The influence of gravity and light on developmental polarity of single cells of Ceratopteris richardii gametophytes.

The gametophyte generation of Ceratopteris richardii is being used in a number of laboratories as a model plant system for developmental studies (Chasan, 1992). It is an especially valuable system for the study of the effect of gravity on plants in that the detection and response to gravity take place in the single-celled spore during its germination. The most visible evidence of the control of C. richardii spore polarity by gravity is that the direction of growth of the primary rhizoid, when it emerges, is downward, in line with the vector of gravity (Edwards and Roux, 1994). However, the direction of rhizoid growth is determined prior to its actual emergence through the spore coat. The influence of gravity on this growth polarity is, in fact, determined irreversibly during a “polaritydetermination window”-a period that occurs after germination has been initiated by exposure of spores to light, but before the first cellular division. Gravity does not appear to have an independent effect on the later events of prothallus and secondary rhizoid growth in the gametophyte, for the growth of prothallial cells follows the same direction as that of the first prothallial cell, and the growth of secondary rhizoids follows the same direction as that of the primary rhizoid, no matter how the gametophyte is oriented. The polarity-determination window is specified by a simple assay in which the spore position is changed by 180” at various times after the initiation of germination by light, but before rhizoid emergence, and then the subsequent direction of rhizoid emergence and growth is recorded (Edwards and Roux, 1994). Turning the spore