Pollen–pistil interaction is a basic process in the reproductive biology of flowering plants and has been the subject of intense fundamental research that has a pronounced practical value. The phytohormones ethylene (ET) and cytokinin (CK) together with other hormones such as auxin, gibberellin (GA), jasmonic acid (JA), abscisic acid (ABA), and brassinosteroids (BRs) influence different stages of plant development and growth. Here, we mainly focus on the information about the ET and CK signaling in the progamic phase of fertilization. This signaling occurs during male gametophyte development, including tapetum (TAP) cell death, and pollen tube growth, including synergid programmed cell death (PCD) and self-incompatibility (SI)-induced PCD. ET joins the coordination of successive events in the developing anther, including the TAP development and cell death, anther dehiscence, microspore development, pollen grain maturation, and dehydration. Both ET and CK take part in the regulation of pollen–pistil interaction. ET signaling accompanies adhesion, hydration, and germination of pollen grains in the stigma and growth of pollen tubes in style tissues. Thus, ET production may be implicated in the pollination signaling between organs accumulated in the stigma and transmitted to the style and ovary to ensure successful pollination. Some data suggest that ET and CK signaling are involved in S-RNase-based SI.

Programmed cell death (PCD) is relevant to many aspects in the growth and development of a plant organism. In their reproduction, many flowering plant species possess self-incompatibility (SI), that is an intraspecific reproductive barrier, which is a genetic mechanism ensuring the avoidance of inbreeding depression by preventing self-pollination. This phenomenon enhances intraspecific variation; however, SI is rather a hindrance for some fruit plant species (such as plum, cherry, and peer trees) rather than an advantage in farming. PCD is a factor of the S-RNase–based SI in Petunia hybrida E. Vilm. The growth of self-incompatible pollen tubes (PTs) is arrested with an increase in the activity of caspase-like proteases during the first hours after pollination so that all traits of PCD—plasma membrane integrity damage, DNA degradation/disintegration, and damage of PT structural organization (absence of vacuoles, turgor disturbance, and separation of cell plasma membrane from the cell wall)—are observable by the moment of PT growth arrest. We succeeded in discovering an additional cytological PCD marker, namely, the formation of ricinosomes in self-incompatible PTs at early stages of PCD. SI is removable by treating petunia stigmas with Acetyl-Asp-Glu-Val-Asp-aldehyde (Ac-DEVD-CHO), an inhibitor of caspase-3/DEVDase, 2 h before a self-incompatible pollination. In this process, the level of caspase-3-like protease activity was low, DNA degradation was absent, PTs grew to the ovary, fertilization was successful, and full-fledged seeds were formed.

Research into molecular mechanisms of self-incompatibility (SI) in plants can be observed in representatives of various families, including Solanaceae. Earlier studies of the mechanisms of S-RNase-based SI in petunia (Petunia hybrida E. Vilm.) demonstrate that programmed cell death (PCD) is an SI factor. These studies suggest that the phytohormon cytokinin (CK) is putative activator of caspase-like proteases (CLPs). In this work, data confirming this hypothesis were obtained in two model objects—petunia and tomato (six Solanaceae representatives). The exogenous zeatin treatment of tomato and petunia stigmas before a compatible pollination activates CLPs in the pollen tubes in vivo, as shown via the intravital imaging of CLP activities. CK at any concentration slows down the germination and growth of petunia and tomato male gametophytes both in vitro and in vivo; shifts the pH of the cytoplasm (PHc) to the acid region, thereby creating the optimal conditions for CLP to function and inhibiting the F-actin formation and/or destructing the cytoskeleton in pollen tubes to point foci during SI-induced PCD; and accumulates in style tissues during SI response. The activity of the ISOPENTENYLTRANSFERASE 5 (IPT5) gene at this moment exceeds its activity in a cross-compatible pollination, and the levels of expression of the CKX1 and CKX2 genes (CK OXIDASE/DEHYDROGENASE) are significantly lower in self-incompatible pollination. All this suggests that CK plays a decisive role in the mechanism underlying SI-induced PCD.

Rapid alkalinization factor (RALF) peptides represent a central class of signaling molecules in plants, regulating processes ranging from fertilization to immune responses. These diverse functions are largely mediated by a conserved receptor complex centered on FERONIA kinase (FER). This review summarizes recent advances positioning the RALF-FER signaling pathway as a major regulatory hub integrating intrinsic and extrinsic signals to coordinate growth, development, and stress adaptation. We examine how this pathway controls the polar growth of root hairs and pollen tubes, orchestrates reproductive barriers and fertilization, and modulates immune and abiotic stress signaling through mechanisms involving ROS, Ca²⁺, and apoplast pH. By framing this new knowledge within the broader framework of known RALF-FER mechanisms, we demonstrate how this pathway achieves high signaling specificity. Finally, we discuss critical unresolved issues and suggest future research directions in the emerging field of molecular stress physiology, highlighting the potential for manipulating this pathway for agricultural crop improvement.

A combination of high potential productivity and ecological stability is essential for current cultivars, which is achievable by breeding. Interspecific/intergeneric hybridization remains a key approach to producing new high-yielding and resistant cultivars. Interspecific reproductive barriers (IRBs) appear in the interaction between the pollen and pistil of interspecific/intergeneric hybrids. The mechanisms underlying these hybridization barriers are to a considerable degree unknown. The pollen–pistil interaction is decisive because the pollen of distantly related plant species either is not recognized by stigma cells or is recognized as foreign, preventing pollen tube (PT) germination and/or penetration into the stigma/style/ovary. This review mainly focuses on (1) the pollen–pistil system; (2) IRB classification; (3) similarity and differences in the function of self-incompatibility (SI) barriers and IRBs; and (4) physiological and biochemical control of IRBs and their overcoming. The main goal is to illuminate the physiological, biochemical, and molecular mechanisms underlying the growth arrest of incompatible PTs and their death. In general, this review consolidates the current understanding of the interaction of the male gametophyte with the sporophyte tissues of the pistil and outlines future research directions in the area of plant reproductive biology.