Variation from plant tissue cultures: Biotechnological application to improving salinity tolerance

Zinc tolerance and accumulation in stable cell suspension cultures and in vitro regenerated plants of the emerging model plant Arabidopsis halleri (Brassicaceae)

Arabidopsis halleri is increasingly employed as a model plant for studying heavy metal hyperaccumulation. With the aim of providing valuable tools for studies on cellular physiology and molecular biology of metal tolerance and transport, this study reports the development of successful and highly efficient methods for the in vitro regeneration of A. halleri plants and production of stable cell suspension lines. Plants were regenerated from leaf explants of A. halleri via a three-step procedure: callus induction, somatic embryogenesis and shoot development. Efficiency of callus proliferation and regeneration depended on the initial callus induction media and was optimal in the presence of 1 mg L(-1) 2,4-dichlorophenoxyacetic acid, and 0.05 mg L(-1) benzylaminopurine. Subsequent shoot and root regeneration from callus initiated under these conditions reached levels of 100% efficiency. High friability of the callus supported the development of cell suspension cultures with minimal cellular aggregates. Characterization of regenerated plants and cell cultures determined that they maintained not only the zinc tolerance and requirement of the whole plant but also the ability to accumulate zinc; with plants accumulating up to 50.0 micromoles zinc g(-1) FW, and cell suspension cultures 30.9 micromoles zinc g(-1) DW. Together this work will provide the experimental basis for furthering our knowledge of A. halleri as a model heavy metal hyperaccumulating plant.

Growth, proline accumulation and water relations of NaCl-selected and non-selected callus lines of Dactylis glomerata L

Sodium chloride-tolerant calli were selected from leaf-derived embryogenic calli of Dactylis glomerata L. on agar solidified medium supplemented with 200 mM NaCl, a concentration lethal to non-selected calli. Growth characteristics, water relations and proline accumulation pattern were compared in selected and non-selected lines. The objective was to gain an understanding of the mechanism(s) of tolerance in the NaCl-tolerant line. Growth in the selected line, as expressed in terms of tolerance index (ratio of fresh wt. on NaCl medium:fresh wt. on NaCl free medium x 100), was greater than that of the non-selected line at all levels of NaCl between 50 and 300 mM. There was no significant difference in proline accumulation in the selected and non-selected lines. Maintenance of turgor by osmotic adjustment was observed in the non-selected line despite decreased growth. In contrast, the selected line lost either the need or the ability to adjust osmotically. There was little or no increase in symplastic osmolality in the selected line when exposed to NaCl. Presumably, selection was made for a salt-excluding tissue that has lost the ability to accumulate solutes and adjust turgor with NaCl stress.

Production of sodium-chloride-tolerant Brassica juncea plants by in vitro selection at the somatic embryo level

Somatic embryos, developed from hypocotyl segments of light-grown seedlings of Brassica juncea cv RLM198, were subjected to selection at varying concentrations of sodium chloride (NaCl). Plants were developed from proliferated somatic embryos selected on NaCl-containing medium. The selections were characterized for salt tolerance, esterase isozyme pattern, and proline accumulation. It has been found that: (i) selected tolerant lines showed better root growth, shoot growth, and fresh weight accumulation on salt-containing medium when compared to the control; (ii) salt tolerance was transmitted to the next generation in seed progeny of tolerant plants grown in the absence of exposure to salt; (iii) both the starting material and the tolerant selections accumulated proline, even when grown in salt-free medium. On salt-containing medium, however, the differences in accumulated proline between the control and tolerant lines became more pronounced, and (iv) the patterns of esterase isozymes of two tolerant selections were similar but distinctly different from that of the parental control.

Proline accumulation and sodium sulfate tolerance in callus cultures of Brassica napus L. cv. Westar

Callus cultures of Brassica napus L. cv. Westar were selected which contained 5 - 6 times more proline than unselected callus. Callus pieces from these cultures were able to survive much better after subculture to medium containing 105 mM Na2SO4 than unselected callus, or unselected callus cultured on exogenous proline before or during transfer to the salt. Exogenous proline was rapidly absorbed. In unselected callus there was a peak in proline accumulation ca. 2 days after transfer to Na2SO4, followed by a decline. In contrast proline accumulation in tolerant callus was linear with time, reaching maximum levels at 8 days. Proline levels induced by exposure to salt were maintained in the absence of stress.