Plant Growth‐Promoting Rhizobacteria (PGPR) Reduce Evaporation and Increase Soil Water Retention

Abstract

Enhancement of plant drought stress tolerance by plant growth-promoting rhizobacteria (PGPR) has been increasingly documented in the literature. However, most studies to date have focused on PGPR-root/plant interactions; very little is known about PGPR's role in mediating physiochemical and hydrological changes in the rhizospheric soil that may impact plant drought stress tolerance. Our study aimed to advance mechanistic understanding of PGPR-mediated biophysical changes in the rhizospheric soil that may contribute to plant drought stress tolerance in addition to plant responses. We measured soil water retention characteristics, hydraulic conductivity, and water evaporation in soils with various textures (i.e., pure sand, sandy soil, and clay) as influenced by a representative PGPR (Bacillus subtilis strain UD1022) using the HYPROP system. We found that all PGPR-treated soils held more water and had reduced hydraulic conductivity and accumulative evaporation, compared to their corresponding controls. We discuss three mechanisms, due to B. subtilis incubation or production of extracellular polymeric substances (EPS), that are potentially responsible for the changes in hydraulic properties and soil evaporation: (i) EPS have a large water holding capacity; (ii) EPS alter soil matrix structure and connectivity of pore space; (iii) EPS modify the physicochemical properties of water (surface tension and viscosity). These results clearly demonstrate PGPR's ability to increase water availability to plants by slowing down evaporation and by increasing the time available for plants to make metabolic adjustments to drought stress.Plain Language Summary PGPR is a group of beneficial bacteria known to improve plant growth by, e.g., reducing pathogenic infection and/or promoting drought/salt tolerance. Despite the important role PGPR could potentially play in reducing drought stress to plants, we lack a complete understanding on the mechanisms through which PGPR mediate plant tolerance to drought. This study aimed to advance mechanistic understanding of PGPR-mediated biophysical changes in soil through microbe-soil interactions, to complement better understanding gained from previous studies that focused on microbe-plant interactions. Through laboratory measurements and imaging of water retention in soil, we show that a representative PGPR (B. subtilis UD1022) can increase soil water retention and reduce soil water evaporation. This effect is likely caused by the PGPR's ability to produce extracellular polymeric substances, which have high water holding capacity and can induce changes in soil physical properties. These changes lead to slower evaporation from soil, which can make more water available to plants as well as increase the time available for plants to make metabolic adjustments to drought stress. Our results provide scientific support to recent efforts in promoting application of rhizobacteria isolates as ''underground resource'' to contribute to solving globally challenging issues, e.g., water resource shortage and ...

Microhydrological Niches in Soils: How Mucilage and EPS Alter the Biophysical Properties of the Rhizosphere and Other Biological Hotspots

“…Note that the relative importance of these counteracting processes (i.e., the enhanced retention and connectivity vs. the increasing viscosity) on the unsaturated conductivity is soil texture dependent, as seen in previous studies (Volk et al, 2016;Zheng et al, 2018). Fig.…”

“…For soils embedded with EPS, a similar deceleration in soil drying was explained by the decrease in both saturated hydraulic conductivity and surface tension, which limit capillary rise, causing a discrepancy between evaporative flow and capillary flow and the consequent breakup of the liquid phase. This point marks the transition from Stage I (evaporation from the soil surface) to Stage II of soil drying (Zheng et al, 2018), when evaporation is reduced and controlled by vapor diffusion through the pore space (Lehmann et al, 2008).…”

“…Similar to the effect of mucilage, extracellular polymeric substances (EPS) produced by microorganism buffers fluctuations in soil moisture in hotspots of high biological activity, like the rhizosphere (Kuzyakov and Blagodatskaya, 2015), microbial colonies (Or et al, 2007; Zheng et al, 2018) and biocrusts (Chamizo et al, 2016; Couradeau et al, 2018; Rossi et al, 2018). Biocrusts stand out as an example, arguably the most extended biofilm on the planet (Elbert et al, 2012; Rodriguez‐Caballero et al, 2018).…”

“…In fine‐textured soils, EPS was even shown to increase the unsaturated conductivity (Volk et al, 2016). Along with these modifications, EPS‐treated soils (Chenu, 1993; Zheng et al, 2018), soils inoculated with a strain of Bacillus subtilis (Zheng et al, 2018), and soil micromodels inoculated with a mucoid strain of Sinorhizobium meliloti (Deng et al, 2015) dried slower than unamended control soil. The reduction in drying rates was absent when respective biofilms of S. meliloti were studied outside a porous matrix (Deng et al, 2015).…”

An interactive web-based tool to guide the preparation of neutron imaging experiments at oak ridge national laboratory

“…For the past several decades, neutron radiography and neutron computed tomography (CT) have been proven to be powerful techniques that provide unique insights in many research fields [1][2][3][4][5][6][7], including biology [8][9][10], archeology [11][12][13], additive manufacturing [14][15][16][17][18], energy storage [19][20][21][22][23][24][25][26][27][28], geology [29][30][31][32][33][34], etc. As a result, the neutron imaging community has grown rapidly and is attracting an increasing number of researchers who may not necessarily have neutron expertise.…”