Cytokinin-producing, plant growth-promoting rhizobacteria that confer resistance to drought stress in Platycladus orientalis container seedlings

Abstract

One of the proposed mechanisms through which plant growth-promoting rhizobacteria (PGPR) enhance plant growth is the production of plant growth regulators, especially cytokinin. However, little information is available regarding cytokinin-producing PGPR inoculation on growth and water stress consistence of forest container seedlings under drought condition. This study determined the effects of Bacillus subtilis on hormone concentration, drought resistance, and plant growth under water-stressed conditions. Although no significant difference was observed under well-watered conditions, leaves of inoculated Platycladus orientalis (oriental thuja) seedlings under drought stress had higher relative water content and leaf water potential compared with those of noninoculated ones. Regardless of water supply levels, the root exudates, namely sugars, amino acids and organic acids, significantly increased because of B. subtilis inoculation. Water stress reduced shoot cytokinins by 39.14 %. However, inoculation decreased this deficit to only 10.22 %. The elevated levels of cytokinins in P. orientalis shoot were associated with higher concentration of abscisic acid (ABA). Stomatal conductance was significantly increased by B. subtilis inoculation in well-watered seedlings. However, the promoting effect of cytokinins on stomatal conductance was hampered, possibly by the combined action of elevated cytokinins and ABA. B. subtilis inoculation increased the shoot dry weight of well-watered and drought seedlings by 34.85 and 19.23 %, as well as the root by 15.445 and 13.99 %, respectively. Consequently, the root/shoot ratio significantly decreased, indicative of the greater benefits of PGPR on shoot growth than root. Thus, inoculation of cytokinin-producing PGPR in container seedlings can alleviate the drought stress and interfere with the suppression of shoot growth, showing a real potential to perform as a drought stress inhibitor in arid environments.

Rhizosphere engineering: Enhancing sustainable plant ecosystem productivity

“…Leaves of oriental thuja (Platycladus orientalis) inoculated with Bacillus subtilis increased stomatal conductance and ABA levels in shoots, conferring drought resistance to container-grown plants. Inoculated plants had increased leafwater content and water potential and an increase in cytokinin concentration, which was linked to the increase in ABA levels (Liu et al, 2013). In another study, Phyllobacterium brassicacearum strain STM196, isolated from the rhizosphere of oilseed rape (Brassica napus), was shown to help alleviate drought stress in inoculated Arabidopsis plants by increasing ABA levels and decreasing leaf transpiration, thereby enhancing osmotic stress tolerance (Bresson et al, 2013).…”

Plant-growth-promoting rhizobacteria: drought stress alleviators to ameliorate crop production in drylands

“…PGPR Phyllobacterium brassicacearum strain STM196, isolated from the rhizosphere of Brassica napus, improved osmotic stress tolerance in inoculated Arabidopsis plants by elevating ABA content, leading to decreased leaf transpiration (Bresson et al 2013). Inoculation of Platycladus orientalis container seedlings with cytokininproducing PGPR (Bacillus subtilis) has been reported to interfere with suppression of shoot growth, thus conferring drought stress resistance (Liu et al 2013). Ethylene biosynthesis is increased during drought stress that results in reduced root and shoot growth.…”

Bacillus aryabhattai SRB02 tolerates oxidative and nitrosative stress and promotes the growth of soybean by modulating the production of phytohormones

“…PGPR have been shown to promote the production of growth promoting phytohormones [26] such as IAA [27, 28], cytokinin [29–31] and gibberellins [32–34], inhibit the synthesis of ethylene [28, 35], and regulate endogenous ABA levels in plants [10]. Furthermore, Serratia marcescens strain 90–166 has been shown to protect plants against cucumber mosaic virus (CMV) in a jasmonic acid (JA)-dependent but SA/NPR1-independent manner, demonstrating the activation of a JA signaling pathway by PGPR [36, 37].…”