Nucleotide Sequence of a Soybean Chalcone Synthase Gene with a Possible Role in Ultraviolet-B Sensitivity, Gmchs6

Recent trends in stratospheric ozone depletion and projected increases in solar UV-B radiation (280-320 nm) have intensified studies of the ecological and physiological effects of increased levels of UV-B on higher plants (Caldwell, 1981; Worrest and Caldwell, 1986). Soybean (Glycine max L. Merr) is among the most extensively studied plants because it is a key crop of worldwide importance and because its potential susceptibility to increased levels of solar UV-B has been amply documented (Teramura et al., 1990, and refs. therein). From such studies, a pair of cultivars of contrasting sensitivity to UV-B has been identified. Williams is tolerant to supplemental UV-B fluences simulating a 25% ozone depletion, whereas Essex is sensitive to the same fluences, resulting in reduction of seed yield by 20 to 25%. The possibility that this may be due to differences in UV-B-absorbing compounds has also been noted. Flavonoids are among the best candidates for UV-B-absorbing compounds, and it is well-known that they can function as UV-B protectants because of their strong molecular absorption of UV light and accumulation in the epidermal layer of leaf tissues (Caldwell et al., 1983). It is also well documented that CHS, a key enzyme of flavonoid biosynthesis, plays an important role in the accumulation of flavonoids in leaf epidermis (Cosio et al., 1985; Beerhues et al., 1988). Thus, we reasoned that the molecular basis for the contrasting sensitivity to UV-B between Williams and Essex might include differences in some aspect of a CHS gene. We have already identified seven members of the soybean chs gene family. Although it is not yet known which member of the gene family is responsible for flavonoid accumulation in leaf epidermis, one interesting discovery was made when we compared Southern hybridization patterns of Williams and Essex genomic DNA HindIII digests with CHS gene 1 probe, i.e. the intensity of the 2.0-kb HindIII band, designated as gene 6 (Gmchs6), was 4 times weaker in Essex than Williams. This suggested the possibility that gene dosage differences in gene 6 might account for the difference in UVB tolerance. The possibility that differences in intensity reflected a restriction site polymorphism did not seem very likely because of the apparent absence of an additional band corresponding to this gene on the Southern blot. However, I _ GCTAZ __ r% Z , __ GC_ 'rCC At, ' -'ITC'rTCCAT AAAGG GAAAr__ _ 'rA'_FTA --,_ , _ 7AA TTTAAa _ sf C_ _ TTG_ _TGG_ _ AC_ ATTe_ _AT C _ A-TT_A_TAAT .AT_A_,CAAI 'A,T