Comparative Transcriptome Analysis Reveals Candidate Genes Involved in Isoquinoline Alkaloid Biosynthesis in Stephania tetrandra

The roots of Stephania tetrandra are used as a traditional Chinese medicine. Isoquinoline alkaloids are considered to be the most important and effective components in this herb, but little is known about the molecular mechanism underlying their biosynthesis. In this context, this study aimed to reveal candidate genes related to isoquinoline alkaloid biosynthesis in S. tetrandra. Determination of tetrandrine and fangchinoline in the roots and leaves of S. tetrandra by HPLC showed that the roots had much higher contents of the two isoquinoline alkaloids than the leaves. Thus, a comparative transcriptome analysis of the two tissues was performed to uncover candidate genes involved in isoquinoline alkaloid biosynthesis. A total of 71 674 unigenes was obtained and 31 994 of these were assigned putative functions based on BLAST searches against 6 annotation databases. Among the 79 isoquinoline alkaloid-related unigenes, 51 were differentially expressed, with 42 and 9 genes upregulated and downregulated, respectively, when the roots were compared with the leaves. The upregulated differentially expressed genes were consistent with isoquinoline alkaloid accumulation in roots and thus were deemed key candidate genes for isoquinoline alkaloid biosynthesis in the roots. Moreover, the expression profiles of 10 isoquinoline alkaloid-related differentially expressed genes between roots and leaves were validated by quantitative real-time polymerase chain reaction, which indicated that our transcriptome and gene expression profiles were reliable. This study not only provides a valuable genomic resource for S. tetrandra but also proposes candidate genes involved in isoquinoline alkaloid biosynthesis and transcription factors related to the regulation of isoquinoline alkaloid biosynthesis. The results lay a foundation for further studies on isoquinoline alkaloid biosynthesis in this medicinal plant.

Distant Precursors of Benzylisoquinoline Alkaloids and their Enzymatic Formation

The incorporation rates of labelled tyrosine, DOPA. tyramine. and dopamine have been inves­tigated during the in vivo formation of the protoberberine alkaloid, jatrorrhizine, in callus cul­tures of Berberis canadensis. While tyrosine was equally well incorporated into both the iso­quinoline (54%) and benzyl (46%) portions of the alkaloid, DOPA was almost exclusively (91%) transformed into the isoquinoline moiety. However, tyramine (25%) and to a lesser extent, dopamine (15%) were incorporated into the aldehyde-derived, benzylic half of the isoquinoline molecule as well. In order to investigate further the precursory roles of these compounds, select enzymes involved in tyrosine metabolism in alkaloid-producing cell cultures have been studied. The occurrence of tyrosine decarboxylase, phenolase, transaminase, p-hydroxyphenylpyruvate decarboxylase, amineoxidase and methionine adenosyl transferase was demonstrated in suspen­sion cells of Berberis. These enzymes were partially purified and a preliminary characterization was performed. In the light of these and previous data, the differential metabolism of tyrosine and DOPA in the early steps of isoquinoline alkaloid biosynthesis is discussed. Conclusive evidence as to the biosynthetic origin of the phenylacetaldehydes which furnish the benzylic moiety of the alkaloids is precluded by the presence of both amineoxidase and phenylpyruvate decarboxylase activities in these cultures.

The biosynthesis of protoberberine and related isoquinoline alkaloids

The biosynthesis of berberine and hydrastine (in Hydrastis canadensis), corydaline and protopine (in Corydalis solida), and ochotensimine and protopine (in C. ochotensis), has been investigated by the administration of [3-14C]-3′,4′-dihydroxyphenylalanine ([3-14C]DOPA). In all cases, incorporation of label was predominantly into the isoquinoline portion of the alkaloid. The role of DOPA in the early stages of isoquinoline alkaloid biosynthesis in these plants is discussed in the light of this and other relevant data. In addition, the later stages of corydaline biosynthesis have been studied by the administration of [9-methoxy-14C]palmatine and -tetrahydropalmatine to C. solida, and the origin of the exocyclic carbons of ochotensimine further verified by feeding [methyl-14C,3H]methionine to C. ochotensis.