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Hiroyuki Usuda, Daisuke Nei, Yasuhiro Ito, Nobutaka Nakamura, Yutaka Ishikawa, Hitomi Umehara, Poritosh Roy, Hiroshi Okadome, Manasikan Thammawong, Takeo Shiina, Mamiko Kitagawa, and Takaaki Satake

Ethylene production and the accumulation of the 1-aminocyclopropane-1-carboxylic acid synthase (ACS; EC 4.4.1.14) gene were determined in tomato (Solanum lycopersicum L.) fruit that were dropped from a height of 5 cm. Dropped fruit had higher ethylene production than nondropped controls, and this lasted for at least 10 h. Maximum accumulation of Le-ACS2, one of the members of the Le-ACS multigene family, was achieved 5 h after dropping, and changes in accumulation tracked closely with ethylene production. In comparison with control fruit, substantial accumulation of Le-ACS1A, Le-ACS4, or Le-ACS6 in dropped fruit was not observed. These results indicated that the increased ethylene production following fruit dropping was most likely regulated by Le-ACS2 transcripts. The transfer of dropping stimuli from directly stressed tissues was investigated by measuring Le-ACS2 accumulation at various positions on the dropped fruit. Le-ACS2 was mainly induced in the fruit pericarp, and there was low accumulation in the fruit interior. The Le-ACS2 accumulation linearly decreased with increasing distance along the pericarp from the stressed site. This implied that accumulation of Le-ACS2 was dependent on stress levels, while most ethylene that was derived from dropping was produced at the stressed site. Using levels of Le-ACS2 accumulation, the ethylene production of tomato fruit at mechanically impacted sites was estimated to be about 50 times higher than that of nondropped controls.