), 5, 10, and 15 dS m -1 . In general, salinity depressed the growth of wheat, but inoculation improved the growth and yield of wheat compared with the uninoculated controls. At the high salinity level (15 dS m-1 ) , plant height, root length, plant biomass, and grain yield increased up to 37, 70,116, and 111%, respectively, compared with the control. Results indicated that inoculated plants had higher K + /Na + ratios, relative water contents and chlorophyll content; however, relatively low proline contents compared with controls. The results also showed that intensity of the classical triple response decreased due to inoculation with these strains. Pseudomonas putida (W2) and P. fluorescens (W17) were the most effective strains for alleviating salt stress even at higher salinity levels. Our results suggest that the assay for ACC-deaminase activity could be an efficient approach to screen effective PGPR for increasing the growth and yield of wheat under salt-stressed conditions.

Hafiz Naeem Asghar

Papers

Phytoremediation: Environmentally sustainable way for reclamation of heavy metal polluted soils.

Effectiveness of rhizobacteria containing ACC deaminase for growth promotion of peas (Pisum sativum) under drought conditions.

Relationship between in vitro production of auxins by rhizobacteria and their growth-promoting activities in Brassica juncea L.

Comparative effectiveness of Pseudomonas and Serratia sp. containing ACC-deaminase for improving growth and yield of wheat (Triticum aestivum L.) under salt-stressed conditions.

Rhizobacteria Capable of Producing ACC-deaminase May Mitigate Salt Stress in Wheat