Blue light-induced branching in Vaucheria. Requirement of nuclear accumulation in the irradiated region

When a narrow region of the fresh water coenocytic alga, Vaucheria terrestr is sensu Gotz is irradiated with moderately intense blue light, a branch is induced from the center of the irradiated region after 4-5h. Movement of o rganelles and microtubule bundles during the photocytomorphogenetic respons e were investigated. Chloroplasts in the cortical layer immediately started to accumulate in the blue light-irradiated region and their accumulation a lmost completely finished 30-40 min after the onset of light when the nucle i residing in endoplasm started to accumulate. Accumulation of nuclei was s ynchronized with disorientation and shortening of microtubule bundles, whic h originally run parallel to the cell axis. Not only amiprophos-methyl, a p otent microtubule-decomposing reagent, but also cytochalasin A strongly inh ibited the branch induction. Amiprophos-methyl completely and cytochalasin A mostly destroyed microtubules and completely inhibited nuclear accumulati on, but both drugs allowed the accumulation of chloroplasts in the cortical layer of irradiated region. These indicate that the accumulation of nuclei is indispensable for branch induction while the chloroplast accumulation i s insufficient by itself for branch induction. The ineffectiveness of cytoc halasin A on chloroplast movement brings the conventional view of sliding m ovement of chloroplast on a long actin cable into question. The morphologic al and functional relationship between a nucleus and a microtubular bundle are discussed.