Improvement of salt tolerance in plants by in vitro selection at the cellular level

Abstract

Somaclonal variation is an important source of genetic diversity; physical and chemical stresses applied on tissue cultures can select tolerant cell lines from which improved plants can be regenerated. Calli derived from mature embryos of 3 citrus rootstocks were cultivated on a medium containing 5 or 10 g l^-1NaCl. Tolerant cell lines of Poncirus trifoliata and citrange (P. trifoliata x Citrus sinensis) were selected and their salt tolerance was tested during several months. The growth of plants regenerated from these cell lines was improved and remained high with salt concentrations as high as 10g 1^-1, while that of unselected ones already decreased for 2.5 g 1^-1. Selected cell lines were characterized by their ability of maintaining a normal content of K⁺and Ca⁺⁺ in spite of increasing concentrations of Na⁺in the medium. Na⁺ and Cl^-accumulation was slower in selected cells of both rootstocks, but final concentrations were lower in citrange than in Poncirus. In rice, selection was performed by culturing calli for 4 to 6 months on media containing 15 or 17.5 g 1^-1NaCl. A few secondary proliferations appeared after 3 or 4 months on necrotic calli and were used for plant regeneration. After a selection using a salt concentration of 15 g l^-1for 4 months, 11% of the regenerated plants appeared more tolerant than the control, while salt tolerance was not improved in the progenies of plants derived from calli recovered in more selective conditions (17.5 g 1^-1 for 6 months). In vitro selection for aluminium tolerance is difficult, because of the low solubility of aluminium salts in normal conditions. Rice cell lines were selected on several adapted culture media and plants tolerant to aluminium were regenerated, not only from cells cultured in the presence of aluminium salts, but also on several modified aluminium-free media, suggesting that the cell improvement could be due to their capability of using lower calcium and phosphorus concentrations rather than to a better tolerance to toxic ions.