Differences may concern the SL receptor itself, since only D14-like sequences have been found in the moss genome. A study of a knock-out mutant for the CCD8 gene, established that SLs regulate P. patens protonema branching, and control plant size as quorum-sensing like molecules very likely by controlling caulonema radial extension. However, a better understanding of how SLs inhibit protonema extension in moss is needed, and the cellular effects of SLs have yet to be described, particularly whether SLs inhibit cell division and/ or cell elongation. The feedback control on SL synthesis genes, previously characterized in vascular plants, has also been highlighted in moss because PpCCD7 transcripts are upregulated in the SL-deficient Ppccd8 mutant and SL application decreased PpCCD7 transcript levels. Exploring the links between the chemical structure of SL molecules and their activity on moss filament cells is useful for determining structural requirements for bioactivity. Comparison of those requirements with regard to hormonal bioactivity in vascular plants and non-vascular plants and with regard to other functions of SL in the rhizosphere may give indications on SL reception in the different systems. To date the SL-receptor has been identified only for the hormonal function in vascular plants. Structure-activity relationship studies have already been performed for the main known functions of SLs in vascular plants. Various natural SLs or synthetic analogs have been tested for their activity as a plant hormone or as a stimulant of parasitic plant seed germination or AM hyphal branching. For all SL functions, the D ring is essential for bioactivity. Although modifications of the tricyclic lactone have no major effect on pea branching, the ABC ring is essential for AM hyphal branching. In pea, some analogs are very active on pea buds but are poorly recognized by parasitic plant seeds, opening the possibility for the use of SLs in agronomy. Natural SLs found in moss and SL analogs with modified ABC rings or D ring with strong bioactivity for the control of shoot branching but not for AM hyphal branching have been tested on moss. We investigated the cellular effects of SLs on moss in the light and in the dark. Dark-grown moss filaments show negative gravitropism. Since only caulonema filaments grow in dark, caulonema length and caulonema cell sizes can be easily quantified in dark culture conditions. In addition, the use of the SL-deficient Ppccd8 mutant make it possible to better characterize the effect of exogenous SL added to the growth medium, since this effect is enhanced in comparison with the wild type which contains endogenous SLs, and as observed in other SAR studies on vascular plants. Here, we show that SLs control filament extension by decreasing the caulonema cell division rate with a slight effect on cell elongation.