Phytohormone-Mediated Ethylene Biosynthesis in Arabidopsis thaliana
The plant hormone ethylene affects various physiological aspects of plant growth and development, including seed development, dormancy, germination, flowering, senescence, abscission, and fruit ripening. To exert these diverse roles of ethylene in plants, the tight regulation of ethylene biosynthesis is absolutely required, and in fact, ethylene biosynthesis is adjusted by transcriptional and post-transcriptional control of the biosynthetic enzymes to regulate the amount of ethylene produced in plants. The interaction between ethylene with other phytohormones also plays a key factor in controlling the ethylene biosynthesis. Previous studies demonstrated that cytokinin and brassinosteroid control the protein stability of ACC synthases (ACS), a generally rate-limiting enzyme in ethylene biosynthesis, and hence regulates the ethylene production in plants. This suggests that the post-transcriptional regulation of ethylene is an important way of modulating ethylene biosynthesis. In addition to cytokinin and brassinosteroid, ethylene also interacts with other phytohormones, raising questions if the crosstalk between ethylene and the phytohormones also control the stability of ACS proteins. In order to understand their influences in regulating the ethylene biosynthesis through the alteration of ACS protein stability, we utilized a dexamethasone (DEX) inducible system that allowed us to study the post-transcriptional regulation of ACS proteins in response to various phytohormones. After phytohormone treatment, we measured ethylene produced from the transgenic lines overexpressing different subtypes of ACS proteins as a proxy of changes in ACS protein stability. We found that gibberellic acid, auxin, jasmonic acid, abscisic acid, and salicylic acid, are also capable of controlling ethylene biosynthesis in dark-grown seedlings either positively or negatively, resulting from differential regulation of various ACS isoforms. It is known that 14-3-3, an evolutionally well conserved signaling molecule involving in many important biological processes, controls ethylene biosynthesis by stabilizing ACS proteins. A recent proteomics study to identify potential 14-3-3 interacting proteins has demonstrated that 14-3-3 exists as a complex with a Seven in Absentia (SINAT) protein, an RING-containing E3 ligase, as well as a subset of ACS proteins, indicating the functional relationship between the molecules in the complex. Therefore, we determined to investigate the function of SINAT E3 ligase in ethylene biosynthesis. To examine the role of SINAT proteins in ethylene biosynthesis, we generated high-order loss-of-function sinat mutants and analyzed their roles in ethylene biosynthesis. As a result, we found that a subset of SINAT proteins (SINAT1 and SINAT2) are positive regulators in brassinosteroid-mediated ethylene biosynthesis pathway, while the rest of SINAT proteins (SINAT3, SINAT4 and SINAT 5) plays a negative role in the pathway. The double mutant of sinat1sinat2 showed a substantially reduced ethylene biosynthesis in response to brassinosteroid in the dark. In contrast, the sinat3sinat5 double mutant exhibited an increase in ethylene biosynthesis upon brassinosteroid treatment. These results indicate that SINAT E3 ligases are potential regulator for ethylene biosynthesis likely through modulating the ACS proteins or their stability regulators. Based on our studies, we revealed that various phytohormones influences ethylene biosynthesis in dark-grown Arabidopsis seedlings in addition to cytokinin and brassinosteroid. This implies that the phytohormones regulate many conditions of plant growth and development through the controlling the amount of ethylene produced in plant. We also identified and characterized a novel regulatory component, SINAT E3 ligase family proteins in brassinosteroid-mediated ethylene biosynthesis.
Yoon, Purdue University.
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