NaTrxh is an essential protein for pollen rejection in Nicotiana by increasing S-RNase activity

In self-incompatible Solanaceae, the pistil protein S-RNase contributes toS-specific pollen rejection in conspecific crosses, as well as to rejecting pollen from foreign species or whole clades. However, S-RNase alone is not sufficient for either type of pollen rejection. We describe a thioredoxin (Trx) type h fromNicotiana alata, NaTrxh, which interacts with and reduces S-RNasein vitro. Here, we show that expressing a redox-inactive mutant, NaTrxh(SS), suppresses bothS-specific pollen rejection and rejection of pollen fromNicotiana plumbaginifolia. Biochemical experiments provide evidence that NaTrxh specifically reduces the Cys(155)-Cys(185)disulphide bond ofS(C10)-Rnase, resulting in a significant increase of its ribonuclease activity. This reduction and increase in S-RNase activity by NaTrxh helps to explain why S-RNase alone could be insufficient for pollen rejection.