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PLANT PHYSIOLOGY , Vol 101, Issue 2 385-390, Copyright © 1993 by American Society of Plant Biologists

ENVIRONMENTAL AND STRESS PHYSIOLOGY

Differential Reactivity of [beta]-Carotene Isomers from Dunaliella bardawil Toward Oxygen Radicals

C. Jimenez and U. Pick
Department of Biochemistry, The Weizmann Institute of Science, 76100 Rehovot, Israel

Dunaliella bardawil accumulates massive amounts of [beta]-carotene in two isoforms, a 9-cis and an all-trans stereoisomer, when grown under high irradiance, as a means to protect the cells against photoinhibition (A. Ben-Amotz, A. Shaish, M. Avron [1989] Plant Physiol 91: 1040-1043). The purpose of this work has been to find out if the mechanism of protection involves scavenging of reactive oxygen species. For this purpose high- and low-[beta]-carotene-containing cells were compared with respect to their sensitivity to several external oxidants [H2O2, methyl viologen, rose bengal, and 2,2[prime]-azobis(2-amidinopropane)HCl]. All oxidants induce a light-stimulated degradation of [beta]-carotene and of chlorophyll. The degradation of [beta]-carotene precedes that of chlorophyll, indicating that it is more reactive toward oxidants. The 9-cis [beta]-carotene is degraded faster than the all-trans stereoisomer when exposed to oxidants, both in intact cells and in isolated [beta]-carotene globules, indicating that it is a more effective scavenger of reactive oxygen species. Comparison of the sensitivity to different oxidants, between high- and low-[beta]-carotene-containing cells, reveals similar rates of chlorophyll and [beta]-carotene degradation in the two populations. Survival tests toward H2O2 and rose bengal show that high-[beta]-carotene cells have a similar sensitivity toward H2O2 but are more resistant toward rose bengal, a photoactivated generator of singlet oxygen, possibly due to masking of the latter by [beta]-carotene. These results suggest that the protection mechanism of massively accumulated [beta]-carotene in Dunaliella against photoinhibition is not due to scavenging of reactive oxygen species.


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S. C. Orset and A. J. Young
Exposure to Low Irradiances Favors the Synthesis of 9-cis beta ,beta -Carotene in Dunaliella salina (Teod.)
Plant Physiology, February 1, 2000; 122(2): 609 - 618.
[Abstract] [Full Text]


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