Inhibition of ethylene biosynthesis in carnation petals by cytokinin

The RhLOL1-RhILR3 module mediates cytokinin-induced petal abscission in rose

In many plant species, petal abscission can be considered the final step of petal senescence. Cytokinins (CKs) are powerful suppressors of petal senescence; however, their role in petal abscission is ambiguous. Here, we observed that, in rose (Rosa hybrida), biologically active CK is accumulated during petal abscission and acts as an accelerator of the abscission process. Using a combination of reverse genetics, and molecular and biochemical techniques, we explored the roles of a LESION SIMULATING DISEASE1 (LSD1) family member RhLOL1 interacting with a bHLH transcription factor RhILR3 in CK-induced petal abscission. Silencing RhLOL1 delays rose petal abscission, while the overexpression of its ortholog SlLOL1 in tomato (Solanum lycopersicum) promotes pedicel abscission, indicating the conserved function of LOL1 in activating plant floral organ abscission. In addition, we identify a bHLH transcription factor, RhILR3, that interacts with RhLOL1. We show that RhILR3 binds to the promoters of the auxin signaling repressor auxin/indole-3-acetic acid (Aux/IAA) genes to inhibit their expression; however, the interaction of RhLOL1 with RhILR3 activates the expression of the Aux/IAA genes including RhIAA4-1. Silencing RhIAA4-1 delays rose petal abscission. Our results thus reveal a RhLOL1-RhILR3 regulatory module involved in CK-induced petal abscission via the regulation of the expression of the Aux/IAA genes.

Silencing of FOREVER YOUNG FLOWER-Like Genes from Phalaenopsis Orchids Promotes Flower Senescence and Abscission

Ectopic expression of FOREVER YOUNG FLOWER (FYF) delays floral senescence and abscission in transgenic Arabidopsis. To analyze the FYF function in Phalaenopsis orchids, two FYF-like genes (PaFYF1/2) were identified. PaFYF1/2 were highly expressed in young Phalaenopsis flowers, and their expression decreased significantly afterward until flower senescence. This pattern was strongly correlated with the process of flower senescence and revealed that PaFYF1/2 function to suppress senescence/abscission during early flower development. Interestingly, in flowers, PaFYF1 was consistently expressed less in petals than in lips/sepals, whereas PaFYF2 was expressed relatively evenly in all flower organs. This difference suggests a regulatory modification of the functions of PaFYF1 and PaFYF2 during Phalaenopsis flower evolution. Delayed flower senescence and abscission, which were unaffected by ethylene treatment, were observed in 35S::PaFYF1/2 and 35S::PaFYF1/2 + SRDX transgenic Arabidopsis plants due to the downregulation of the ethylene signaling and abscission-associated genes EDF1-4, IDA and BOP1/2. These results suggest a possible repressor role for Phalaenopsis PaFYF1/2 in controlling floral senescence/abscission by suppressing ethylene signaling and abscission-associated genes. To further validate the function of PaFYF1/2, PaFYF1/2-VIGS (virus-induced gene silencing) Phalaenopsis were generated and analyzed. Promotion of senescence and abscission was observed in PaFYF1/2-VIGS Phalaenopsis flowers by the upregulation of PeEDF1/2, PeSAG39 and PeBOP1/2 expression, the early occurrence of greening according to their increased chlorophyll content and the reduction in water content in flower organs. Our results support that PaFYF1/2 function as transcriptional repressors to prohibit flower senescence and abscission in Phalaenopsis.

RhERF113 Functions in Ethylene-Induced Petal Senescence by Modulating Cytokinin Content in Rose

In rose (Rosa hybrida), flower senescence is accelerated by ethylene and delayed by cytokinins (CTKs). However, the effectors that regulate these processes are not currently understood. In this study, we identified an APETALA2/ethylene-responsive factor (AP2/ERF) gene, RhERF113, which was induced by ethylene and up-regulated during flower senescence in most floral organs, including sepal, petal, stamen and pistil. The virus-induced gene silencing (VIGS) of RhERF113 expression accelerated rose flower senescence, which was accompanied by a lower CTK content in the flowers. This accelerated senescence could be restored by exogenous CTK treatment. Moreover, the expression levels of genes related to CTK biosynthesis and signaling, including ISOPENTENYL TRANSFERASE 5 (RhIPT5), RhIPT8, HISTIDINE KINASE 2 (RhHK2), RhHK3, CYTOKININ RESPONSE REGULATOR 3 (RhCRR3), RhCRR5, RhCRR8, HOMEOBOX PROTEIN 6 (RhHB6) and PATHOGENESIS-RELATED 10.1 (RhPR10.1), were decreased in the RhERF113-silenced rose flowers. Taken together, our results demonstrate that RhERF113 delays ethylene-induced flower senescence by increasing the CTK content of the floral tissues.

Adenine type and diphenyl urea derived cytokinins improve the postharvest performance of Iris germanica L. cut scapes

An experiment was designed to evaluate the effect of various adenine derived cytokinins (kinetin and 6-benzylaminopurine) and diphenyl urea cytokinin (thidiazuron) on the postharvest performance of cut scapes of Iris germanica. Flower scapes were harvested with the oldest bud at '1 day before anthesis stage', brought to laboratory under water, cut to a uniform length of 35 cm, divided into three sets viz., kinetin (KIN), 6-benzyl aminopurine (BAP) and thidiazuron (TDZ). Each set of scapes was treated with a particular cytokinin alone or in combination with 0.1 M sucrose. TDZ was effective than KIN and BAP in improving the postharvest life of the I. germanica scapes by 5.4 days as compared to the control (untreated scapes held in distilled water). This was because of the minimum percentage of bud abortion by TDZ application. Cytokinin application resulted in increased antioxidant activity, higher protein and phenolic content, besides a decrease in specific protease activity and α-amino acids in the tepal tissues. Application of TDZ resulted in the maximum increase in the superoxide dismutase, catalase and ascorbate peroxidase activity in the tepal tissues. The scapes treated with BAP and KIN maintained higher carbohydrate content in the tissue samples as compared to control and TDZ treated scapes. TDZ and BAP application resulted in increased membrane stability because of the decreased lipoxygenase activity which prevented membrane lipid peroxidation. Among the cytokinins tested, TDZ proved to be the promising cytokinin in improving the postharvest performance of beautiful flowers of I. germanica scapes.

Petal senescence: a hormone view

Flowers are highly complex organs that have evolved to enhance the reproductive success of angiosperms. As a key component of flowers, petals play a vital role in attracting pollinators and ensuring successful pollination. Having fulfilled this function, petals senesce through a process that involves many physiological and biochemical changes that also occur during leaf senescence. However, petal senescence is distinct, due to the abundance of secondary metabolites in petals and the fact that petal senescence is irreversible. Various phytohormones are involved in regulating petal senescence, and are thought to act both synergistically and antagonistically. In this regard, there appears to be developmental point during which such regulatory signals are sensed and senescence is initiated. Here, we review current understanding of petal senescence, and discuss associated regulatory mechanisms involving hormone interactions and epigenetic regulation.

An Ethylene-Induced Regulatory Module Delays Flower Senescence by Regulating Cytokinin Content

In many plant species, including rose (Rosa hybrida), flower senescence is promoted by the gaseous hormone ethylene and inhibited by the cytokinin (CTK) class of hormones. However, the molecular mechanisms underlying these antagonistic effects are not well understood. In this study, we characterized the association between a pathogenesis-related PR-10 family gene from rose (RhPR10.1) and the hormonal regulation of flower senescence. Quantitative reverse transcription PCR analysis showed that RhPR10.1 was expressed at high levels during senescence in different floral organs, including petal, sepal, receptacle, stamen, and pistil, and that expression was induced by ethylene treatment. Silencing of RhPR10.1 expression in rose plants by virus-induced gene silencing accelerated flower senescence, which was accompanied by a higher ion leakage rate in the petals, as well as increased expression of the senescence marker gene RhSAG12 CTK content and the expression of three CTK signaling pathway genes were reduced in RhPR10.1-silenced plants, and the accelerated rate of petal senescence that was apparent in the RhPR10.1-silenced plants was restored to normal levels by CTK treatment. Finally, RhHB6, a homeodomain-Leu zipper I transcription factor, was observed to bind to the RhPR10.1 promoter, and silencing of its expression also promoted flower senescence. Our results reveal an ethylene-induced RhHB6-RhPR10.1 regulatory module that functions as a brake of ethylene-promoted senescence through increasing the CTK content.

Interaction of plant growth regulators and reactive oxygen species to regulate petal senescence in wallflowers (Erysimum linifolium)

BACKGROUND

In many species floral senescence is coordinated by ethylene. Endogenous levels rise, and exogenous application accelerates senescence. Furthermore, floral senescence is often associated with increased reactive oxygen species, and is delayed by exogenously applied cytokinin. However, how these processes are linked remains largely unresolved. Erysimum linifolium (wallflower) provides an excellent model for understanding these interactions due to its easily staged flowers and close taxonomic relationship to Arabidopsis. This has facilitated microarray analysis of gene expression during petal senescence and provided gene markers for following the effects of treatments on different regulatory pathways.

RESULTS

In detached Erysimum linifolium (wallflower) flowers ethylene production peaks in open flowers. Furthermore senescence is delayed by treatments with the ethylene signalling inhibitor silver thiosulphate, and accelerated with ethylene released by 2-chloroethylphosphonic acid. Both treatments with exogenous cytokinin, or 6-methyl purine (which is an inhibitor of cytokinin oxidase), delay petal senescence. However, treatment with cytokinin also increases ethylene biosynthesis. Despite the similar effects on senescence, transcript abundance of gene markers is affected differentially by the treatments. A significant rise in transcript abundance of WLS73 (a putative aminocyclopropanecarboxylate oxidase) was abolished by cytokinin or 6-methyl purine treatments. In contrast, WFSAG12 transcript (a senescence marker) continued to accumulate significantly, albeit at a reduced rate. Silver thiosulphate suppressed the increase in transcript abundance both of WFSAG12 and WLS73. Activity of reactive oxygen species scavenging enzymes changed during senescence. Treatments that increased cytokinin levels, or inhibited ethylene action, reduced accumulation of hydrogen peroxide. Furthermore, although auxin levels rose with senescence, treatments that delayed early senescence did not affect transcript abundance of WPS46, an auxin-induced gene.

CONCLUSIONS

A model for the interaction between cytokinins, ethylene, reactive oxygen species and auxin in the regulation of floral senescence in wallflowers is proposed. The combined increase in ethylene and reduction in cytokinin triggers the initiation of senescence and these two plant growth regulators directly or indirectly result in increased reactive oxygen species levels. A fall in conjugated auxin and/or the total auxin pool eventually triggers abscission.

Ethylene resistance in flowering ornamental plants - improvements and future perspectives

Various strategies of plant breeding have been attempted in order to improve the ethylene resistance of flowering ornamental plants. These approaches span from conventional techniques such as simple cross-pollination to new breeding techniques which modify the plants genetically such as precise genome-editing. The main strategies target the ethylene pathway directly; others focus on changing the ethylene pathway indirectly via pathways that are known to be antagonistic to the ethylene pathway, e.g. increasing cytokinin levels. Many of the known elements of the ethylene pathway have been addressed experimentally with the aim of modulating the overall response of the plant to ethylene. Elements of the ethylene pathway that appear particularly promising in this respect include ethylene receptors as ETR1, and transcription factors such as EIN3. Both direct and indirect approaches seem to be successful, nevertheless, although genetic transformation using recombinant DNA has the ability to save much time in the breeding process, they are not readily used by breeders yet. This is primarily due to legislative issues, economic issues, difficulties of implementing this technology in some ornamental plants, as well as how these techniques are publically perceived, particularly in Europe. Recently, newer and more precise genome-editing techniques have become available and they are already being implemented in some crops. New breeding techniques may help change the current situation and pave the way toward a legal and public acceptance if products of these technologies are indistinguishable from plants obtained by conventional techniques.

Illuminating light, cytokinin, and ethylene signalling crosstalk in plant development

Integrating important environmental signals with intrinsic developmental programmes is a crucial adaptive requirement for plant growth, survival, and reproduction. Key environmental cues include changes in several light variables, while important intrinsic (and highly interactive) regulators of many developmental processes include the phytohormones cytokinins (CKs) and ethylene. Here, we discuss the latest discoveries regarding the molecular mechanisms mediating CK/ethylene crosstalk at diverse levels of biosynthetic and metabolic pathways and their complex interactions with light. Furthermore, we summarize evidence indicating that multiple hormonal and light signals are integrated in the multistep phosphorelay (MSP) pathway, a backbone signalling pathway in plants. Inter alia, there are strong overlaps in subcellular localizations and functional similarities in components of these pathways, including receptors and various downstream agents. We highlight recent research demonstrating the importance of CK/ethylene/light crosstalk in selected aspects of plant development, particularly seed germination and early seedling development. The findings clearly demonstrate the crucial integration of plant responses to phytohormones and adaptive responses to environmental cues. Finally, we tentatively identify key future challenges to refine our understanding of the molecular mechanisms mediating crosstalk between light and hormonal signals, and their integration during plant life cycles.

From models to ornamentals: how is flower senescence regulated?

Floral senescence involves an ordered set of events coordinated at the plant, flower, organ and cellular level. This review assesses our current understanding of the input signals, signal transduction and cellular processes that regulate petal senescence and cell death. In many species a visible sign of petal senescence is wilting. This is accompanied by remobilization of nutrients from the flower to the developing ovary or to other parts of the plant. In other species, petals abscise while still turgid. Coordinating signals for floral senescence also vary across species. In some species ethylene acts as a central regulator, in others floral senescence is ethylene insensitive and other growth regulators are implicated. Due to the variability in this coordination and sequence of events across species, identifying suitable models to study petal senescence has been challenging, and the best candidates are reviewed. Transcriptomic studies provide an overview of the MAP kinases and transcription factors that are activated during petal senescence in several species including Arabidopsis. Our understanding of downstream regulators such as autophagy genes and proteases is also improving. This gives us insights into possible signalling cascades that regulate initiation of senescence and coordination of cell death processes. It also identifies the gaps in our knowledge such as the role of microRNAs. Finally future prospects for using all this information from model to non-model species to extend vase life in ornamental species is reviewed.

Overproduction of cytokinins in petunia flowers transformed with P(SAG12)-IPT delays corolla senescence and decreases sensitivity to ethylene

Plant senescence is regulated by a coordinated genetic program mediated in part by changes in ethylene, abscisic acid (ABA), and cytokinin content. Transgenic plants with delayed senescence are useful for studying interactions between these signaling mechanisms. Expression of ipt, a cytokinin biosynthetic gene from Agrobacterium tumefaciens, under the control of the promoter from a senescence-associated gene (SAG12) has been one approach used to delay senescence. We transformed petunia (Petunia x hybrida cv V26) with P(SAG12)-IPT. Two independently transformed lines with extended flower longevity (I-1-7-22 and I-3-18-34) were used to study the effects of elevated cytokinin content on ethylene synthesis and sensitivity and ABA accumulation in petunia corollas. Floral senescence in these lines was delayed 6 to 10 d relative to wild-type (WT) flowers. Ipt transcripts increased in abundance after pollination and were accompanied by increased cytokinin accumulation. Endogenous ethylene production was induced by pollination in both WT and IPT corollas, but this increase was delayed in IPT flowers. Flowers from IPT plants were less sensitive to exogenous ethylene and required longer treatment times to induce endogenous ethylene production, corolla senescence, and up-regulation of the senescence-related Cys protease phcp1. Accumulation of ABA, another hormone regulating flower senescence, was significantly greater in WT corollas, confirming that floral senescence was delayed in IPT plants. These results extend our understanding of the hormone interactions that regulate flower senescence and provide a means of increasing flower longevity.