A Gene Encoding a Cinnamyl Alcohol Dehydrogenase Homolog in Arabidopsis thaliana

CAD (EC 1.1.1.195) catalyzes the reduction of cinnamyl aldehydes to produce p-coumaryl, coniferyl, and sinapyl alcohols, which constitute the precursors for lignin biosynthesis. Antisense inhibition of CAD activity in tobacco (Nicotiana tabacum) resulted in altered lignin composition, demonstrating that the enzyme is a useful target for manipulation of lignification (Halpin et al., 1994). CAD activity is induced by funga1 elicitors, and inhibitor studies have shown that CAD is causally involved in rust resistance in wheat (Triticum aestivum) (Moerschbacher et al., 1990), indicating an important role in plant-pathogen interactions. Isozymes of CAD have been resolved based on differences in substrate specificity (Goffner et al., 1992; Hawkins and Boudet, 1994). A CAD isozyme referred to as CAD2 was purified to homogeneity from xylem-enriched fractions of Eucalyptus gunnii Hook seedlings (Goffner et al., 1992). CAD2 is a dimer of 43-kD subunits with high affinity for p-coumaryl, coniferyl, and sinapyl aldehydes. Recently, cDNA and genomic clones of the CAD2-type isozyme were isolated from N. tabacum (Knight et al., 1992), Eucalyptus (Grima-Pettenati et al., 1993), Picea abies L. (Galliano et al., 1993), and Aralia cordata (Hibino et al., 1993). cDNA and genomic CAD clones were isolated from Arabidopsis thaliana for future investigations of tissue-specific expression of the CAD gene(s). A. thaliana was used as a model plant species because of its highly characterized diploid genetics and amenability to transformation and genetic analyses of gene regulation. A CAD cDNA clone from Medicago sativa was used as a heterologous probe to isolate a nearly full-length A. thaliana cDNA, which was sequenced (GenBank accession No. L37884). The cDNA of 1380 bp exhibited a single open reading frame of 357 amino acids, which was about 50% identical with the sequence of two N. tabacum CAD cDNAs (Knight et al., 1992) and about 55% identical with two pathogen-response genes of previ-