One of the most exciting scientific advances in recent decades has been the realization that the diverse and immensely active microbial communities are not only ‘passengers’ with plants, but instead play an important role in plant growth, development and resistance to biotic and abiotic stresses. A picture is emerging where plant roots act as ‘gatekeepers’ to screen soil bacteria from the rhizosphere and rhizoplane. This typically results in root endophytic microbiome dominated by Proteobacteria, Actinobacteria and to a lesser extent Bacteroidetes and Firmicutes, but Acidobacteria and Gemmatimonadetes being almost depleted. A synthesis of available data suggest that motility, plant cell-wall degradation ability and reactive oxygen species scavenging seem to be crucial traits for successful endophytic colonization and establishment of bacteria. Recent studies provide solid evidence that these bacteria serve host functions such as improving of plant nutrients through acquisition of nutrients from soil and nitrogen fixation in leaves. Additionally, some endophytes can engage ‘priming’ plants which elicit a faster and stronger plant defense once pathogens attack. Due to these plant growth-promoting effects, endophytic bacteria are being widely explored for their use in the improvement of crop performance. Updating the insights into the mechanism of endophytic bacterial colonization and interactions with plants is an important step in potentially manipulating endophytic bacteria/microbiome for viable strategies to improve agricultural production.

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Inner Plant Values: Diversity, Colonization and Benefits from Endophytic Bacteria.

Biomass production for bioenergy using marginal lands

Halotolerant plant growth–promoting bacteria: Prospects for alleviating salinity stress in plants

Halo-tolerant plant growth promoting rhizobacteria for improving productivity and remediation of saline soils.

Soil salinity is a major limiting factor for crop productivity worldwide and is continuously increasing owing to climate change. A wide range of studies and practices have been performed to induce salt tolerance mechanisms in plants, but their result in crop improvement has been limited due to lack of time and money. In the current scenario, there is increasing attention towards habitat-imposed plant stress tolerance driven by plant-associated microbes, either rhizospheric and/or endophytic. These microbes play a key role in protecting plants against various environmental stresses. Therefore, the use of plant growth-promoting microbes in agriculture is a low-cost and eco-friendly technology to enhance crop productivity in saline areas. In the present review, the authors describe the functionality of endophytic bacteria and their modes of action to enhance salinity tolerance in plants, with special reference to osmotic and ionic stress management. There is concrete evidence that endophytic bacteria serve host functions, such as improving osmolytes, anti-oxidant and phytohormonal signaling and enhancing plant nutrient uptake efficiency. More research on endophytes has enabled us to gain insights into the mechanism of colonization and their interactions with plants. With this information in mind, the authors tried to solve the following questions: (1) how do benign endophytes ameliorate salt stress in plants? (2) What type of physiological changes incur in plants under salt stress conditions? And (3), what type of determinants produced by endophytes will be helpful in plant growth promotion under salt stress?

Endophytic Bacteria in Plant Salt Stress Tolerance: Current and Future Prospects

Halophytes are plants that are adapted to grow in saline soils, and have been widely studied for their physiological and molecular characteristics, but little is known about their associated microbiomes. Bacteria were isolated from the rhizosphere and as root endophytes of Salicornia rubra, Sarcocornia utahensis, and Allenrolfea occidentalis, three native Utah halophytes. A total of 41 independent isolates were identified by 16S rRNA gene sequencing analysis. Isolates were tested for maximum salt tolerance, and some were able to grow in the presence of up to 4 M NaCl. Pigmentation, Gram stain characteristics, optimal temperature for growth, and biofilm formation of each isolate aided in species identification. Some variation in the bacterial population was observed in samples collected at different times of the year, while most of the genera were present regardless of the sampling time. Halomonas, Bacillus, and Kushneria species were consistently isolated both from the soil and as endophytes from roots of all three plant species at all collection times. Non-culturable bacterial species were analyzed by Illumina DNA sequencing. The most commonly identified bacteria were from several phyla commonly found in soil or extreme environments: Acidobacteria, Actinobacteria, Bacteroidetes, Chloroflexi, and Gamma- and Delta-Proteobacteria. Isolates were tested for the ability to stimulate growth of alfalfa under saline conditions. This screening led to the identification of one Halomonas and one Bacillus isolate that, when used to inoculate young alfalfa seedlings, stimulate plant growth in the presence of 1% NaCl, a level that significantly inhibits growth of uninoculated plants. The same bacteria used in the inoculation were recovered from surface sterilized alfalfa roots, indicating the ability of the inoculum to become established as an endophyte. The results with these isolates have exciting promise for enhancing the growth of inoculated alfalfa in salty soil.

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Salt-Tolerant Halophyte Rhizosphere Bacteria Stimulate Growth of Alfalfa in Salty Soil.

Switchgrass (Panicum virgatum L.) is a perennial warm season grass capable of growth on marginal lands without major inputs of water and fertilizers. However, full utilization of its agronomic potential as a bioenergy crop requires improvement of its stand establishment, disease resistance, and prevention of the biomass yield variation from year to year. Our program focuses on the utilization of beneficial bacterial endophytes to enhance switchgrass performance under a low-input production system on marginal lands. We demonstrated earlier that inoculation of switchgrass cv. Alamo with a growth promoting endophyte, Burkholderia phytofirmans strain PsJN (PsJN), can significantly enhance seedling vigor and plant growth under both in vitro and greenhouse conditions. In this study, we tested the effects of PsJN bacterization of switchgrass seedlings on stand establishment, plant growth, and biomass yield in three field experiments conducted over 2 years on highly fertile prime land, and on a former tobacco farm with low fertility soil. PsJN bacterization improved growth and development of switchgrass seedlings, significantly stimulated plant root and shoot growth, and tiller number on the low fertility soil (p < 0.001), and enhanced biomass accumulation on both poor (p < 0.001) and rich (p < 0.05) soils, with more effective stimulation of plant growth in low fertility soil than in high fertility soil. The study indicates the potential for the use of PsJN and/or other beneficial bacterial endophytes in the development of low-input switchgrass feedstock production systems.

Switchgrass Field Performance on Two Soils as Affected by Bacterization of Seedlings with Burkholderia phytofirmans Strain PsJN

OpenCMP is a computational multiphysics software package based on the finite element method (Ferziger & Perić, 2002). It is primarily intended for physicochemical processes in which fluid convection plays a significant role. OpenCMP uses the NGSolve finite element library (Schöberl, n.d.) for spatial discretization and provides a configuration file-based interface for pre-implemented models and time discretization schemes. It also integrates with Netgen (Schöberl, n.d.) and Gmsh (Geuzaine & Remacle, 2009) for geometry construction and meshing. Additionally, it provides users with built-in functionality for post-processing, error analysis, and data export for visualisation using Netgen (Schöberl, n.d.) or ParaView (Ahrens et al., 2005).

J. Scott Lowman

Papers

Salt-Tolerant Halophyte Rhizosphere Bacteria Stimulate Growth of Alfalfa in Salty Soil.

Switchgrass Field Performance on Two Soils as Affected by Bacterization of Seedlings with Burkholderia phytofirmans Strain PsJN

OpenCMP: An Open-Source Computational Multiphysics Package