NEW POTENTIAL MARKERS OF IN-VITRO TOMATO MORPHOGENESIS IDENTIFIED BY MESSENGER-RNA DIFFERENTIAL DISPLAY

The identification of plant genes involved in early phases of in vitro morphogenesis can not only contribute to our understanding of the pro cesses underlying growth regulator-controlled determination but also p rovide novel markers for evaluating the outcome of in vitro regenerati on experiments. To search for such genes and to monitor changes in gen e expression accompanying in vitro regeneration, we have adapted the m RNA differential display technique to the comparative analysis of a mo del system of tomato cotyledons that can be driven selectively toward either shoot or callus formation by means of previously determined gro wth regulator supplementations. Hormone-independent transcriptional mo dulation (mainly down-regulation) has been found to be the most common event, indicating that a non-specific reprogramming of gene expressio n quantitatively predominates during the early phases of in vitro cult ure. However, cDNA fragments representative of genes that are either d own-regulated or induced in a programme-specific manner could also be identified, and two of them (G35, G36) were further characterized. One of these cDNA fragments, G35, corresponds to an mRNA that is down-reg ulated much earlier in callus- (day 2) than in shoot-determined explan ts (day 6). The other, G36, identifies an mRNA that is transiently exp ressed in shoot-determined explants only, well before any macroscopic signs of differentiation become apparent, and thus exhibits typical fe atures of a morphogenetic marker.