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

Summary In self‐incompatible Solanaceae, the pistil protein S‐RNase contributes to S‐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 from Nicotiana alata, NaTrxh, which interacts with and reduces S‐RNase in vitro. Here, we show that expressing a redox‐inactive mutant, NaTrxhSS, suppresses both S‐specific pollen rejection and rejection of pollen from Nicotiana plumbaginifolia. Biochemical experiments provide evidence that NaTrxh specifically reduces the Cys155‐Cys185 disulphide bond of SC10‐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. Significance Statement S‐RNase is very well known for its role in S‐RNase‐based self‐incompatibility and in some types of interspecific pollen rejection. Through a dominant‐negative approach, we show that a thioredoxin (Trx) type h from Nicotiana alata, NaTrxh, is required for both types of pollen rejection. This is a significant advance, although what sets our new contribution apart is that we show that NaTrxh reduces an S‐RNase disulphide and increases its activity, providing a remarkable level of insight into the biochemical mechanism of S‐RNase‐dependent pollen rejection.