Rhizodeposits, root exudates, and root border cells are vital components of the rhizosphere that significantly affect root colonization capacity and multiplication of rhizosphere microbes, as well as secretion of organic bioactive compounds. The rhizosphere is an ecological niche, in which beneficial bacteria compete with other microbiota for organic carbon compounds and interact with plants through root colonization activity to the soil. Some of these root-colonizing beneficial rhizobacteria also colonize endophytically and multiply inside plant roots. In the rhizosphere, these components contribute to complex physiological processes, including cell growth, cell differentiation, and suppression of plant pathogenic microbes. Understanding how rhizodeposits, root exudates, and root border cells interact in the rhizosphere in the presence of rhizobacterial populations is necessary to decipher their synergistic role for the improvement of plant health. This review highlights the diversity of plant growth-promoting rhizobacteria (PGPR) genera, their functions, and the interactions with rhizodeposits in the rhizosphere.Agriculture 2019, 9, 142 2 of 13 in the rhizosphere [12,13], and these interactions that influence plant growth and crop yields [14] can be root-root, root-insect, and root-microbe interactions [15]. The role of the rhizosphere is pivotal for plant growth-promotion, nutrition, and crop quality [16] because of the importance of plant-microbe interactions in the rhizosphere carbon sequestration, nutrient cycling, and ecosystem functioning [17]. In addition, the rhizosphere is where plant roots communicate with beneficial rhizobacteria for energy and nutrition. Plant growth-promoting rhizobacteria (PGPR) may affect plant growth, development, and disease suppression by one or more direct or indirect mechanisms. Bacterial genera such as Bacillus and Pseudomonas have been extensively studied and utilized as biocontrol agents, biofertilizers, and also have been shown to trigger induced systemic resistance (ISR) [18][19][20][21][22][23][24]. In this review, we discuss the importance, functions, and effects of root-derived organic molecules secreted in the rhizosphere and their interactions with plant growth-promoting rhizobacteria (PGPR) for enhancing plant growth and biological control of plant pathogens. PGPR Diversity in the RhizosphereThe plant rhizosphere contains diverse rhizobacterial species with the potential to enhance plant growth and biological control activity. PGPR genera present in the rhizosphere include Agrobacterium,
To understand the growth-promoting and disease-inhibiting activities of plant growth-promoting rhizobacteria (PGPR) strains, the genomes of 12 Bacillus subtilis group strains with PGPR activity were sequenced and analyzed. These B. subtilis strains exhibited high genomic diversity, whereas the genomes of B. amyloliquefaciens strains (a member of the B. subtilis group) are highly conserved. A pairwise BLASTp matrix revealed that gene family similarity among Bacillus genomes ranges from 32 to 90%, with 2839 genes within the core genome of B. amyloliquefaciens subsp. plantarum. Comparative genomic analyses of B. amyloliquefaciens strains identified genes that are linked with biological control and colonization of roots and/or leaves, including 73 genes uniquely associated with subsp. plantarum strains that have predicted functions related to signaling, transportation, secondary metabolite production, and carbon source utilization. Although B. amyloliquefaciens subsp. plantarum strains contain gene clusters that encode many different secondary metabolites, only polyketide biosynthetic clusters that encode difficidin and macrolactin are conserved within this subspecies. To evaluate their role in plant pathogen biocontrol, genes involved in secondary metabolite biosynthesis were deleted in a B. amyloliquefaciens subsp. plantarum strain, revealing that difficidin expression is critical in reducing the severity of disease, caused by Xanthomonas axonopodis pv. vesicatoria in tomato plants. This study defines genomic features of PGPR strains and links them with biocontrol activity and with host colonization.
Plant growth-promoting rhizobacteria (PGPR) are increasingly used in crops worldwide. While selected PGPR strains can reproducibly promote plant growth under controlled greenhouse conditions, their efficacy in the field is often more variable. Our overall aim was to determine if pectin or orange peel (OP) amendments to Bacillus velezensis (Bv) PGPR strains could increase soybean growth and nodulation by Bradyrhizobium japonicum in greenhouse and field experiments to reduce variability. The treatments included untreated soybean seeds planted in field soil that contained Bv PGPR strains and non-inoculated controls with and without 0.1% (w/v) pectin or (1 or 10 mg/200 μL) orange peel (OP) amendment. In greenhouse and field tests, 35 and 55 days after planting (DAP), the plants were removed from pots, washed, and analyzed for treatment effects. In greenhouse trials, the rhizobial inoculant was not added with Bv strains and pectin or OP amendment, but in the field trial, a commercial B. japonicum inoculant was used with Bv strains and pectin amendment. In the greenhouse tests, soybean seeds inoculated with Bv AP193 and pectin had significantly increased soybean shoot length, dry weight, and nodulation by indigenous Bradyrhizobium compared to AP193 without pectin. In the field trial, pectin with Bv AP193 significantly increased the shoot length, dry weight, and nodulation of a commercial Bradyrhizobium japonicum compared to Bv AP193 without pectin. In greenhouse tests, OP amendment with AP193 at 10 mg significantly increased the dry weight of shoots and roots compared to AP193 without OP amendment. The results demonstrate that pectin-rich amendments can enhance Bv-mediated soybean growth promotion and nodulation by indigenous and inoculated B. japonicum.
Rotor flux and torque of an induction motor (IM) are decoupled to obtain performance of DC motor. The decoupling strategy has been developed in terms of stator current components where the core loss is neglected. Many different controllers including fuzzy logic controller (FLC) with neglecting core loss have been designed to control the speed of induction motor. The outcome of investigation about the effect of core loss on indirect field oriented control (IFOC) has been concluded that the actual flux and torque are not reached to the reference flux and torque if core loss is neglected. Thus, the purpose of this paper is to propose a fuzzy logic speed controller of induction motor where flux and torque decoupling strategy is decoupled in terms of magnetizing current instead of stator current to alleviate the effects of core loss. The performances of proposed fuzzy-logic-based controller have been verified by computer simulation. The simulation of speed control of IM using PI and FLC are performed. The simulation study for high-performance control of IM drive shows the superiority of the proposed fuzzy logic controller over the conventional PI controller
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