Inoculation with plant growth-promoting rhizobacteria can increase plant salt resistance. We aimed to reveal bacterial effects on the formation of apoplastic barriers and hormone concentration in relation to maintaining ion homeostasis and growth of salt-stressed plants. The rhizosphere of a durum wheat variety was inoculated with cytokinin-producing Bacillus subtilis and auxin-producing Pseudomonas mandelii strains. Plant growth, deposition of lignin and suberin and concentrations of sodium, potassium, phosphorus and hormones were studied in the plants exposed to salinity. Accumulation of sodium inhibited plant growth accompanied by a decline in potassium in roots and phosphorus in shoots of the salt-stressed plants. Inoculation with both bacterial strains resulted in faster appearance of Casparian bands in root endodermis and an increased growth of salt-stressed plants. B. subtilis prevented the decline in both potassium and phosphorus concentrations and increased concentration of cytokinins in salt-stressed plants. P. mandelii decreased the level of sodium accumulation and increased the concentration of auxin. Growth promotion was greater in plants inoculated with B. subtilis. Increased ion homeostasis may be related to the capacity of bacteria to accelerate the formation of Casparian bands preventing uncontrolled diffusion of solutes through the apoplast. We discuss the relative impacts of the decline in Na accumulation and maintenance of K and P content for growth improvement of salt-stressed plants and their possible relation to the changes in hormone concentration in plants.
Plant organogenesis remains one of the most essential questions of plant developmental biology. Callus tissue in vitro is a valuable tool for the studies on hormonal aspects of plant organogenesis, especially its early events, and immunohistochemical analysis is one of the few approaches offering information on the localization and role of hormones during organ development. The localization of endogenous zeatin and indole-3-acetic acid was investigated during simultaneous bud and root formation in calluses derived from immature embryos of wheat (Triticum aestivum L.). Calluses were induced on Murashige and Skoog (MS) medium supplemented with 2.0 mg L −1 2,4-dichlorophenoxyacetic acid. To stimulate simultaneous bud and root formation, calluses were transferred onto MS medium supplemented with 0.2 mg L −1 kinetin and 0.2 mg L −1 indoleacetic acid. Strong immunostaining for both hormones was detected in proliferating callus tissue, in developing meristematic centers and meristematic zones (whose cells were shown to be involved in organ formation), and at the sites of shoot and root apex initiation. During further development, shoot apexes with leaf primordia were heavily immunostained for zeatin, while immunostaining for indole-3-acetic acid was more intense at the sites of leaf primordia initiation and incipient primordia themselves. In the developing roots, immunostaining for both hormones reached a maximum in the root apex and gradually declined with increasing distance from the apex. Cells of developing procambial strands were also strongly stained for both zeatin and indole-3-acetic acid. These data suggest considerable similarity between patterns of hormone distribution in organs in vitro and in vivo. Thus, callus culture is a convenient and useful model for the study of fundamental biological questions such as how hormones regulate development.
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