Submitted to: Plant Molecular Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/1/2001
Publication Date: 5/1/2001
Citation: Interpretive Summary: Plants have many ways of dealing with diseases and pests. One ways is through the production of chemicals that can raise the resistance/defense of the plant to diseases and pests. Another is to change and strengthen the physical barriers by increased production of lignins. The phenylpropanoid pathway of plants is involved with both defense mechanisms. By understanding how this pathway works it should be possible to produce plants that have superior resistant/defense mechanisms resulting in reduced applications of insecticides, fungicides, bactericides, etc. during production and storage. We have looked at one of the first enzymes, i.e., cinnamate 4-hydroxylase (C4H), in the phenylpropanoid pathway in Valencia oranges and found that there are two forms (isoforms) of the enzyme present. One appears to be induced by wounding (C4H1) while the other (C4H2) is constitutive and is slightly induced by wounding. Although shown for other phenylpropanoid pathway enzymes, differential expression of C4H isoforms is a new observation. This may provide a way to control different aspects of induction of defense systems in plants yielding tools to combat losses due to disease and pests.
Technical Abstract: Two different full-length cDNAs for cinnamate 4-hydroxylase (C4H1 and C4H2) were isolated from Citrus sinensis Osbeck cv. `Valencia' libraries. C4H1 (1708 bp) and C4H2 (1871 bp) share only 65% identity on nucleotide and 66% identity on amino acid level, respectively. C4H1 is most homologous to a cinnamate 4-hydroxylase sequence from French bean (Phaseolus vulgaris), but codes for a unique N-terminus. C4H2 shows highest similarity to a poplar (Populus kitakamiensis) sequence, but also shows a unique N-terminus. The two genes are expressed differentially in orange flavedo, C4H2 is constitutive, C4H1 is wound-induced. Using competitive RT-PCR, the mRNA for both genes in wounded and non-treated tissue was quantified. C4H1 is strongly wound-inducible from `not detectable' to about 35 fg mRNA per 50 ng total RNA 8 h after wounding. The first detectable C4H1 mRNA was found 4 h after wounding. After reaching peak levels four hours later the levels slightly declined, but stayed elevated until the end of the experiment (48 h). C4H2 is expressed 3-10 times higher than wound-induced C4H1 even in the control sample; wounding transiently increases the level of expression another 2-3 times. The existence of different N-termini and their effects on the possible role of both genes in phenylpropanoid pathways is discussed.