beta-lactam antibiotics inhibit chloroplast division in a moss (Physcomitrella patens) but not in tomato (Lycopersicon esculentum)

Photoautotrophic suspension cultures of moss protonema of Physcomitrella patens (Hedw.) B.S.G. and of tomato (Lycopersicon esculentum L.), respectively, were treated with three different p-lactam antibiotics. Ampicillin, cefotaxime and penicillin in the range from 10 mu mol/L to 1 mmol/L had no visible effects on tomato cells but led to moss cells with giant chloroplasts. According to electron micrographs these cells are morphologically normal but contain 2-3 macrochloroplasts with well developed inner membranes. As only chloroplasts in dividing tip cells were affected, we conclude thar p-lactam antibiotics specifically inhibit the plastid division process in the moss. Major proteins were not affected by antibiotic treatments as judged by SDS-PAGE and subsequent silver stain. The inhibitory effect on chloroplast division could not: be compensated for by exogenous cytokinin (5 x 10(-6) mol/L i(6) Ade). However, upon removal of the antibiotics, macrochloroplasts started to divide. As division of bacteria and of cyanelles is sensitive to p-lactam antibiotics, we suggest that mosses conserved an ancient component of the chloroplast division mechanism that was subsequently modified or substituted during plastid evolution in land plants.