Oxidative burst in Chenopodium rubrum suspension cells: Induction by auxinand osmotic changes

Lowering of the osmotic value of the medium has been reported previously to induce an oxidative burst in plant cells, This has been explained by a seq uence of events, including solute influx, cellular swelling and the activat ion of stretch-regulated channels, triggering the production of reactive ox ygen species, Moreover, it is known that the plant growth hormone auxin ind uces protoplast swelling, Together, these findings prompted the hypothesis that plant cells can respond to auxin-treatment with an oxidative burst. We tested this hypothesis using suspension cultured cells from Chenopodium ru brum L, and 2,7-dichlorodihydrofluorescein as the indicator for reactive ox ygen species, An auxin-induced oxidative burst was found similar to an osmo tically induced burst. Osmotic treatment consisted of a shift from 110 mOsm to 40 mOsm, The naturally occurring halogenated auxin 4-chloroindole-3-ace tic acid was the most active compound tested, giving maximum rates of indic ator oxidation corresponding to the formation of 4 x 10(-15) mol H2O2 cell( -1) min(-1). Auxin analogous (10 muM) exhibited the following order of effe ctiveness: 4-chloride indoleacetic acid (100'%), indoleacetic acid (80%), 2 ,4-dichlorophenoxyacetic acid (75%), 2-naphthylacetic acid (52%) and 1-naph thylacetic acid (47%). Benzoic acid (23%) was used as a control, Fusicoccin (35%) showed only slight stimulation in conjunction with complex kinetics, The detection of oxidative burst responses to 10 nM indoleacetic acid reve aled a high sensitivity of the assay for auxin, Cell-free medium from aged batch cultures and light were also found to stimulate the production of rea ctive oxygen species. These data indicate that reactive oxygen species can transduce and integrate developmental and environmental signals and thus pl ay a general role in plant growth regulation.