Aminooxyacetic acid (АОА), inhibitor of 1-aminocyclopropane-1-carboxilic acid (AСС) synthesis, suppresses self-incompatibility-induced programmed cell death in self-incompatible Petunia hybrida L. pollen tubes

Special Issue "Phytohormones 2022-2023"

The hormonal system plays a decisive role in controlling plant growth and development [...].

Ac-DEVD-CHO (caspase-3/DEVDase inhibitor) suppresses self-incompatibility-induced programmed cell death in the pollen tubes of petunia (Petunia hybrida E. Vilm.)

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.

Comparative physiochemical and transcriptomic analysis reveals the influences of cross-pollination on ovary and fruit development in pummelo (Citrus maxima)

'Shuijingmiyou' pummelo (SJ), one of the most popular fruits in Yunnan province of China, is of relatively low fruit shape (FS) quality. In this study, we compared the FS promoting effects of cross pollinations using pollens from seven pummelo varieties, and found that 'Guanximiyou' pummelo (GX) cross-pollination showed the best FS promoting effects on SJ fruits by shortening its fruit neck. To explore the underlying mechanism, physiochemical and transcriptomic differences between self- and cross-pollinated SJ ovaries (SJO and GXO) were investigated. Higher salicylic acid, gibberellin and indole acetic acid contents and superoxide dismutase, peroxidase and catalase activities, and lower polyphenol oxidase activity were determined in GXO compared with SJO. Enrichment analysis of the identified 578 differentially expressed genes (123 up-regulated and 455 down-regulated) in GXO showed that genes involved in solute transport, RNA biosynthesis, phytohormone action and cell wall organization were significantly enriched. The results obtained in this study will be helpful in understanding the influences of cross-pollination on pummelo ovary and fruit development, and can provide the basis for clarifying the underlying mechanism of cross-pollination improved fruit quality.

KED gene expression in early response to wounding stress in tomato plants

The wounding-responsive KED gene, named for its coding for a lysine (K), glutamic acid (E), and aspartic acid (D)-rich protein, is widely present among land plants. However, little is known about its regulation or function. In this study, we found that transcription of the tomato (Solanum lycopersicum) KED gene, SlKED, was rapidly and transiently elevated by wounding or ethephon treatment. Compared to the wild-type plants, the CRISPR/Cas9-mediated SlKED knockout plants did not exhibit altered expression patterns for genes involved in hormone biosynthesis or stress signaling, suggesting a lack of pleiotropic effect on other stress-responsive genes. Conversely, jasmonic acid did not appear to directly regulate SlKED expression. Wounded leaves of the KED-lacking plants exhibited higher binding of Evans blue dye than the wild-type, indicating a possible role for KED in healing damaged tissues. The SlKED knockout plants showed a similar dietary effect as the wild-type on the larval growth of tobacco hornworm. But a higher frequency of larval mandible (mouth) movement was recorded during the first 2 minutes of feeding on the wounded KED-lacking SlKED knockout plants than on the wounded KED-producing wild-type plants, probably reflecting an initial differential response by the feeding larvae to the SlKED knockout plants. Our findings suggest that SlKED may be an ethylene-mediated early responder to mechanical stress in tomato, acting downstream of the wound stress response pathways. Although its possible involvement in response to other biotic and abiotic stresses is still unclear, we propose that SlKED may play a role in plant's rapid, short-term, early wounding responses, such as in cellular damage healing.

Hormonal Signaling during dPCD: Cytokinin as the Determinant of RNase-Based Self-Incompatibility in Solanaceae

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.

Hormonal Signaling in the Progamic Phase of Fertilization in Plants

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 E. 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.

Initiation and Execution of Programmed Cell Death and Regulation of Reactive Oxygen Species in Plants

Programmed cell death (PCD) plays crucial roles in plant development and defence response. Reactive oxygen species (ROS) are produced during normal plant growth, and high ROS concentrations can change the antioxidant status of cells, leading to spontaneous cell death. In addition, ROS function as signalling molecules to improve plant stress tolerance, and they induce PCD under different conditions. This review describes the mechanisms underlying plant PCD, the key functions of mitochondria and chloroplasts in PCD, and the relationship between mitochondria and chloroplasts during PCD. Additionally, the review discusses the factors that regulate PCD. Most importantly, in this review, we summarise the sites of production of ROS and discuss the roles of ROS that not only trigger multiple signalling pathways leading to PCD but also participate in the execution of PCD, highlighting the importance of ROS in PCD.

Caspase-like proteases and the phytohormone cytokinin as determinants of S-RNAse-based self-incompatibility-induced PCD in Petunia hybrida L

S-RNAse-based self-incompatibility (SI) in petunia (Petunia hybrida L.) is a self-/non-self-recognition system underlying the pistil rejection of self-pollen. Using different methods, including a TUNEL assay, we have recently shown that programmed cell death (PCD) is a factor of the SI in petunia. Here, we show that the growth of self-incompatible pollen tubes in the style tissues during 4 h after pollination is accompanied by five-sixfold increase in a caspase-like protease (CLP) activity. Exogenous cytokinin (CK) inhibits the pollen tube growth and stimulates the CLP activity in compatible pollen tubes. The actin depolymerization with latrunculin B induces a sharp drop in the CLP activity in self-incompatible pollen tubes and its increase in compatible pollen tubes. Altogether, our results suggest that a CLP is involved in the SI-induced PCD and that CK is a putative activator of the CLP. We assume that CK provokes acidification of the cytosol and thus promotes the activation of a CLP. Thus, our results suggest that CK and CLP are involved in the S-RNAse-based SI-induced PCD in petunia. Potential relations between these components in PCD signaling are discussed.

The regulatory framework of developmentally programmed cell death in floral organs: A review

Developmentally programmed cell death (dPCD) is a tightly controlled biological process. In recent years, vital roles of dPCD on regulating floral organ growth and development have been reported. It is well known that flower is an essential organ for reproduction and a turning point of plants' life cycle. Hence, uncovering the complex molecular networks which regulates dPCD processes in floral organs is utmost important. So far, our understanding of dPCD on floral organ growth and development is just starting. Herein, we summarize the important factors that involved in the tapetal degeneration, pollen tube rupture, receptive synergid cell death, nucellar degradation, and antipodal cell degradation. Meanwhile, the known factors that involved in transmitting tract formation and self-incompatibility-induced PCD were also introduced. Furthermore, the genes that associated with anther dehiscence and petal senescence and abscission were reviewed as well. The functions of various types of factors involved in floral dPCD processes are highlighted principally. The regulatory panorama described here can provide us some insights about flower-specific dPCD process.

Two Self-Incompatibility Sites Occur Simultaneously in the Same Acianthera Species (Orchidaceae, Pleurothallidinae)

In most species of Pleurothallidinae, the self-incompatibility site occurs in the stylar canal inside the column, which is typical of gametophytic self-incompatibility (GSI). However, in some species of Acianthera, incompatible pollen tubes with anomalous morphology reach the ovary, as those are obstructed in the column. We investigated if a distinct self-incompatibility (SI) system is acting on the ovary of A. johannensis, which is a species with partial self-incompatibility, contrasting with a full SI species, A. fabiobarrosii. We analyzed the morphology and development of pollen tubes in the column, ovary, and fruit using light, epifluorescence, and transmission electron microscopy. Our results show that the main reaction site in A. johannensis is in the stylar canal inside the column, which was also recorded in A. fabiobarrosii. Morphological and cytological characteristics of the pollen tubes with obstructed growth in the column indicated a process of programmed cell death in these tubes, showing a possible GSI reaction. In addition, partially self-incompatible individuals of A. johannensis exhibit a second SI site in the ovary. We suggest that this self-incompatibility site in the ovary is only an extension of GSI that acts in the column, differing from the typical late-acting self-incompatibility system recorded in other plant groups.