Auxin redistributes upwards in graviresponding gynophores of the peanut plant

The peanut (Arachis hypogaea L.) produces flowers aerially, but buries the recently fertilized ovules into the soil, where fruit and seed development occur. The young seeds are carried down into the soil at the tip of a speci alized organ called the gynophore. Although the gynophore has a typical sho ot anatomy, it responds positively to gravity like a root. In this study, w e explore the role of the plant growth regulator indole-3-acetic acid (IAA) in the growth and the gravitropic response of the peanut gynophore. With a n immunolocalization technique using an IAA monoclonal antibody, we localiz ed IAA within the tissues of vertically oriented and gravistimulated gynoph ores. We found that in vertically oriented gynophores, IAA labeling occurs in the periphery of the gynophore, in the entire cortex and epidermis. With in 20 min of horizontal reorientation, the IAA signal gradually increases i n the upper cortex/epidermis and diminishes in the lower cortex/epidermis. At 1.5 h after gravistimulation, all of the IAA immunolocalization signal i s detected in the upper cortex and epidermis - none is detected in the lowe r side. Growth rate measurements also indicate that after 1-2 h of reorient ation, the growth rate maximum on the upper side corresponds temporally and spatially to the growth rate minimum on the lower side. Experiments using radioactively labeled IAA corroborate an upper-side redistribution of this hormone upon horizontal reorientation. These results are analyzed with resp ect to the current theories of plant gravitropic response, and a model for a possible gravity-induced IAA redistribution from the lower to the upper s ide of the peanut gynophore is proposed.