acid, 2,4-dichlorophenoxyacetic acid, or 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), ... partial pressure of ethylene in fungus-inoculated or 2,4,5-T-treated carrot ...
Plant Physiol. (1969) 44, 235-241
Ethylene-induced Isocoumarin Formation in Carrot Root Tissue E. Chalutz, J. E. DeVay, and E. C. Maxie Departments of Plant Pathology and Pomology, University of California, Dav-is, California 95616 Received October 8, 1968. A bstract. The concentrations of 3.methvl-6.methoxv-8-hydroxy-3,4-dihydroisocoumarin (MMHD) formed in carrot roots inoculated with certain fungi or treated with indole-3-acetic acid, 2,4-dichlorophenoxyacetic acid, or 2,4,5-trichlorophenoxyacetic acid (2,4,5-T), were related to the amount of ethylene produced by the root tissue. Ethylene applied exogenously in concentrations above 0.3 ppm induced the formation of MMHD in carrot root discs. Continued production of MMHD required the continued presence of ethylene. The amounts of MMHD in the discs were reduced by CO,. an inhibitor of ethylene action, and by reduction of the partial pressure of ethylene in fungus-inoculated or 2,4,5-T-treated carrot root discs. The results indicate that ethylene is required for the induction of MMHD formation by carrot root tissue.
Materials and Methods
The compound 3-methyl-6-methoxy-8-hydroxy-3,4dihydroisocoumarin (MMHD) has been isolated from carrot (Daucus carota L.) root tissue inoculated with Ceratocystis fimobriata Ell. and Halst. and several other fungi (2,11,13,21). MMHD is also formed in the tissue following treatment with certain chemicals (2) and has been isolated from healthy carrot roots in storage (16, 24). MMHD production can be induced in healthy carrot roots by adding ethylene to the atmosphere of stored carrots. Carlton et al. (7) showed that MMHD formation is induced when an air stream containing 100 ppm ethylene is passed over carrot roots stored at 20. MIMHD was found in low concentration after a 2-week storage period, and increased witlh storage time. Thev also reported that MMHD formation is prevented by short anaerobic treatments. It has been reported (17) that MMHD has fungistatic properties and is involved in resistance of carrot roots to C. fimitbriata, a nonpathogen of carrots. Allen (2, 13) thought it unlikely that ethylene was involved in the induction. of MIMHD in carrot roots inoculated with C. fimibriata, since only C. fimbriata produced ethylene in pure culture among several fungi tested whereas all \-ere capable of inducing MMHD formation. It has been shown, however, that the rate and amounts of ethylene produced in pture cultur-e by several isolates of C. fimiibriata are not related to those produced by carrot roots inoculated with the same isolates (9). Moreover. certain fungi that produce no detectable ethylene in culture induice ethylene production \-hen inoculated on carrot tissue (8). The experiments reported here were done to evaluate the relation between ethvlene and 'MMHD formation in carrot roots,
Fungits Isolates. Single-spore cultuires of 5 isolates of C. fimibriata were used. Isolate 43, from a bark canker oIn a cherry tree (Prumus avium L.), was pathogenic on almond (P. aniygdalus L.), causing bark can¶kers (15). Isolate 104. from black rot lesions on sweet potato [Ipomitoca batatas (L.) Lam.], was pathogenic on both sweet potato and almond trees (15). Tsolates Asl, Ptl anid EC1 were respectively obtained from trees of quaking aspen (Populus tremiuloides Michx.) plane (Platanus spp.). and coffee (Coffea arabica L.). Also used were 2 isolates of Helminthosporium carbonum Ullstrup, Carb and Tenn. 49, and 1 isolate of Fusarium oxysporutm f. lycopersci (Sacc.) Snyder and Hansen, race 2. Methods of Inoculationis. Fresh carrot roots (var. 'Long Imperator') w,ere washed in detergent and water and surface-sterilized w-ith 70 % ethanol. Uniform 1-cm-thick discs were partially immersed for 30 sec in a spore suspension (1 X 105 spores/ml) and then placed on moist filter paper, inoculatedsurface-up, in sterile modified Petri plates. The plates which were gas-tight had a tube. approximately 2 cm long, welded to a 13-mm opening in the cover, plugged with cotton, and sealed with a rubber stopper for a 12-hr period each day l(8). Control discs of carrots were dipped in an autoclaved spore sis_pension or in sterile distilled Nvater. Chemnicals Tested. Aqueous solutions were prepared of indole-3-acetic acid (1AA). 2,4-dichlorophenoxyacetic acid (2,4-D), 2,4,5-trichlorophenoxyacetic acid (2,4,5-T). HgCl.. KCN. alnd potassium phosphate, at concentrations from 2.5 to 500 ppm. When necessarv, soluition pH was adjulsted to 6.8
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PHYSIOLOGY
with 0.1 N K IOH and 0.05) H.SO1. Chenmicals which did not dii-solve readilv in wN-ater were first dissolved in 2 ml 95 % et-l:n-ol. In stuch cases the same proportion of ethanol w%as
