Sulfide recycling in mitochondria during active ethylene biosynthesis in etiolated rice seedlings

Abstract

Sulfide recycling is needed in ripening apple to sustain cyanide detoxification under a high ethylene production rate (2-5 nmol/g/h). Cyanide detoxification is facilitated by β-cyanoalanine synthase (CAS) which coverts cysteine to β-cyanoalanine, also yielding hydrogen sulfide in the β-displacement reaction. To sustain cysteine supply for the detoxification, cysteine is regenerated by O-acetylserine sulfhydrylase (OASS) utilizing hydrogen sulfide. This couple of reactions may provide sulfide recycling in higher plants because hydrogen sulfide is not detected to be released from apple fruits during ripening. However both CAS and OASS belong to the same gene family with high sequence homology and may have overlapping catalytic functions. Rice seedlings also produce ethylene in high rate (2 nmol/g/h) and co-product cyanide is removed by the action of OsCAS localized inside mitochondria. We have confirmed that OsCAS mainly function as cyanide detoxification but not cysteine synthesis based on the CAS/OASS activity ratio and its incapability to interact with serine acetyltransferase (SAT) to form the cysteine synthase complex. Based on our search results, the rice genome contains 9 members in the CAS/OASS gene family. OsCAS seems to be the sole sequence encoding CAS and the sole sequence encoding protein predicted to be localized to mitochondria. This raises the questions of how cysteine can be synthesized in rice mitochondria; recycled from hydrogen sulfide as a result of CAS action; and whether cysteine can be imported to mitochondria from cytoplasm in rice.