Growth and hydrogen sulfide emission of photoheterotrophic tobacco cells supplied with l-cysteine as sole sulfur source

Growth of photoheterotrophic tobacco cells (Nicotiana tabacum L. var."Samsun") in suspension cultures is inhibited in the presence of 0.8 mM L-cysteine as sole sulfur source, when inoculated at densities up to 9.5 mg dry weight. Under these conditions, hydrogen sulfide is emitted in amounts increasing with increasing inoculation densities. An inoculum of 19 mg dry weight, however, enables a moderate growth of the cells (doubling time 178 h) while reducing the emission of hydrogen sulfide. Feeding of 0.4 mM L-leucine to the suspension did not only reduce hydrogen sulfide emission at all inoculation densities applied; it also caused exponential growth (doubling time 110 h) at inoculation densities of 9.5 mg and upwards. Initial and final rate of growth of tobacco cells supplied with L-cysteine as sulfur source increased, hydrogen sulfide emission decreased with increasing L-leucine concentrations in the medium. These effects are not specific for L-leucine, but are also observed when L-is oleucine, L-threonine, or L-valine were added to the suspensions. Support of growth and suppression of hydrogen sulfide emission by L-leucine is accompanied by a reduction of the initial as well as the final rate of uptake of L-cysteine by the cells. (IFU)