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Mini Review Dna Damage, Repair and Mutagenesis In ISRAEL JOURNAL OF BOTANY, Vol. 36, 1987, pp. 1-14 MINI REVIEW DNA DAMAGE, REPAIR AND MUTAGENESIS IN HIGHER PLANTS VALERY N. SOYFERI Chertanovskaya Street 39, Bldg. 2, Apt. 131, Moscow, USSR There are at least three types of DNA repair: photoreactiva,ion, excision repair and postreplication repair. The phenomenon of DNA repair was first discovered only 20 years ago, in bacteria (Boyce & Howard-Flanders, 1964; Setlow & Carrier, 1964). It was later found that pyrimidine dimers are also excised from the DNA in mammalian cells (Regan & Carrier, 1974). Postreplication repair was observed at about the same time (Rupp & Howard-Flanders, 1968). Numerous attempts to find the specialized repair systems in plants were unsuccessful, although early experimental findings pointed to the presence of repair in plants (Higgins & Sheard, 1927; Bawden & Kleczkowski, 1952). The first direct evidence for the presence of photoreactivation in individual cells of plants was found by Trosko and Mansour (1968). The same process was demonstrated in whole plant seedlings by Soyfer and Cieminis (1973, 1974). The first attempts to find excision repair in higher plants were, however, unsuccessful (Trosko & Mansour, 1968, 1969). Negative results were also obtained in the first searches for repair replication in Chlamydomonas (Swinton & Hanawalt, 1973) and in Viciafaba(Wolff & Cleaver, 1973). By the end of 1973 it was suggested that higher plants, for some intrinsic reason, had either not acquired repair mechanisms during evolution or they had lost them. This point of view was most clearly stated by Wolff and Cleaver ( 1973). In this review, experiments addressing various problems of DNA repair and mutagenesis in grass pea and barley seeds are discussed. Damage of DNA by Ultraviolet Irradiation There were two basic reasol).s for the negative results obtained in the early experiments investigating excision repair in higher plants: (I) the level of incorporation of radioactive precursors into DNA was very low, and (2) high doses of irradiation of plants or cells completely inhibited the process of pyrimidine dimer eXCISIOn. 1 Professor V.N. Soyfer was dismissed from his job after applying for a visa to emigrate from USSR. Correspondence should be addressed to Prof. A. Halevy, Faculty of Agriculture, The Hebrew University of Jerusalem, P.O.B. 12, Rehovot 76100, Israel. Received May 9, 1985 2 V.N.SOYFER Isr. J. Bot. In our early experiments with Lathyrus sativus L. (grass pea) we found that incorporation increased markedly if the seedlings were grown from isolated embryos which had been placed in radioactive medium immediately after their removal from the seeds (Soyfer et al., 1973, 1974). For an appropriate choice of noninhibiting doses of U_Y -irradiation the knowledge of the dose dependence of dimerization of pyrimidines is crucial. The dose-effect curves, both for pure DNA in solution and for intact chromosomal DNA in whole seedlings, are shown in Fig. I. While the in-solution dimerization curve is linear for a broad range of doses (Fig. l A), the curve for the experiments in vivo shows at least three components with different slopes (Fig. l B). The maximum quantity of UV -dimerized thymines in the seedlings in vivo was half that obtained by irradiation of pure DNA (0.6 and 1.3% of thymine dimers to thymines, respectively). It is important to know whether natural sunlight could also induce thymine dimers in the DNA of higher plants. Although shortwave UV light (shorter than 280 nm) is intensely absorbed by the ozone layer of the atmosphere, light of wavelengths from 280 to 400 nm (which can also cause genetic damage) does reach the earth's surface. Its effects on DNA extracted from L. sativus seedlings were studied by placing 14G-labeled plant DNA in an open petri dish and in a quartz cuvette and leaving both in direct sunlight under a cloudless sky at 29°C. After 10 min of illumination of DNA in the open dish a significant amount of pyrimidine in vitro in vivo 06 A 1.0 1- I­ ~ 1- 'I- 0.3 I- 4 6 8 10 20 30 40 Dose of UV- irradiation (min) Fig. I. Thymine dimer induction in DNA of seedlings. The percentage thymine dimers to thymines ratio (TT1 T) is plotted against the dose of UV -irradiation. .
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