Pathway and control of sucrose import into initiating cotton fibre cells

Our aim is to unravel the mechanisms controlling fibre cell initiation from the epidermis of cotton (Gossypium hirsutum L.) ovules. We compared the development of fibres and trichomes in wild type cotton and a fibreless seed (fls) mutant, and determined the cellular pathway of sucrose transport into fibre initials on the day of anthesis. Although fibre initiation is inhibited in the fls mutant, leading to the fibreless phenotype, trichome development in other parts of the plant is normal. Confocal imaging analysis revealed that the fluorescent molecule, 5(6)-carboxyfluorescein, which is transported symplastically, moved readily from the integument phloem into initiating fibres. Plasmolysis studies showed that the fibre initials and adjacent non-initiating ovule epidermal cells have similar osmotic potential. Immunolocalisation analysis showed the absence of sucrose transporter proteins in the initiating fibre, but their abundance in the transfer cell precursors at the innermost integument. These results (i) demonstrate that fibre cell initiation is controlled by unique mechanism(s) that differ from that for normal trichome development; (ii) show a symplastic pathway of sucrose import into initiating fibres and strengthen the current opinion that sucrose synthase is likely to be the key enzyme mobilising sucrose into initiating fibres; and (iii) suggest that the initial protrusion of the fibre cells above the ovule surface is largely achieved by increased cell wall extensibility rather than higher turgor as is commonly thought.