DIFFERENTIATION, GROWTH AND MORPHOGENESIS - ACETABULARIA AS A MODEL SYSTEM

The aim of this paper is to review the present knowledge of the main a spects of differentiation of Acetabularia, a unicellular, eukaryotic o rganism, and to underline the multiple control pathways modulated by c ircadian rhythmicity. Growth and morphogenesis are sequentially progra mmed. Timing of cap differentiation is highly dependent on external co nditions. The importance of the sequence of processes is shown by expe rimental disregulation. The alga is a highly polarize cell, both in mo rphology and in the relative concentrations of a number of the molecul es it contains. Apical cap differentiation is regulated at the post-tr anscriptional level and could also depend in part on polyamines and on proteolytic activity. Acetabularia displays a number of circadian rhy thms (CR). These rhythms form an elaborate biological time structure ( also called temporal morphology, or morphology in time as opposed to m orphology in space): the distribution in the 24 h cycle of the peaks a nd troughs of the oscillating functions. The oscillations display fixe d relations both with the other functions and with external conditions (such as the transition from dark to light). Interestingly, the CR mo dulate Acetabularia's development, which is influenced by photoperiod; we present preliminary experiments suggesting that disruption of temp oral morphology is deleterious to morphogenesis. Induction of growth a nd of morphogenesis are totally dependent on blue light. However, blue light receptors in plants are probably multiple, but we present argum ents suggesting that flavin-cytochrome b and the associated SHAM-sensi tive molecule are present in Acetabularia plasma membrane and are invo lved in blue light perception. Agents interfering with different steps of signal perception and transduction show that at least some of thes e steps are temporally regulated. According to recent experiments from our laboratory, the existence of a redox signalling mechanism appears to be highly probable. The phyto hormones (or plant regulators), auxi n (indole acetic acid), abscisic acid and ethylene, exert cell-regulat ory functions and are involved in Acetabularia differentiation. They a lso modulate at least some circadian rhythms. Finally, circadian rhyth ms intervene in differentiation and are proposed to have an integrativ e function.