ASGSB 1999 Annual Meeting Abstracts

TIP-FOCUSED CALCIUM GRADIENTS IN GRAVIRESPONDING CHARA RHIZOIDS. J.M. Fasano¹, E.B. Blancaflor², and S.Gilroy¹. ¹Dept. Of Biology, Penn State University, and ²Plant Biology Division, S.R. Noble Foundation, Ardmore, OK.

The algae Chara vulgaris possesses anchoring structures (rhizoids) which extend by tip growth. Other tip-growing systems in higher plants (root hairs, pollen tubes) have an apical gradient of Ca²⁺ focused on the tip which is involved in growth control. Chara rhizoids may possess similar properties. Rhizoids differ from these other tip-growing systems, however, in that they exhibit positive gravitropism mediated by sedimenting barium crystals (statoliths) located in the tip. The current model of rhizoid gravitropism postulates statolith control of growth by physical blockage of secretory vesicles en route to the tip. It is unclear how this model would relate to potential growth-associated Ca2+ gradients in Chara rhizoids. We have microinjected rhizoids with fluorescent Ca²⁺ indicator dyes, and have established that a tip-focused Ca²⁺ gradient is present. The rhizoids remain capable of growth and gravity-induced curvature. We have also found that rhizoid growth is susceptible to the action of modulators of Ca²⁺ fluxes, such as La³⁺, and have induced curvature by asymmetric application of a Ca²⁺ ionophore. It appears that Chara rhizoids resemble other tip-growing systems in possessing growth-associated Ca²⁺ gradients. We are further characterizing the Ca²⁺ tip gradients of vertically-growing and gravistimulated rhizoids and experimentally manipulating the Ca²⁺ gradient. We are also comparing the kinetics of changes in the gradient, rhizoid curvature, and statolith sedimentation upon gravistimulation to better understand the chain of events that lead to gravity-induced rhizoid curvature. This research was supported by NSF and NASA.

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