The effect of karrikins was also tested. These smoke-derived small compounds are butenolides as are SLs, and share certain components of the SL signaling pathway. These assays should further help pinpoint which parts of SL chemical structure are required to inhibit protonema extension in P. patens compared to those required for hormonal activity in vascular plants, or for activity in the rhizosphere as part of parasitic and symbiotic relationships. Our results on moss indicate that, at the cellular level, the caulonema cell division rate is inhibited by exogenously supplied SLs, in contrast to cell length that is only slightly affected. The higher branching rate of protonemata observed in the Ppccd8 mutant compared to the WT can be attributed to this higher rate of division. More divisions may result in a higher number of caulonemata in the Ppccd8 mutant. This effect of SLs on cell division rather than cell elongation was already suggested by the higher number of cells per chloronema filament at different times after germination, in Ppccd8 in comparison to the WT. In vascular plants, SLs have been reported to inhibit or enhance cell division, depending on the species and the tissue in question. In rice, on WT and SL synthesis mutant plants grown in the dark, addition of GR24 negatively regulates mesocotyl cell division but has no effect on cell elongation. In contrast, in Arabidopsis, SL synthesis mutants show reduced cambium activity and local treatment of stems with GR24 induces cambium-like cell proliferation. Also in Arabidopsis and rice, both SL synthesis and perception mutants show a fewer root meristem cells compared to the WT. In Arabidopsis, the addition of GR24 in the growth medium restores the number of root meristem cells in SL-deficient plants. In pea, SL-deficient mutants are relatively dwarf, and this dwarfism is not due to more frequent branching, but to a deficiency in SLs. Dwarfism of the SL synthesis mutant is maintained even when branching is inhibited, and adding GR24 restores internode length. Internodes in dwarf mutants show fewer epidermal cells whose length is not affected, suggesting that SLs stimulate internode elongation by stimulating cell division. SLs were first identified as phytohormones that inhibit axillary bud outgrowth. This hormonal action further suggests that in vascular plants, SLs can have different roles on cell division in different types of meristem. It has been suggested that SLs function as central modulators in plant architecture regulation, allowing the plant to respond to changing environmental conditions. The pannexins are large-pore ion channels distantly related by sequence to the invertebrate gap junction channels, the innexins, and unrelated by sequence to the connexins. Despite such sequence homology to the innexins, pannexin channels appear not to form gap junctions in vertebrates but instead form large-pore plasma-membrane hemichannels.