Stylar S-RNase induces a PCD syndrome in self-incompatibility of Pyrus pyrifolia
Shaoling Zhang﹡, Jun Wu, Shutian Tao, Juyou Wu, Chunlei Wang, Zhuqing Liu

College of Horticulture, Pear Engineering Research Centre, State Key Laboratory of Crop Genetics and Germplasm Enhancement, Nanjing Agricultural University, Nanjing, China, 210095
Corresponding Author: Tel: (+8625)84396580; Fax: (+8625)84396485; E-mail: slzhang@njau.edu.cn

Abstract
Pear has an S-RNase-based gametophytic self-incompatibility system. Self-S-RNase induced the pear pollen actin cytoskeleton disintegrated into punctate foci, which occurred prior to the arrest of pollen tube growth. The mitochondrial membrane potential (Δψ_mit) was also changed 30 min after SI challenge, indicating the collapse of mitochondrial in incompatible pollen tubes. Western blotting confirmed that the Δψ_mit collapse induced leakage of cytochrome c into the cytosol after SI challenge. Direct evidence of swollen mitochondria was detected after SI challenge by transmission electron microscopy. The degradation of nuclear DNA was observed by both 4, 6-diamidino-2-phenylindole and transferase-mediated dUTP nick-end labeling. These diagnostic features of programmed cell death (PCD) suggested that PCD may specifically occur in incompatible pollen tubes. Moreover, S-RNase specifically disrupted tip-localized ROS of incompatible pollen tubes. The mitochondria ROS disruption was related to the mitochondria alteration, and cell walls ROS disruption was related to the decrease of NADPH quantity. The downstream reaction of tip-localized ROS disruption not only inhibited the Ca²⁺ current, depolymerized the actin cytoskeleton, and it also induced nuclear DNA degradation.

Key Words: Pear; self-incompatibility; pollen tube