Abstract
BackgroundMicrotubules, polymerized from alpha and beta-tubulin monomers, play a fundamental role in plant morphogenesis, determining the cell division plane, the direction of cell expansion and the deposition of cell wall material. During polarized pollen tube elongation, microtubules serve as tracks for vesicular transport and deposition of proteins/lipids at the tip membrane. Such functions are controlled by cortical microtubule arrays. Aim of this study was to first characterize the flax β-tubulin family by sequence and phylogenetic analysis and to investigate differential expression of β-tubulin genes possibly related to fibre elongation and to flower development.ResultsWe report the cloning and characterization of the complete flax β-tubulin gene family: exon-intron organization, duplicated gene comparison, phylogenetic analysis and expression pattern during stem and hypocotyl elongation and during flower development. Sequence analysis of the fourteen expressed β-tubulin genes revealed that the recent whole genome duplication of the flax genome was followed by massive retention of duplicated tubulin genes. Expression analysis showed that β-tubulin mRNA profiles gradually changed along with phloem fibre development in both the stem and hypocotyl. In flowers, changes in relative tubulin transcript levels took place at anthesis in anthers, but not in carpels.ConclusionsPhylogenetic analysis supports the origin of extant plant β-tubulin genes from four ancestral genes pre-dating angiosperm separation. Expression analysis suggests that particular tubulin subpopulations are more suitable to sustain different microtubule functions such as cell elongation, cell wall thickening or pollen tube growth. Tubulin genes possibly related to different microtubule functions were identified as candidate for more detailed studies.
Highlights
Microtubules, polymerized from alpha and beta-tubulin monomers, play a fundamental role in plant morphogenesis, determining the cell division plane, the direction of cell expansion and the deposition of cell wall material
Flax stems and hypocotyls have been used as a model to identify, through microarray studies, transcripts expressed during phloem fibre differentiation [5, 6], and to study the expression of genes involved in cell wall development, such as glycosyl hydrolases (GH35) [7], cellulose synthase (CESA) [8, 9] and chitinase-like (CTL) genes [8]
Internal primer pairs Fex1-Rex1 and Fin2-Rin2, annealing to the ORF at the first and second intron borders respectively, have been abundantly tested because they are at the base of the genomic profiling method called tubulin-based polymorphism (TBP), exploiting on the different length of β-tubulin introns [29, 36]
Summary
Microtubules, polymerized from alpha and beta-tubulin monomers, play a fundamental role in plant morphogenesis, determining the cell division plane, the direction of cell expansion and the deposition of cell wall material. Downstream the SP, the galactan-enriched matrix (Gn-layer) of the secondary cell wall is gradually modified into a mature cellulosic gelatinous-layer (Glayer), showing the typical crystalline morphology of thickened cell wall, characterized by microfibril axial orientation, xylan absence, and a low content of lignin [3] This change requires transcription of specific βgalactosidase genes (LuBGAL1 and LuBGAL2) [4] and βgalactosidase activity, involved in remodelling of the cell wall matrix. Phloem fibre differentiation proceeds during hypocotyl elongation, where the onset of secondary cell wall thickening follows cell elongation and is marked by an increase in β-galactosidase activity and up-regulation of corresponding genes along time [5] For this reason, flax stems and hypocotyls have been used as a model to identify, through microarray studies, transcripts expressed during phloem fibre differentiation [5, 6], and to study the expression of genes involved in cell wall development, such as glycosyl hydrolases (GH35) [7], cellulose synthase (CESA) [8, 9] and chitinase-like (CTL) genes [8]
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