Association of spectrin-like proteins with the actin-organized aggregate of endoplasmic reticulum in the Spitzenkörper of gravitropically tip-growing plant cells.
Spectrin-like epitopes were immunochemically detected and immunofluorescently localized in gravitropically tip-growing rhizoids and protonemata of characean algae. Antiserum against spectrin from chicken erythrocytes showed cross-reactivity with rhizoid proteins at molecular masses of about 170 and 195 kD. Confocal microscopy revealed a distinct spherical labeling of spectrin-like proteins in the apices of both cell types tightly associated with an apical actin array and a specific subdomain of endoplasmic reticulum (ER), the ER aggregate. The presence of spectrin-like epitopes, the ER aggregate, and the actin cytoskeleton are strictly correlated with active tip growth. Application of cytochalasin D and A23187 has shown that interfering with actin or with the calcium gradient, which cause the disintegration of the ER aggregate and abolish tip growth, inhibits labeling of spectrin-like proteins. At the beginning of the graviresponse in rhizoids the labeling of spectrin-like proteins remained in its symmetrical position at the cell tip, but was clearly displaced to the upper flank in gravistimulated protonemata. These findings support the hypothesis that a displacement of the Spitzenkörper is required for the negative gravitropic response in protonemata, but not for the positive gravitropic response in rhizoids. It is evident that the actin/spectrin system plays a role in maintaining the organization of the ER aggregate and represents an essential part in the mechanism of gravitropic tip growth.
Braun
M
M
Botanisches Institut, Universität Bonn, Venusbergweg 22, D-53115 Bonn, Germany. mbraun@uni-bonn.de
D000199:Actins / Q000378:metabolism*; D000818:Animals; D000906:Antibodies; D002645:Chickens; D003429:Cross Reactions; D004721:Endoplasmic Reticulum / Q000502:physiology* / Q000648:ultrastructure; D004912:Erythrocytes / Q000378:metabolism; D018522:Gravitropism / Q000502:physiology*; D019684:Magnoliopsida / Q000166:cytology* / Q000254:growth & development*; D018613:Microscopy, Confocal; D010940:Plant Proteins / Q000378:metabolism*; D013049:Spectrin / Q000378:metabolism*
D000199:Actins; D000906:Antibodies; D010940:Plant Proteins; D013049:Spectrin