COTTON (GOSSYPIUM-HIRSUTUM) SEED TRICHOMES EXPAND VIA DIFFUSE GROWINGMECHANISM

The ultrastructure of cotton (Gossypium hirsutum) seed trichomes was i nvestigated to obtain insight into their growth behavior during the ph ase of rapid cell elongation. A mold and cast method of scanning elect ron microscopy was used to record the morphological changes in the ovu lar surface during the initiation and elongation of trichomes. A rapid freeze-fixation and freeze-substitution protocol was used to study th e cytological features of trichomes at 2 days after anthesis. At the c ytological level, attention was primarily focused on determining wheth er the seed trichomes display features that are characteristic of othe r tip-growing plant cells, including organelle zonation, polarized dep osition of cell wall, axial orientation of microtubules, and microfila ment meshwork at the cell apex. Our results show that cotton seed tric homes do not share any ultrastructural characteristic with other tip-g rowing plant cells. Rather, they show all the characteristics of cells that undergo diffuse growth. The roles of actin microfilaments and mi crotubules were also investigated through an in vitro administration o f cytochalasin D and colchicine. Although the disruption of actin fila ments did not stop trichome growth, disruption of microtubules did pre vent polarized cell expansion. Based on these results, cotton seed tri chomes are not tip-growing cells but expand via diffuse growth.