USE OF RADIATION-INDUCED TUMORS TO STUDY GROWTH-CONTROL IN ARABIDOPSIS

Tumors on both animals and plants arise as a result of loss of growth control, and in animal systems have been a source of many insights int o regulatory mechanisms. We have attempted to develop a similar paradi gm in plants by using ionizing radiation to induce tumors in Arabidops is thaliana. From the primary tumors we have established a number of h ormone autonomous tissue lines in culture which present a variety of p henotypes: leafy, rooty, and undifferentiated. These lines also differ from one another in their hormone content and in their response to ex ogenous hormones. Some tumor tissue lines have higher levels of free o r conjugated auxin or cytokinin, whereas others do not. Some lines sho w the same growth response as normal callus when hormones are supplied in the growth medium, whereas others have increased or decreased sens itivity to hormones. Using subtraction hybridization, we have identifi ed a number of cDNAs whose expression is enhanced in the tumors compar ed with hormone-dependent callus. Several of these cDNAs have been ide ntified by homology with sequences in Genbank, and include a membrane channel protein, a lipid transfer protein and hydroxyproline- and glyc ine-rich proteins. The expression of these relatively abundant protein s may well be a consequence rather than a cause of tumorigenicity. Som e of the less abundant transcripts remain unidentified. Two of the tum ors described have shown secondary changes in culture, leading to fast er-growing cell lines. In one case this was associated with an increas e in free indole-3-acetic acid (IAA), a reduction in IAA conjugates an d an increased sensitivity to naphthylphthalamic acid. In a second ins tance, the primary tumor had increased levels of cytokinin but was cyt okinin insensitive while the fast-growing variants retained high cytok inin levels, had become cytokinin sensitive and acquired a shooty phen otype. Ways in which this approach to understanding plant hormones and growth control may be developed are discussed.