Abstract
Microtubule organization is intimately associated with cellulose microfibril deposition, central to plant secondary cell wall development. We have determined that a relatively large suite of eight alpha-TUBULIN (TUA) and 20 beta-TUBULIN (TUB) genes is expressed in the woody perennial Populus. A number of features, including gene number, alpha:beta gene representation, amino acid changes at the C terminus, and transcript abundance in wood-forming tissue, distinguish the Populus tubulin suite from that of Arabidopsis thaliana. Five of the eight Populus TUAs are unusual in that they contain a C-terminal methionine, glutamic acid, or glutamine, instead of the more typical, and potentially regulatory, C-terminal tyrosine. Both C-terminal Y-type (TUA1) and M-type (TUA5) TUAs were highly expressed in wood-forming tissues and pollen, while the Y-type TUA6 and TUA8 were abundant only in pollen. Transcripts of the disproportionately expanded TUB family were present at comparatively low levels, with phylogenetically distinct classes predominating in xylem and pollen. When tension wood induction was used as a model system to examine changes in tubulin gene expression under conditions of augmented cellulose deposition, xylem-abundant TUA and TUB genes were up-regulated. Immunolocalization of TUA and TUB in xylem and phloem fibers of stems further supported the notion of heavy microtubule involvement during cellulose microfibril deposition in secondary walls. The high degree of sequence diversity, differential expansion, and differential regulation of Populus TUA and TUB families may confer flexibility in cell wall formation that is of adaptive significance to the woody perennial growth habit.
Highlights
Microtubule organization is intimately associated with cellulose microfibril deposition, central to plant secondary cell wall development
Polymers of the a-TUBULIN (TUA) and b-TUBULIN (TUB) proteins comprise dynamic arrays of cortical microtubules (MTs) that are continually reorganizing in response to developmental and environmental cues (e.g. Wasteneys, 2004), and are postulated to guide the deposition of cellulose microfibrils during cell wall formation in plants (Ledbetter and Porter, 1963)
Functional heterogeneity of tubulin subunits within species, organs, or even cells is further manifest as spatiotemporally distinct gene products, many of which are subject to posttranslational modification (PTM; for review, see Luduena, 1998; McKean et al, 2001)
Summary
Microtubule organization is intimately associated with cellulose microfibril deposition, central to plant secondary cell wall development. Strict temporal association of specific tubulin transcripts during cotton (Gossypium hirsutum) fiber development (Whittaker and Triplett, 1999; Li et al, 2002) or Zinnia tracheary element differentiation (Yoshimura et al, 1996) is consistent with the These data suggest a basis for examining the role of tubulin isoforms during secondary wall development in wood-forming tissues of trees, a process characterized by orderly deposition of extensive microfibril arrays. Their encoded protein sequences are more diverse than in any other species reported so far, especially at the hypervariable C terminus Both the TUA and TUB families contain spatiotemporally distinct isoforms, a small minority of which are highly expressed in wood-forming tissues undergoing secondary cell wall thickening. Over the course of tree ontogeny, such flexibility of expression might reasonably be expected to contribute adaptively to perennial cellulose deposition and vascular development
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