A multi-responsive gene encoding 1-aminocyclopropane-1-carboxylate synthase (ACS6) in mature Arabidopsis leaves

Physiological and biochemical studies have provided evidence that mechanical strain (touch)-induced modifications in plant growth and development may be due to ethylene. In order to better understand the involvement of ethylene in touch-induced responses, we identified and characterized an Arabidopsis cDNA (ACS6) encoding 1-aminocyclopropane-1-carboxylic acid (ACC) synthase which is an important regulatory enzyme in the ethylene biosynthetic pathway. Northern analysis showed that ACS6 was induced by touch in the leaves of 3-week old light-grown plants within 5 min and reached maximum transcription at 15 min. ACC, which is the product of ACC synthase and the immediate precursor to ethylene, exhibited a dramatic rise between 15 and 30 min after touch stimulation. Experiments with multiple touch treatments showed that a saturation in gene expression was obtained with one touch treatment and subsequent touch stimulations were progressively less effective in promoting ACS6 expression. Additional characterization of ACS6 gene expression indicated that the gene is also induced by wounding, and by treatment with LiCl, NaCl, CuCl₂, auxin, cycloheximide (CHX), aminooxyacetic acid (AOA) and ethylene. ACC levels were also increased in response to each of these treatments with the exception of CHX and AOA which resulted in a decrease and no effect, respectively. Our results show that ACS6 is rapidly turned on in response to touch which is followed by an increase in ACC which is the immediate precursor to ethylene, thereby providing evidence that it is responsible for touch-inducible ethylene production in light-grown Arabidopsis plants. The identification and characterization of ACS6 now provides us with a tool to better understand the involvement of ethylene produced in response to external stimuli as well as during plant growth and development.