Control of gravimorphogenesis by auxin: accumulation pattern of CS-IAA1 mRNA in cucumber seedlings grown in space and on the ground

Cucumber (Cucumis sativus L.) seedlings grown in microgravity developed a p eg on each side of the transition zone between hypocotyl and root, whereas seedlings grown in a horizontal position on the ground developed a peg on t he concave side of the gravitropically bending transition zone. The morphol ogical features of the space-grown seedlings were similar to those of seedl ings grown in a vertical position on the ground with their radicles pointin g down: both became two-pegged seedlings. Morphogenesis of cucumber seedlin gs is thus inhibited by gravity. Analysis by in-situ hybridization of an au xin-inducible gene, CS-IAA1, showed that its mRNA accumulated to a much gre ater extent on the lower side of the transition zone in the horizontally pl aced seedlings on the ground just prior to and during the initiation period of peg formation. On the other hand, when seedlings were grown in microgra vity or in a vertical position on the ground, accumulation of CS-IAA1 mRNA occurred all around the transition zone. Accumulation of CS-IAA1 mRNA in ho rizontally grown seedlings appreciably decreased on the upper side of the t ransition zone and increased on the lower side upon gravistimulation, compa red with the two-pegged seedlings. Application of IAA to seedlings in a hor izontal position caused the development of a peg on each side of the transi tion zone, or a collar-like protuberance, depending on the concentration us ed. These results suggest that upon gravistimulation the auxin concentratio n on the upper side of the horizontally placed transition zone is reduced t o a level below the threshold value necessary for peg formation. Space-grow n seedlings of cucumber might develop two pegs symmetrically because the au xin level in the entire transition zone is maintained above the threshold. This spaceflight experiment verified for the first time that auxin does not redistribute in microgravity.