DIFFERENTIAL EXPRESSION OF MYROSINASE GENE FAMILIES

In mature seeds of Brassica napus three major and three minor myrosina se isoenzymes were identified earlier. These myrosinases are known to be encoded by at least two different families of myrosinase genes, den oted MA and MB. In the work described in this paper the presence of di fferent myrosinase isoenzymes in embryos, seedlings, and vegetative ma ture tissues of B. napus was studied and related to the expression of myrosinase MA and MB genes in the same tissues to facilitate future fu nctional studies of these enzymes. In developing seeds, myrosinases of 75, 73, 70, 68, 66, and 65 kD were present. During seedling developme nt there was a turnover of the myrosinase pool such that in 5-d-old se edlings the 75-, 70-, 66-, and 65-kD myrosinases were present, with th e 70- and 75-kD myrosinases predominating. In 21-d-old seedlings the s ame myrosinases were present, but the 66- and 65-kD myrosinase species were most abundant. At flowering the mature organs of the plant conta ined only a 72-kD myrosinase. MA genes were expressed only in developi ng seeds, whereas MB genes were most highly expressed in seeds, seedli ng cotyledons, young leaves, and to a lesser extent other organs of th e mature plant. During embryogenesis of B. napus, myrosinase MA and MB gene transcripts started to accumulate approximately 20 d after polli nation and reached their highest level approximately 15 d later. MB tr anscripts accumulated to about 3 times the amount of MA transcripts. I n situ hybridization analysis of B. napus embryos showed that MA trans cripts were present predominantly in myrosin cells in the axis, wherea s MB genes were expressed in myrosin cells of the entire embryo. The e mbryo axis contained 75-, 70-, and 65-kD myrosinases, whereas the coty ledons contained mainly 70- and 65-kD myrosinases. Amino acid sequenci ng revealed the 75-kD myrosinase to be encoded by the MA gene family. The high degree of cell and tissue specificity of the expression of my rosinase genes suggests that studies of their transcription should pro vide interesting information concerning a complex type of gene regulat ion.