Chlamydomonas shortens its flagella by activating axonemal disassembly, stimulating IFT particle trafficking, and blocking anterograde cargo loading.

Almost all eukaryotic cells form cilia/flagella, maintain them at their genetically specified lengths, and shorten them. Here, we define the cellular mechanisms that bring about shortening of flagella prior to meiotic cell division and in response to environmental cues in the biflagellated green alga Chlamydomonas. We show that the flagellar shortening pathway is distinct from the one that enforces transient shortening essential for length control. During flagellar shortening, disassembly of the axoneme is stimulated over the basal rate, and the rate of entry into flagella of intraflagellar transport (IFT) particles is increased. Moreover, the particles entering the disassembling flagella lack cargo. Thus, flagellar shortening depends on the interplay between dynamic properties of the axoneme and the IFT machinery; a cell triggered to shorten its flagellum activates disassembly of the axoneme and stimulates entry into the flagellum of IFT particles possessing empty cargo binding sites available to retrieve the disassembled components.