EXPANSINS IN DEEP-WATER RICE INTERNODES

Cell walls of deepwater rice (Oryza sativa L.) internodes undergo long -term extension (creep) when placed under tension in acidic buffers. T his is indicative of the action of the cell wall-loosening protein exp ansin. Wall extension had a pH optimum of around 4.0 and was abolished by boiling. Acid-induced extension of boiled cell walls could be reco nstituted by addition of salt-extracted rice or cucumber cell wall pro teins. Cucumber expansin antibody recognized a single protein band of 24.5-kD apparent molecular mass on immunoblots of rice cell wall prote ins. Expansins were partially purified by concanavalin A affinity chro matography and sulfopropyl (SP) cation-exchange chromatography. The la tter yielded two peaks with extension activity (SP20 and SP29), and im munoblot analysis showed that both of these active fractions contained expansin of 24.5-kD molecular mass. The N-terminal amino acid sequenc e of SP20 expansin is identical to that deduced from the rice expansin cDNA Os-EXP1. The N-terminal amino acid sequence of SP29 expansin mat ches that deduced from the rice expansin cDNA Os-EXP2 in six of eight amino acids. Our results show that two expansins occur in the cell wal ls of rice internodes and that they may mediate acid-induced wall exte nsion.