Effect of Dropping on Le-ACS2 Accumulation Around the Mechanically Stressed Site of the Tomato Fruit

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 ,o rLe-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.