Abstract
BackgroundSeveral members of the R2R3-MYB family of transcription factors act as regulators of lignin and phenylpropanoid metabolism during wood formation in angiosperm and gymnosperm plants. The angiosperm Arabidopsis has over one hundred R2R3-MYBs genes; however, only a few members of this family have been discovered in gymnosperms.ResultsWe isolated and characterised full-length cDNAs encoding R2R3-MYB genes from the gymnosperms white spruce, Picea glauca (13 sequences), and loblolly pine, Pinus taeda L. (five sequences). Sequence similarities and phylogenetic analyses placed the spruce and pine sequences in diverse subgroups of the large R2R3-MYB family, although several of the sequences clustered closely together. We searched the highly variable C-terminal region of diverse plant MYBs for conserved amino acid sequences and identified 20 motifs in the spruce MYBs, nine of which have not previously been reported and three of which are specific to conifers. The number and length of the introns in spruce MYB genes varied significantly, but their positions were well conserved relative to angiosperm MYB genes. Quantitative RTPCR of MYB genes transcript abundance in root and stem tissues revealed diverse expression patterns; three MYB genes were preferentially expressed in secondary xylem, whereas others were preferentially expressed in phloem or were ubiquitous. The MYB genes expressed in xylem, and three others, were up-regulated in the compression wood of leaning trees within 76 hours of induction.ConclusionOur survey of 18 conifer R2R3-MYB genes clearly showed a gene family structure similar to that of Arabidopsis. Three of the sequences are likely to play a role in lignin metabolism and/or wood formation in gymnosperm trees, including a close homolog of the loblolly pine PtMYB4, shown to regulate lignin biosynthesis in transgenic tobacco.
Highlights
Several members of the R2R3-MYB family of transcription factors act as regulators of lignin and phenylpropanoid metabolism during wood formation in angiosperm and gymnosperm plants
Insights into the regulation of lignin biosynthesis during vascular development of plants are being derived from angiosperm model plants like Arabidopsis thaliana and from investigations unravelling the molecular basis of wood formation in trees like Populus
We found a motif similar to that involved in the interaction with basic helix-loop-helix proteins in Arabidopsis ([DE]L × 2 [RK] × 3L × 6L × 3R; [28]) in the R3 repeat of three spruce MYBs (PgMYB5, 10 and 13) as well as in PmMBF1 ([12]; Fig. 1)
Summary
Several members of the R2R3-MYB family of transcription factors act as regulators of lignin and phenylpropanoid metabolism during wood formation in angiosperm and gymnosperm plants. Members of the R2R3-MYB transcription factor family have been implicated as regulators of phenylpropanoid and lignin metabolism [1] as well as pattern formation and differentiation of primary and secondary vascular tissues, (reviewed by [3]). The loblolly pine genes PtMYB1 and PtMYB4 were shown to be transcriptional activators which have the ability to regulate lignin synthesis enzymes [10,11] They are expressed in xylem tissues, bind AC elements and activate transcription in transient assays in yeast or plant cells [10,11,13]. The number of MYB transcription factors that may participate in regulating lignification in gymnosperms, and their potential roles remain an open question
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