System Analysis of the Ciliary Response to Red Light in Chlamydomonas reinhardtii

Abstract

To understand signal processing networks within cells we study the motile green alga Chlamydomonas reinhardtii. This organism swims with two cilia and has an eye with rhodopsin (peak sensitivity 500 nm, green) controlling the direction of its swimming (phototaxis). Here we report on a red light receptor (peak sensitivity 670 nm) that influences the frequency of ciliary beating, which we monitor by observing a single cell held on a micropipette with a quadrant photodiode. For square-wave on-off stimuli, the recorded beating frequency shows the on-step latency is strongly light intensity dependent, ranging from 700 ms at 1.3 W/m2 to 200 ms at 50 W/m2. This long delay suggests a diffusive step in the signaling pathway and contrasts with the sub-milliseconds latency of green light stimuli. The response amplitude is sustained for the full length of the on-step and is also light intensity dependent. However, the off-step latency seems light independent ranging from 400 to 1600 ms. Following the light off, the beating frequency drops more rapidly and farther the greater the prior light intensity. Products appear to be synthesized in proportion to the light intensity, but once the light is off these products in the cilium run out after about a second. Since the beating frequency is feedback regulated, the red-light elevation of the beating frequency is compensated by a counter molecular change in the cilia. Consequently, when these products run out, there is a rapid declined in beating frequency that takes about 10 s to recover. In addition to steps, sine and white-noise stimuli have been used to refine the response function.