Abstract
BackgroundKnowledge of plant secondary cell wall (SCW) regulation and deposition is mainly based on the Arabidopsis model of a ‘typical’ lignocellulosic SCW. However, SCWs in other plants can vary from this. The SCW of mature cotton seed fibres is highly cellulosic and lacks lignification whereas xylem SCWs are lignocellulosic. We used cotton as a model to study different SCWs and the expression of the genes involved in their formation via RNA deep sequencing and chemical analysis of stem and seed fibre.ResultsTranscriptome comparisons from cotton xylem and pith as well as from a developmental series of seed fibres revealed tissue-specific and developmentally regulated expression of several NAC transcription factors some of which are likely to be important as top tier regulators of SCW formation in xylem and/or seed fibre. A so far undescribed hierarchy was identified between the top tier NAC transcription factors SND1-like and NST1/2 in cotton. Key SCW MYB transcription factors, homologs of Arabidopsis MYB46/83, were practically absent in cotton stem xylem. Lack of expression of other lignin-specific MYBs in seed fibre relative to xylem could account for the lack of lignin deposition in seed fibre. Expression of a MYB103 homolog correlated with temporal expression of SCW CesAs and cellulose synthesis in seed fibres. FLAs were highly expressed and may be important structural components of seed fibre SCWs. Finally, we made the unexpected observation that cell walls in the pith of cotton stems contained lignin and had a higher S:G ratio than in xylem, despite that tissue’s lacking many of the gene transcripts normally associated with lignin biosynthesis.ConclusionsOur study in cotton confirmed some features of the currently accepted gene regulatory cascade for ‘typical’ plant SCWs, but also revealed substantial differences, especially with key downstream NACs and MYBs. The lignocellulosic SCW of cotton xylem appears to be achieved differently from that in Arabidopsis. Pith cell walls in cotton stems are compositionally very different from that reported for other plant species, including Arabidopsis. The current definition of a ‘typical’ primary or secondary cell wall might not be applicable to all cell types in all plant species.
Highlights
Knowledge of plant secondary cell wall (SCW) regulation and deposition is mainly based on the Arabidopsis model of a ‘typical’ lignocellulosic SCW
Previous work had suggested that SCWs of mature seed fibres [54] and those of cotton stem pith [53] may be lignified, so we measured acetyl bromide lignin (ABL) in the cell walls from xylem, pith, and mature seed fibre and characterised the lignins by NMR (Fig. 1, Additional file 3)
If lignin does exist in cotton seed fibre it has an unusual composition in that it is not composed of the typical “G” and “S” subunits found in ‘normal’ dicot lignins
Summary
Knowledge of plant secondary cell wall (SCW) regulation and deposition is mainly based on the Arabidopsis model of a ‘typical’ lignocellulosic SCW. Development of secondary cell walls (SCWs) was a key event during the evolution of land plants. Pollen release and fertilisation, as well as seed dispersal are facilitated by specialised SCWs. Plants can deposit localised ‘SCW-like’ structures in response to pathogen attack, or in specialised cells types such as transfer cells. Our most comprehensive understanding on how SCWs are made has mainly come from detailed work over the last decade on xylem vessels and fibres in Arabidopsis thaliana [1,2,3] and other plants such as poplar [4], rice [5], grasses [6], spruce [7]. SCWs are deposited between the cell’s external primary cell wall (PCW) and the plasma membrane, often once cell expansion has ceased, and are usually orders of magnitude thicker than the PCW
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