Regulation of ethylene biosynthesis via ACC synthase protein stability

Abstract

The gaseous plant hormone ethylene is involved in many plant processes including germination, leaf and flower abcission, cell elongation and inhibition, plant defense and fruit ripening. Ethylene synthesis can be altered in response to stimuli to effect these plant processes and the fitness of a plant in its environment. Although there are several points in ethylene biosynthesis that can be regulated under differing conditions, one of the key components in ethylene output is the stability of ACC synthase (ACS) protein, which tends to correlate positively with ethylene production. In Arabidopsis, there are nine ACS proteins that fall into three groups based on sequence homology specifically of the C-terminal region. My research focuses on Type-2 ACS proteins, which have an intermediate length C-terminal region with a putative CDPK phosphorylation site. Both application of cytokinin and mutations found in the C-terminal region of the Type-2 ACS proteins result in an increase in protein stability and an increase in ethylene synthesis. To more fully understand ethylene biosynthesis regulation, I have investigated several different aspects of ACS protein stability. The three main areas studied include the effect other phytohormones in ACS protein stability, the interaction E3 ligase components with ACS proteins and their effect on ethylene production, and identification of possible novel interactors and suppressors of ACS protein stability. The phytohormone work shows that brassinosteroid stabilizes multiple ACS proteins and that the stabilization is C-terminus-dependent. I also showed that cytokinin-induced ACS stability requires functional AHK, AHP and the type-B ARR1 and this stabilization occurs by a non-C-terminal dependent mechanism. The second aspect of my work showed that the BTB-adaptor proteins, ETO1 and EOL1 were specific to type-2 ACS in Arabidopsis, and the level of ethylene synthesis is shown to be most increased by elimination of ETO1, then EOL2 and finally EOL1. The last and most preliminary element of my reasearch is the identification of possible suppressors of an inducible myc-ACS5 overexpression transgenic plant line. These suppressors may be direct interactors and novel components in the ACS stability mechanism.

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