gamma-Radiation-induced
tumors of Arabidopsis thaliana L. have been produced as a novel approach to isolation of genes that regulate plant development.
Tumors excised from irradiated plants are
hormone autonomous in culture and have been maintained on
hormone-free medium for up to 4 years. Five
tumor tissue lines having different morphologies and growth rates were analyzed for
auxin,
cytokinin, and
1-aminocyclopropane-1-carboxylic acid (ACC) content,
ethylene production, and response to exogenous growth regulators. Normal tissues and two crown gall tissue lines were analyzed for comparison. Rosettes and whole seedlings each contained approximately 30 nanograms. (gram fresh weight)(-1) free
indoleacetic acid (IAA), 150 nanograms. (gram fresh weight)(-1)
ester-conjugated IAA, and 10 to 20 micrograms. (gram fresh weight)(-1)
amide-conjugated IAA. The crown gall lines contained similar amounts of free and
ester-conjugated IAA but less
amide conjugates. Whereas three of the radiation-induced
tumor lines had IAA profiles similar to normal tissues, one line had 10- to 100-fold more free IAA and three- to 10-fold less
amide-conjugated IAA. The fifth line had normal free IAA levels but more conjugated IAA than control tissues. Whole seedlings contained approximately 2 nanograms. (gram fresh weight)(-1) of both
zeatin riboside and
isopentenyladenosine. The crown gall lines had 100- to 1000-fold higher levels of each
cytokinin. In contrast, the three radiation-induced
tumor lines analyzed contained
cytokinin levels similar to the control tissue. The radiation-induced
tumor tissues produced very little
ethylene, although each contained relatively high levels of ACC. Normal callus contained similar amounts of ACC but produced several times more
ethylene than the radiation-induced
tumor lines. Each of the radiation-induced
tumor tissues displayed a unique set of responses to exogenously supplied growth regulators. Only one
tumor line showed the same response as normal callus to both
auxin and
cytokinin feeding. In some cases, one or more
tumor lines showed increased sensitivity to certain
growth substances. In other cases, growth regulator feeding had no significant effect on
tumor tissue growth. Morphology of the radiation-induced
tumor tissues generally did not correlate with
auxin to
cytokinin ratio in the expected manner. The results suggest that a different primary genetic event led to the formation of each
tumor and that growth and differentiation in the
tumor tissue lines are uncoupled from the normal hormonal controls.