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 described a thioredoxin h from Nicotiana alata, NaTrxh, which interacts with and reduces S-RNase in vitro. Here, we show that expressing a redox-inactive mutant, NaTrxh SS , suppresses both S-specific pollen rejection and rejection of pollen from N. plumbaginifolia.Biochemical experiments provide evidence that NaTrxh specifically reduces the Cys 155 -Cys 185 disulphide bond of S 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. INTRODUCTIONSince flowering plants rely on extrinsic vectors, such as insects and wind to disperse pollen, they often deploy pollen-pistil interactions to control mating success. For example, selfincompatibility (SI) mechanisms favour outcrossing by preventing self-fertilization and interspecific pollen rejection mechanisms help to avoid crosses between species.Self-incompatible species in Solanaceae, Plantaginaeae, and Rosaceae display S-RNasebased SI, which is genetically controlled by the S-locus (de Nettancourt, 1989). Self-crosses are prevented because pollen with an S-haplotype identical to the pistillate parent is
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