Abstract
Nicotiana alata pollen tubes are a widely used model for studies of polarized tip growth and cell wall synthesis in plants. To better understand these processes, RNA-Seq and de novo assembly methods were used to produce a transcriptome of N. alata pollen grains. Notable in the reconstructed transcriptome were sequences encoding proteins that are involved in the synthesis and remodelling of xyloglucan, a cell wall polysaccharide previously not thought to be deposited in Nicotiana pollen tube walls. Expression of several xyloglucan-related genes in actively growing pollen tubes was confirmed and xyloglucan epitopes were detected in the wall with carbohydrate-specific antibodies: the major xyloglucan oligosaccharides found in N. alata pollen grains and tubes were fucosylated, an unusual structure for the Solanaceae, the family to which Nicotiana belongs. Finally, carbohydrate linkages consistent with xyloglucan were identified chemically in the walls of N. alata pollen grains and pollen tubes grown in culture. The presence of a fucosylated xyloglucan in Nicotiana pollen tube walls was thus confirmed. The consequences of this discovery to models of pollen tube growth dynamics and more generally to polarised tip-growing cells in plants are discussed.
Highlights
Nicotiana pollen tubes (N. tabacum and N. alata) are a widely used and a well-characterised system for studying polar cell growth and cell wall synthesis in plants [1,2,3]
It is apparent that forming the pollen tube cell wall requires precise control over the spatial distribution of the various glycosyl synthases and transferases needed to make the limited number of polysaccharides that are found in the wall, which is predominantly composed of the (1,3)-β-D-glucan callose and lesser amounts of cellulose, a neutral pectic arabinan and acidic pectins [5,6]
The transcriptome included contigs matching previously studied pollen-expressed genes from N. alata related to cell wall synthesis such as NaGSL1 (N. alata GLUCAN SYNTHASE-LIKE 1), which encodes the putative callose synthase [8], and NaCSLD1 (N. alata CELLULOSE SYNTHASE-LIKE D1), which encodes the putative cellulose synthase [7]
Summary
Nicotiana pollen tubes (N. tabacum and N. alata) are a widely used and a well-characterised system for studying polar cell growth and cell wall synthesis in plants [1,2,3]. At intervals along distal regions of the pollen tube shank are transverse callose-containing cross-walls called plugs that act to seal the cytoplasmic living portion of the pollen tube, containing the sperm cells, off from spent portions of the tube further back towards the grain Given this structure, it is apparent that forming the pollen tube cell wall requires precise control over the spatial distribution of the various glycosyl synthases and transferases needed to make the limited number of polysaccharides that are found in the wall, which is predominantly composed of the (1,3)-β-D-glucan callose and lesser amounts of cellulose, a neutral pectic arabinan and acidic pectins [5,6]. Only callose and cellulose have been associated with candidate genes and enzymes in pollen tubes [7,8]
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