The RING-type E3 ligase RIE1 sustains leaf longevity by specifically targeting AtACS7 to fine-tune ethylene production in <i>Arabidopsis</i>

Ethylene is widely recognized as a positive regulator of leaf senescence. However, how plants coordinate the biosynthesis of ethylene to meet the requirements of senescence progression has not been determined. The rate-limiting enzyme in the ethylene biosynthesis pathway is ACC synthase. AtACS7 was previously considered one of the major contributors to the synthesis of "senescence ethylene" in <i>Arabidopsis</i>. However, the "brake signal" that fine-tunes the expression of AtACS7 to ensure optimal ethylene production during leaf development has yet to be identified. In the present study, the RING-H2 zinc-finger protein RIE1 was found to specifically interact with and ubiquitinate AtACS7, among all functional ACSs in <i>Arabidopsis</i>, to promote its degradation. Overexpression of <i>RIE1</i> markedly decreased ethylene biosynthesis and delayed leaf senescence, whereas loss of function of <i>RIE1</i> significantly increased ethylene emission and accelerated leaf senescence. The ethylene-related phenotypes of <i>RIE1</i> overexpressing or knockout mutants were effectively rescued by the ethylene precursor ACC or the competitive inhibitor of ACS, respectively. In particular, AtACS7-induced precocious leaf senescence was strongly enhanced by the loss of <i>RIE1</i> but was significantly attenuated by the overexpression of <i>RIE1</i>. The specific regions of interaction between AtACS7 and RIE1, as well as the major ubiquitination sites of AtACS7, were further investigated. All results demonstrated that RIE1 functions as an important modulator of ethylene biosynthesis during leaf development by specifically targeting AtACS7 for degradation, thereby enabling plants to produce the optimal levels of ethylene needed.