Gravity susception by buoyancy: Floating lipid globules in sporangiophores of Phycomyces

Abstract

To elucidate the mechanisms of gravity susception that operate in the sporangiophore of Phycomyces blakesleeanus, we characterized the function and topography of a large apical complex of lipid globules. Stage-1 sporangiophores (without sporangium) possess a roughly spherical complex of 100-200 large lipid globules whose center is localized 110 μm below the apex. The complex of lipid globules (CLG) is rather stable and is kept in place by positioning forces that resist centrifugal accelerations of up to 150 g. The lipid globules possess an average diameter of 2 to 2.5 μm and a density of 0.791 g cm -3, which is below that of typical plant oleosomes. The potential energy which is generated by the buoyancy of a CLG of 100 globules is in the order of 10-17 to 10-16 J, which is 4 to 5 orders of magnitude above thermal noise. The formation of lipid globules can be supressed by raising stage-1 sporangiophores for 24 hs at 5°C. Sporangiophores with a reduced number of lipid globules display gravitropic bending angles that are 3 to 4 times smaller than those of sporangiophores with the normal number of lipid globules. The results suggest that the lipid globules function as gravisusceptors of Phycomyces and that buoyancy is the physical principle for their mode of action. The globules contain β-carotene and two distinct fluorescing pigments that are, however, dispensible for graviperception.