A 13C-pulse-labeling study of phytoalexin biosynthesis in hypersensitively responding cotton cotyledons
1995
Gorski, P.M. | Vickstrom, T.E. | Pierce, M.L. | Essenberg, M.
Time courses of multiplication of an incompatible strain of Xanthomonas campestris pv. malvacearum, accumulation of phytoalexins, and development of phytoalexin-rich, yellow-green fluorescent, hypersensitively necrotic cells were followed in cotton cotyledons. Occasionally, hypersensitively responding palisade cells were seen to pass through a stage in which the yellow-green fluorescence, due to the phytoalexins lacinilene C and its methyl ether, was confined to the cytoplasmic layer surrounding an intact vacuole, but more often, when cells became yellow-green fluorescent, the fluorescence appeared throughout the cell. The period of most rapid increase in fluorescent cell numbers, 48-72 h post-inoculation, was also the period of most rapid increase in phytoalexin content and the rate of bacterial multiplication began to slow down at this time. Stable 13C isotope incorporation from a 4 h pulse with [13C2]acetate into the phytoalexin 2,7-dihydroxycadalene and its methyl ether was measured by direct probe electron impact-mass spectrometry to monitor their biosynthesis at 2, 3, 4, and 6 days after inoculation. Highest incorporation rates occurred during the period of most rapid increase in numbers of fluorescent cells, consistent with the hypothesis that these cells achieve a burst of phytoalexin biosynthesis causing them to become fluorescent as they die. In addition, biosynthesis was still active when fluorescent cell numbers approached their maximum and was appreciably above zero 1 and 2 days later, which suggests the possibility that healthy cells neighboring the hypersensitively responding ones contribute to the biosynthesis of phytoalexins in cotton.
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