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 de velopment 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 inhibi ted in the fls mutant, leading to the fibreless phenotype, trichome develop ment in other parts of the plant is normal. Confocal imaging analysis revea led that the fluorescent molecule, 5(6)-carboxyfluorescein, which is transp orted symplastically, moved readily from the integument phloem into initiat ing fibres. Plasmolysis studies showed that the fibre initials and adjacent non-initiating ovule epidermal cells have similar osmotic potential. Immun olocalisation analysis showed the absence of sucrose transporter proteins i n 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 impo rt into initiating fibres and strengthen the current opinion that sucrose s ynthase is likely to be the key enzyme mobilising sucrose into initiating f ibres; and (iii) suggest that the initial protrusion of the fibre cells abo ve the ovule surface is largely achieved by increased cell wall extensibili ty rather than higher turgor as is commonly thought.