Abstract
The pollen tube is a cellular protuberance formed by the pollen grain, or male gametophyte, in flowering plants. Its principal metabolic activity is the synthesis and assembly of cell wall material, which must be precisely coordinated to sustain the characteristic rapid growth rate and to ensure geometrically correct and efficient cellular morphogenesis. Unlike other model species, the cell wall of the Arabidopsis (Arabidopsis thaliana) pollen tube has not been described in detail. We used immunohistochemistry and quantitative image analysis to provide a detailed profile of the spatial distribution of the major cell wall polymers composing the Arabidopsis pollen tube cell wall. Comparison with predictions made by a mechanical model for pollen tube growth revealed the importance of pectin deesterification in determining the cell diameter. Scanning electron microscopy demonstrated that cellulose microfibrils are oriented in near longitudinal orientation in the Arabidopsis pollen tube cell wall, consistent with a linear arrangement of cellulose synthase CESA6 in the plasma membrane. The cellulose label was also found inside cytoplasmic vesicles and might originate from an early activation of cellulose synthases prior to their insertion into the plasma membrane or from recycling of short cellulose polymers by endocytosis. A series of strategic enzymatic treatments also suggests that pectins, cellulose, and callose are highly cross linked to each other.
Highlights
Upon contact with the stigma, the pollen grain swells through water uptake and develops a cellular protrusion, the pollen tube
Unlike most other plant cells, cellulose is not very abundant representing only 10% of total neutral polysaccharides in Nicotiana alata pollen tubes, whereas callose accounts for more than 80% in this species (Schlüpmann et al, 1994)
We describe the biochemical composition of the Arabidopsis pollen tube cell wall 75 grown in in vitro conditions using immuno-cytochemical labelling coupled with epi76 fluorescence and electron microscopic techniques
Summary
Upon contact with the stigma, the pollen grain swells through water uptake and develops a cellular protrusion, the pollen tube. To fullfil its biological function, the pollen tube has to (1) adhere to and invade transmitting tissues (Hill and Lord, 1987; Lennon et al, 1998), (2) provide physical protection to the sperm cells, and (3) control its own shape and invasive behaviour (Parre and Geitmann, 2005b; Geitmann and Steer, 2006). For all of these functions, the pollen tube cell wall plays an important regulatory and structural role. Unlike most other plant cells, cellulose is not very abundant representing only 10% of total neutral polysaccharides in Nicotiana alata pollen tubes, whereas callose accounts for more than 80% in this species (Schlüpmann et al, 1994)
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