Studies on submerged cotton fiber growth : induction and characterization, effects of Congo Red and auxin
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Induction of growth of submerged cotton (Gossypium hirsutum L.) fibers from cultured ovules has been investigated for the first time. Both exogenous plant hormone levels and the age of the ovules at induction play important roles in induction of submerged cotton fiber growth. The diameter of submerged fibers was about same as that of air-grown fibers but was smaller than that of fibers grown in vivo. Submerged fibers were shorter in the fiber length, stronger in the tensile strength, and they had thicker secondary cell walls and smaller crystallite sizes compared with air-grown fibers and fibers in vivo. Helical secondary cell wall thickenings were exclusively found in submerged fibers. Congo Red is a natural dye that has a high affinity for the biopolymer cellulose. The addition of Congo Red to the culture medium had an influence only on submerged cotton fibers and not on air-grown cotton fibers. When Congo Red was applied in the early primary wall stage, fiber cell elongation was inhibited, but amyloplast production was induced. When Congo Red was applied in late primary wall or early secondary wall stage (about 14-16 DPA), the effects were less severe, but a significant increase in birefringence of secondary cell walls was observed. In both conditions of treatment with Congo Red in the primary wall and the secondary wall stages, a "nodulation" occurred on the wall surface. Neither cellobiohydrolase CBH I or CBH II had affinity for the external wall materials, implying that there was no cellulose present or binding sites for CBH had been occupied by Congo Red. X-ray diffraction data showed that Congo Red decreased the crystallite size of cellulose in submerged cotton fibers. The preliminary investigation with auxin (indole-3-acetic acid) depletion in the culture medium was to study whether or not amyloplasts were produced under this condition. No amyloplasts were observed in submerged fibers grown in the auxindepleted medium, but cellulose microfibrils in the secondary cell wall were greatly disorganized. Possibly, indole-3-acetic acid might play an important role in regulating the arrays of microtubules, which, in turn, may help to organize the patterns of cellulose deposition.