Caspase-Like Proteases as Contributors in the Functioning of Reproductive Barriers During Distant Hybridization in Solanaceae.

Distant hybridization is an important breeding strategy that can effectively expand the genetic pool for developing new, high-yielding, and stress-resistant cultivars. However, reproductive isolation often hinders the application of this approach. Intergeneric reproductive barriers (IRBs) manifest during pollen-pistil interactions in plants belonging to different genera, yet the mechanisms underlying these barriers remain largely unknown. In this study, we investigated pollen-pistil interactions during the progamic phase of fertilization in three Solanaceae species using several intergeneric cross combinations. Pollen of all tested species germinated on the stigma surface, grew through the stylar transmitting tissue, and ceased elongation at a distance of 300-920 µm therefrom due to the action of a reproductive barrier. We suggest that programmed cell death (PCD) is a factor contributing to IRBs in Solanaceae. The obtained data indicate that, in all cross combinations, the rejection of incompatible pollen tubes (PTs) is accompanied by increased activity of caspase-like proteases (CLPs), as confirmed by in vivo visualization of active CLPs and spectrophotometric measurement of their activity. Specifically, incompatible PT growth in stylar tissues during the first 4 h after pollination was accompanied by a five- to six-fold increase in caspase-3-like protease activity. Notably, the arrest of PT growth in distant hybridization shows striking similarity to that observed during S-RNase-based self-incompatibility (SI) in Solanaceae. We anticipate that further research will ultimately provide insights into the regulation of reproductive barriers during distant hybridization.