Baby Shaharoona

Effect of plant growth promoting rhizobacteria containing ACC-deaminase on maize (Zea mays L.) growth under axenic conditions and on nodulation in mung bean (Vigna radiata L.)

Aims:  This study was conducted to test the hypothesis that the bacterial strains possessing 1‐aminocyclopropane‐1‐carboxylic acid (ACC)‐deaminase activity may also promote growth of inoculated plants and could increase nodulation in legumes upon co‐inoculation with rhizobia.

Inoculation with Pseudomonas spp. Containing ACC-Deaminase Partially Eliminates the Effects of Drought Stress on Growth, Yield, and Ripening of Pea (Pisum sativum L.)

Abstract Two preselected plant growth promoting rhizobacteria (PGPR) containing 1-aminocyclopropane-1-carboxylate (ACC)-deaminase (EC 4.1.99.4) were used to investigate their potential to ameliorate the effects of drought stress on growth, yield, and ripening of pea ( Pisum sativum L.). Inoculated and uninoculated (control) seeds of pea cultivar 2000 were sown in pots (four seeds pot −1 ) and placed in a wire house. The plants were exposed to drought stress at different stages of growth (vegetative, flowering, and pod formation) by skipping the respective irrigation. Results revealed that inoculation of peas with PGPR containing ACC-deaminase significantly decreased the “drought stress imposed effects” on the growth and yield of peas. Exposure of plants to drought stress at vegetative growth stage significantly decreased shoot growth by 41% in the case of uninoculated plants, whereas, by only 18% in the case of inoculated plants compared to nonstressed uninoculated control. Grain yield was decreased when plants were exposed to drought stress at the flowering and pod formation stage, but inoculation resulted in better grain yield (up to 62% and 40% higher, respectively) than the respective uninoculated nonstressed control. Ripening of pods was also delayed in plants inoculated with PGPR, which may imply decreased endogenous ethylene production in inoculated plants. This premise is further supported by the observation that inoculation with PGPR reduced the intensity of classical “triple” response in etiolated pea seedlings, caused by externally applied ACC. It is very probable that the drought stress induced inhibitory effects of ethylene could be partially or completely eliminated by inoculation with PGPR containing ACC-deaminase.

Fertilizer-dependent efficiency of Pseudomonads for improving growth, yield, and nutrient use efficiency of wheat (Triticum aestivum L.)

Acquisition of nutrients by plants is primarily dependent on root growth and bioavailability of nutrients in the rooting medium. Most of the beneficial bacteria enhance root growth, but their effectiveness could be influenced by the nutrient status around the roots. In this study, two 1-aminocyclopropane-1-carboxylate (ACC)-deaminase containing plant-growth-promoting rhizobacteria (PGPR), Pseudomonas fluorescens and P. fluorescens biotype F were tested for their effect on growth, yield, and nutrient use efficiency of wheat under simultaneously varying levels of all the three major nutrients N, P, and K (at 0%, 25%, 50%, 75%, and 100% of recommended doses). Results of pot and field trials revealed that the efficacy of these strains for improving growth and yield of wheat reduced with the increasing rates of NPK added to the soil. In most of the cases, significant negative linear correlations were recorded between percentage increases in growth and yield parameters of wheat caused by inoculation and increasing levels of applied NPK fertilizers. It is highly likely that under low fertilizer application, the ACC-deaminase activity of PGPR might have caused reduction in the synthesis of stress (nutrient)-induced inhibitory levels of ethylene in the roots through ACC hydrolysis into NH3 and α-ketobutyrate. The results of this study imply that these Pseudomonads could be employed in combination with appropriate doses of fertilizers for better plant growth and savings of fertilizers.

Plant Growth Promoting Rhizobacteria and Sustainable Agriculture

The diverse groups of bacteria in close association with roots and capable of stimulating plant growth by any mechanism(s) of action are referred to as plant growth-promoting rhizobacteria (PGPR). They affect plant growth and development directly or indirectly either by releasing plant growth regulators (PGRs) or other biologically active substances, altering endogenous levels of PGRs, enhancing availability and uptake of nutrients through fixation and mobilization, reducing harmful effects of pathogenic microorganisms on plants and/or by employing multiple mechanisms of action. Recently, PGPR have received more attention for use as a biofertilizer for the sustainability of agro-ecosystems. Selection of efficient PGPR strains based on well-defined mechanism(s) for the formulation of biofertilizers is vital for achieving consistent and reproducible results under field conditions. Numerous studies have suggested that PGPR-based biofertilizers could be used as effective supplements to chemical fertilizers to promote crop yields on sustainable basis. Various aspects of PGPR biotechnology are reviewed and discussed.

Comparative effectiveness of Bacillus spp. possessing either dual or single growth-promoting traits for improving phosphorus uptake, growth and yield of wheat (Triticum aestivum L.)

The aim of this study was to compare the effectiveness of Bacillus spp. simultaneously carrying dual traits such as P solubilization and ACC deaminase activity with Bacillus spp. having any one of these traits for improving growth, yield and P uptake by wheat crop. Six Bacillus strains having predominantly either ACC deaminase activity (KA1 and KA2) or P solubilizing activity (KP3 and KP4) or simultaneously both of these traits (KAP5 and KAP6) were evaluated for improving growth of wheat cv. Bhakar-2002 using rock phosphate (RP) as an exclusive P-source. Under axenic conditions, the bacterial strains with dual plant growth-promoting activities were superior in improving growth of wheat as compared to the strains possessing single trait. Similarly, these dual traits bacterial strains were more effective than single trait strains under soil conditions (pot trial) in increasing root weight (up to 3.9-fold) and root elongation (up to 3.8-fold), dry shoot weight (up to 37.6%), number of tillers (up to 56%), grain yield (up to 38.5%) and P uptake in grain (up to 77.4%) of wheat grown in the presence of P applied as diammonium phosphate (DAP), RP (rock phosphate) or RP-enriched compost. An almost similar trend was observed when the same trial was repeated under field conditions. Inoculation in the presence of RP-enriched compost resulted in promoting various growth parameters almost comparable to that recorded in the case of DAP. It was concluded that the simultaneous presence of two superior plant growth-promoting traits in the bacteria could have an additive effect not only on growth and yield of wheat but also on P uptake. The performance of Bacillus strains possessing dual traits was distinctly superior to that of the single trait strains. These bacteria exhibited an excellent effectiveness in utilizing RP as the source of P in the growth medium as well as in soil.

Manipulation of Ethylene Synthesis in Roots Through Bacterial ACC Deaminase for Improving Nodulation in Legumes

Symbiotic association between rhizobia and legumes results in the development of unique structures on roots, called nodules. Nodulation is a very complex process involving a variety of genes that control NOD factors (bacterial signaling molecules), which are essential for the establishment, maintenance and regulation of this process and development of root nodules. Ethylene is an established potent plant hormone that is also known for its negative role in nodulation. Ethylene is produced endogenously in all plant tissues, particularly in response to both biotic and abiotic stresses. Exogenous application of ethylene and ethylene-releasing compounds are known to inhibit the formation and functioning of nodules. While inhibitors of ethylene synthesis or its physiological action enhance nodulation in legumes, some rhizobial strains also nodulate the host plant intensively, most likely by lowering endogenous ethylene levels in roots through their 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. Co-inoculation with ACC deaminase containing plant growth promoting rhizobacteria plus rhizobia has been shown to further promote nodulation compared to rhizobia alone. Transgenic rhizobia or legume plants with expression of bacterial ACC deaminase could be another viable option to alleviate the negative effects of ethylene on nodulation. Several studies have well documented the role of ethylene and bacterial ACC deaminase in development of nodules on legume roots and will be the primary focus of this critical review.

Role of Ethylene and Plant Growth-Promoting Rhizobacteria in Stressed Crop Plants

Ethylene is a gaseous plant growth hormone produced endogenously by almost all plants and plays a key role in inducing multifarious physiological changes in plants at molecular level. Endogenous production of ethylene is accelerated substantially in response to biotic and abiotic stresses which adversely affects the root growth and consequently the growth of the plant as a whole. Certain plant growth promoting rhizobacteria (PGPR) contain a vital enzyme, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, which regulates ethylene production by metabolizing ACC into α-ketobutyrate and ammonia. Inoculation with such PGPR could be helpful in sustaining plant growth and development under stress conditions by reducing stress-induced ethylene production. In this chapter, all aspects of such PGPR regarding alleviation of “stresses imposed effects” on plants will be discussed.