Phytohormone Mediation of Interactions Between Plants and Non-Symbiotic Growth Promoting Bacteria Under Edaphic Stresses

Abstract

The capacity of rhizoshere bacteria to influence plant hormonal status, by bacterial production or metabolism of hormones, is considered an important mechanism by which they promote plant growth, and productivity. Nevertheless, inoculating these bacteria into the plant rhizosphere may produce beneficial or detrimental results depending on bacterial effects on hormone composition and quantity in planta, and the environmental conditions under which the plants are growing. This review considers some effects of bacterial hormone production or metabolism on root growth and development and shoot physiological processes. We analyze how these changes in root and shoot growth and function help plants adapt to their growth conditions, especially as these change from optimal to stressful. Consistent effects are addressed, along with plant responses to specific environmental stresses: drought, salinity, and soil contamination (with petroleum in particular).

Salt-Tolerant Plant Growth Promoting Rhizobacteria for Enhancing Crop Productivity of Saline Soils

“…The ST-PGPR utilize an array of mechanisms (Figure 3) which directly or indirectly take part in amelioration of salt stress in crop plants (Egamberdieva et al, 2016;Hashem et al, 2016). Studies confirmed that ST-PGPR produce various types of phytohormones, such as auxins, gibberellins, cytokinins (Dodd et al, 2010), synthesize ACC deaminase (Glick et al, 2007), produce secondary compounds such as exopolysaccharides (Upadhyay et al, 2012;Timmusk et al, 2014) and osmolytes (proline, trehalose, and glycine betaines) (Bano and Fatima, 2009;Upadhyay and Singh, 2015), regulate plant defense systems and activate plant's antioxidative enzymes under salt stress (Hashem et al, 2016).…”

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

“…Sandhya et al [117] found that a quarter of pseudomonads that were isolated from plants growing in arid and semi-arid areas showed increased cytokinin production under osmotic stress conditions. Elucidating the role of cytokinins under saline stress, a hypothesis illustrates that may be limiting the root proliferation leads to reduced uptake of salts by plants, but the larger picture of cytokinin driven tolerance mechanism is still unknown [118] .…”

Capacity of Pseudomonas Strains to Degrade Hydrocarbons, Produce Auxins and Maintain Plant Growth under Normal Conditions and in the Presence of Petroleum Contaminants

“…Han et al [20] showed that the plant hormone brassinolide could promote the photosynthesis of Robinia pseudoacacia seedlings in petroleum-stress conditions, thereby counteracting its adverse effects. Microorganisms can directly influence plant growth by synthesizing growth-stimulating hormones [21][22][23]. Nevertheless, only a small number of publications have addressed the synthesis of hormones by plant growth promoting rhizobacteria (PGPR) under conditions of oil pollution and their phytoremedial effect on plants.…”