The immediate-early ethylene response gene OsARD1 encodes an acireductone dioxygenase involved in recycling of the ethylene precursor S-adenosylmethionine.

ABSTRACT

Methylthioadenosine (MTA) is formed as a by-product of ethylene biosynthesis from S-adenosyl-L-methionine (AdoMet). The methionine cycle regenerates AdoMet from MTA. In two independent differential screens for submergence-induced genes and for 1-aminocyclopropane-1-carboxylic acid (ACC)-induced genes from deepwater rice (Oryza sativa L.) we identified an acireductone dioxygenase (ARD). OsARD1 is a metal-binding protein that belongs to the cupin superfamily. Acireductone dioxygenases are unique proteins that can acquire two different activities depending on the metal ion bound. Ectopically expressed apo-OsARD1 preferentially binds Fe(2+) and reconstituted Fe-OsARD1 catalyzed the formation of 2-keto-pentanoate and formate from the model substrate 1,2-dihydroxy-3-ketopent-1-ene and dioxygen, indicating that OsARD1 is capable of catalyzing the penultimate step in the methionine cycle. Two highly homologous ARD genes were identified in rice. OsARD1 mRNA levels showed a rapid, early and transient increase upon submergence and after treatment with ethylene-releasing compounds. The second gene from rice, OsARD2, is constitutively expressed. Accumulation of OsARD1 transcript was observed in the same internodal tissues, i.e. the meristem and elongation zone, which were previously shown to synthesize ethylene. OsARD1 transcripts accumulated in the presence of cycloheximide, an inhibitor of protein synthesis, indicating that OsARD1 is a primary ethylene response gene. Promoter analysis suggests that immediate-early regulation of OsARD1 by ethylene may involve an EIN3-like transcription factor. OsARD1 is induced by low levels of ethylene. We propose that early feedback activation of the methionine cycle by low levels of ethylene ensures the high and continuous rates of ethylene synthesis required for long-term ethylene-mediated submergence adaptation without depleting the tissue of AdoMet.