Characterization of ethylene biosynthesis in cotton (Gossypium hirsutum L.) fruiting form as a means to improve early season fruit set

Abstract

The metabolism of I-aminocyclopropane-l-carboxylic acid (ACC) was investigated in cotton fruiting forms harvested at four stages (3 day pre-anthesis, 1 day preanthesis, anthesis, 3 day post-anthesis). Comparing bracts, the ACC concentration was at its highest level in those of 3 day pre-anthesis fruiting forms. This level fell in 1 day pre-anthesis bracts and remained low at anthesis due to an increase in ACC oxidase activity and a concomitant decrease in ACC synthase activity. At anthesis, ACC synthase activity fell to its minimum and ACC oxidase activity fell to its starting activity. In comparison to the ovary samples, ACC levels in bracts were higher, ACC synthase activity was similar but ACC oxidase activity was about fivefold lower for 3 day pre and 1 day pre-anthesis samples. This suggest that the bracts play a minor role in the overall ethylene production at anthesis. The ACC concentration of the ovary dipped somewhat at 1 day pre-anthesis, then reached its maximum at anthesis. This level then fell back to its starting point at 3 day post-anthesis. Several factors led to the sharp increase in ACC content in the ovary at anthesis. First, there was a concomitant increase in 1(malonyamino)-cyclopropane-l-carboxylic acid (MACC) concentration and ACC oxidase activity in the 1 day peranthesis flower bud, which led to a subtle decrease in ACC content. Second, ACC synthase activity rose sharply at this point and reached its maximum at anthesis. Next, ACC oxidase activity fell sharply and reached its starting point at anthesis. These data clearly show that the ovary is the tissue responsible for the increase in ethylene production at anthesis. Since ACC synthase and ACC oxidase activities were minimal in the bracts at all stages of flower development and both ACC synthase and oxidase activities of the ovary were maximal just prior to and at anthesis, this may suggest that the expression of the ACC synthase gene in the ovary tissue is significantly different than the ACC synthase gene in the bracts.