Copper and zinc ions induce both stylar RNase inhibition and fruit set in Japanese pear

• Stylar RNase activity of ‘Kosui’ pear was inhibited by copper and zinc ions.
• Copper ion induced fruit set more than 30% following self-pollination.
• Copper ion may cause fruit set through reducing stylar RNase activity.
• These cations seem not to bind strongly with stylar RNase.
• Copper ion may change the stereostructure of RNase protein.

When inhibitory effects on stylar RNase activity were investigated in the Japanese pear (Pyrus pyrifolia Nakai), CuSO4 and ZnSO4 were significantly effective, and the inhibition was dose-dependent. Since MgSO4 and CaCl2 did not show any inhibition, the active ions would be Cu2+ and Zn2+. Because Cu2+ separated clearly from the stylar proteins when protein–CuSO4 mixture was loaded on Sephadex G-10 column chromatography, the cation may not be combined strongly with the protein. The RNase activity of the protein, furthermore, was almost completely recovered when the proteins were isolated from the mixture by (NH4)2SO4 precipitation. Copper ions strongly reduced the RNase A activity, moderately RNase S and weakly RNase T1 at 1 mM, while it did not affect the RNase B. The CuSO4 application at 2 mM induced more than 30% fruit set following self-pollination but Na2SO4 and K2SO4 did not, suggests that Cu2+ causes fruit set through reducing stylar RNase activity.