The olefin gas ethylene, as one of the five major classes of plant hormones, has a wide range of physiological effects on plants. The production of ethylene by plants is tightly regulated and cannot be uncoupled from its actions. The present study is aimed to examine the biosynthesis of ethylene and its role(s) in plant gravitropism. Firstly, we studied ethylene biosynthesis. We successfully isolated and sequenced one ACC synthase gene (MAACS5) and one ACC oxidase gene (MAACOl) from the banana plant. Both MAACS5 and MAACOl mRNAs are ethylene-inducible. Furthermore, an ACC synthase and ACC oxidase fusion gene, ACSO, was constructed from a soybean ACC synthase and a tomato ACC oxidase. A concurrent production of ACC and ethylene by this fusion gene was observed in yeast cells and insect c...[ Read more ]

The olefin gas ethylene, as one of the five major classes of plant hormones, has a wide range of physiological effects on plants. The production of ethylene by plants is tightly regulated and cannot be uncoupled from its actions. The present study is aimed to examine the biosynthesis of ethylene and its role(s) in plant gravitropism. Firstly, we studied ethylene biosynthesis. We successfully isolated and sequenced one ACC synthase gene (MAACS5) and one ACC oxidase gene (MAACOl) from the banana plant. Both MAACS5 and MAACOl mRNAs are ethylene-inducible. Furthermore, an ACC synthase and ACC oxidase fusion gene, ACSO, was constructed from a soybean ACC synthase and a tomato ACC oxidase. A concurrent production of ACC and ethylene by this fusion gene was observed in yeast cells and insect cells. Secondly, we studied ethylene effects on Arabidopsis shoots gravitropism. We found that ethylene can have both stimulatory and inhibitory effects on the gravitropic responses of Arabidopsis inflorescence stems and hypocotyls. Long-term pretreatments of inflorescence stems with one ppm of ethylene for twelve hours or of hypocotyls with five ppm of ethylene for forty-eight hours greatly enhanced the gravitropic responses of inflorescence stems and hypocotyls, respectively; while short-term pretreatments of inflorescence stems and hypocotyls with one ppm of ethylene inhibited the gravitropic response. This is the first time that both stimulatory and inhibitory effects of ethylene are observed on gravitropism at the same time. The ethylene overproducing mutant etol also showed a faster gravitropic response in the inflorescence stem, further confirming our discovery that long-term ethylene pretreatments are capable of enhancing gravitropism. Thirdly, we tried to elucidate ethylene's actions in gravitropism by studying ethylene's effects on the gravitropism of ethylene perception and response mutants etrl and ctrl. Both long-term and short-term ethylene pretreatments failed to alter the shoots gravitropic response of the etrl mutant; long-term ethylene pretreatment was still able to enhance the gravitropic response of ctrl while short-term ethylene pretreatment had no effect on the ctrl shoots gravitropism. The results reveal that ethylene's actions on Arabidupsis shoot gravitropism go through the ethylene receptor ETRl. However, ethylene signal branches after ETRI, the inhibitory signaling goes through CTRl while the stimulatory signaling goes through an alternative signaling component. Finally, possible involvement of auxin in ethylene's modulation in gravitropism was explored by studying the gravitropic response of three of auxin mutants, auxl, axr1 and axr2. The responses of auxl and axr1 mutants upon ethylene treatments were similar to those of wild type, indicating that both mutations does not interfere with ethylene's modulation of gravitropic response in Arabidopsis inflorescence stem. However, ethylene treatment had no effect on the gravitropic response of axr2 mutant, suggesting that axr2 mutation interferes with ethylene's modulation of gravitropic response. A model of ethylene's actions on Arabidopsis gravitropism was proposed based both on our study and literature review.