Gravimorphogenesis: gravity-regulated formation of the peg in cucumber seedlings.

Formation and/or positioning of a protuberance, or peg, in cucumber (Cucumis sativus L.) seedlings is controlled by gravity. The gravisensing apparatus for this gravimorphogenesis is possibly shared with that of gravitropism. Sheath cells in the transition zone between stem and root contain sedimentable amyloplasts, but amyloplasts in cortical cells (peg cells) do not sediment uniformly. These putative statoliths appear before peg initiation becomes visible on the lower side of the transition zone. The increased level of auxin on the lower side of the transition zone, which may occur following graviperception, could be a factor responsible for peg development. This system may be a useful model in spaceflight experiments for understanding the mechanisms underlying gravity-regulated formation of the peg, and its interaction with gravitropism, auxin physiology and cell growth polarity.