ISOLATION AND CHARACTERIZATION OF HORMONE-AUTONOMOUS TUMORS OF ARABIDOPSIS-THALIANA

In order to study the molecular genetics of factors controlling plant cell growth, we have isolated and begun to characterize a set of tumours on the small crucifer Arabidopsis thaliana. Seeds or seedlings were exposed to Co-60 gamma radiation and, 30-60 d after germination, tumours developed either on the hypocotyl or in the region of the apical meristem of about 1% of the plants. When excised and placed in culture, some of these tumours were found to be capable of hormone-independent growth. The tumours exhibit a number of different phenotypes, varying in colour, texture, and degree of differentiation. Some tumours appear to be completely undifferentiated, one consistently produces roots, and others show the sporadic appearance of shoots or leaflets. Doubling times of the tumours on hormone-free medium range from approximately 2 d to 9 d. We propose that these tumours arose due to heritable changes in the genome which result in altered expression of important growth-regulatory genes. Preliminary investigations of gene expression in the tumours have led to the identification of an mRNA that is abundant in all of the tumours, differentially expressed in plant organs and hormone-dependent callus grown on different auxins, and which encodes a putative glycine-rich protein.