Michael S. Reid

Effects of 1-MCP on the vase life and ethylene response of cut flowers

Pretreatment for 6 h with low concentrations of 1-MCP (1-Methylcyclopropene, formerly designated as SIS-X), a cyclic ethylene analog, inhibits the normal wilting response of cut carnations exposed continuously to 0.4 μl·l−1 ethylene. The response to 1-MCP was a function of treatment concentration and time. Treatment with 1-MCP was as effective in inhibiting ethylene effects as treatment with the anionic silver thiosulfate complex (STS), the standard commercial treatment. Other ethylene-sensitive cut flowers responded similarly to carnations. In the presence of 1 μl·l−1 ethylene, the vase life of 1-MCP-treated flowers was up to 4 times that of the controls.

Ethylene and flower senescence

The end of the relatively short life of carnations held in air is associated with climacteric rises in ethylene production and respiration, and coordinate rises in activity of the enzymes of the ethylene biosynthetic pathway. Carnation sensescence is associated with derepression of specific genes, increased polyribosome activity, and major changes in patterns of protein synthesis. Isotopic competition assays indicate the presence in carnation petals of ethylene binding activity with the expected characteristics of the physiological ethylene receptor. Inhibition of ethylene production and/or ethylene binding (whether in selected varieties, or by treatment with chemicals) results in longer-lived carnations. Examination of other flowers shows that the carnation is not a universal paradigm for flower senescence. The response to ethylene varies widely, and in many species petal wilting occurs without any apparent involvement of ethylene.

Chalcone synthase as a reporter in virus-induced gene silencing studies of flower senescence

Agrobacterium-mediatedinfection of petunia (Petunia hybrida) plants with tobacco rattle virus (TRV) bearing fragments of Petuniagenes resulted in systemic infection and virus-induced gene silencing (VIGS) of the homologous host genes. Infection with TRV containing a phytoene desaturase (PDS) fragment resulted in reduced abundance of PDS transcripts and typical photobleaching of photosynthetic tissues. Infection with TRV containing a chalcone synthase (CHS) fragment resulted in silencing of anthocyanin production in infected flowers. The silencing phenotype ranged from scattered white spots on the normal purple background to entirely white flowers. Symptoms in the V26 cultivar were a diffuse mosaic, but infection of some purple-flowered commercial cultivars resulted in large white sectors and even entirely white flowers. Abundance of CHS transcripts in the white flowers was less than 4 of that in purple flowers on the same plant. Infection with TRV containing a tandem construct of PDS and CHS resulted in leaf photobleaching and white patterns on the flowers. Transcripts of CHSand PDSwere reduced both in leaves and in flowers confirming simultaneous silencing of both genes by the tandem construct. We tested the effects of infection with TRV containing CHS and a fragment of a petunia gene encoding for 1-aminocyclopropane-1-carboxylate oxidase (ACO4) Abundance of transcripts encoding ACO4 and ACO1 were reduced (by 5 and 20, respectively) in infected flowers. Whether the flowers were treated with ACC or pollinated, the white (silenced) flowers or flower sectors produced less ethylene and senesced later than purple (non-silenced) tissues. These results indicate the value of VIGS with tandem constructs containing CHS as reporter and a target gene as a tool for examining the function of floral-associated genes.

Up-regulation of a cysteine protease accompanies the ethylene-insensitive senescence of daylily (Hemerocallis) flowers

The flowers of daylily (Hemerocallis × hybrida cv. Cradle Song) open at midnight, start to senesce 12 h later, and are completely senescent by the following midnight. Differential screening of a cDNA library constructed from tepals of flowers showing incipient senescence revealed 25 clones that were strongly up-regulated in senescent tepals. Re-screening and interactive Southern analysis of these clones revealed 3 families of up-regulated clones. Transcripts of one clone, SEN10, were not detectable at midnight, but increased dramatically as senescence proceeded. The derived amino acid sequence of the full-length cDNA (SEN102) has strong homology with cysteine proteases that have been reported from other plant tissues. The sequence contains a secretory signal peptide and a probable prosequence upstream of the mature protein. Amino acids critical to the active site and structure of cysteine proteases are conserved, and the C-terminus of the polypeptide has a unique putative endoplasmic reticulum retention signal -RDEL.

Analysis of the expression of two thiolprotease genes from daylily (Hemerocallis spp.) during flower senescence.

A cDNA clone encoding a daylily (Hemerocallis spp.) thiolprotease (SEN11), whose expression is strongly up-regulated in flower tepal senescence, has been isolated. The amino acid sequence, deduced from the nucleotide sequence, showed highest similarity to plant thiolproteases of Vigna mungo, Phaseolus vulgaris and Hemerocallis (SEN102), and contains a putative ER retention signal that has been described in Vigna mungo. SEN102 and SEN11 transcripts were not detectable in flower buds at the opening stage, but two peaks of transcripts were seen after 9 h and 19 h, in both petals and sepals, when wilting symptoms were apparent. The pattern of protease activity migrating on a 26.3 kDa protein was similar to the SEN102 and SEN11 transcript profiles. These two genes were also expressed in stamens and leaves, but their transcripts were undetectable in carpels and rhizomes. The expression of SEN102 was lower in the senescent leaf than in the green leaf. The pattern of expression of these genes suggests their involvement in the protein hydrolysis occurring in tepals at the late senescence stage, whereas in leaves they could be involved in the constitutive protein turnover machinery. Exogenous gibberellic acid application to cut flowers increased transcripts of both genes.

Changes in 1-aminocyclopropane-1-carboxylic-acid content of cut carnation flowers in relation to their senescence

The rise in ethylene production accompanying the respiration climacteric and senescence of cut carnation flowers (Dianthus caryophyllus L. cv. White Sim) was associated with a 30-fold increase in the concentration of 1-aminocyclopropane-1-carboxylic acid (ACC) in the petals (initial content 0.3 nmol/g fresh weight). Pretreatment of the flowers with silver thiosulfate (STS) retarded flower senescence and prevented the increase in ACC concentration in the petals. An increase in ACC in the remaining flower parts, which appeared to precede the increase in the petals, was only partially prevented by the STS pretreatment. Addition of aminoxyacetic acid (2 mM) to the solution in which the flowers were kept completely inhibited accumulation of ACC in all flower parts.

Microarray Analysis of the Abscission-Related Transcriptome in the Tomato Flower Abscission Zone in Response to Auxin Depletion

The abscission process is initiated by changes in the auxin gradient across the abscission zone (AZ) and is triggered by ethylene. Although changes in gene expression have been correlated with the ethylene-mediated execution of abscission, there is almost no information on the molecular and biochemical basis of the increased AZ sensitivity to ethylene. We examined transcriptome changes in the tomato (Solanum lycopersicum ‘Shiran 1335’) flower AZ during the rapid acquisition of ethylene sensitivity following flower removal, which depletes the AZ from auxin, with or without preexposure to 1-methylcyclopropene or application of indole-3-acetic acid after flower removal. Microarray analysis using the Affymetrix Tomato GeneChip revealed changes in expression, occurring prior to and during pedicel abscission, of many genes with possible regulatory functions. They included a range of auxin- and ethylene-related transcription factors, other transcription factors and regulatory genes that are transiently induced early, 2 h after flower removal, and a set of novel AZ-specific genes. All gene expressions initiated by flower removal and leading to pedicel abscission were inhibited by indole-3-acetic acid application, while 1-methylcyclopropene pretreatment inhibited only the ethylene-induced expressions, including those induced by wound-associated ethylene signals. These results confirm our hypothesis that acquisition of ethylene sensitivity in the AZ is associated with altered expression of auxin-regulated genes resulting from auxin depletion. Our results shed light on the regulatory control of abscission at the molecular level and further expand our knowledge of auxin-ethylene cross talk during the initial controlling stages of the process.

1-MCP blocks ethylene-induced petal abscission of Pelargonium peltatum but the effect is transient

Continual exposure to 1.5 m ll 1 ethylene caused 100% petal abscission within 2 h from detached flowers of Pelargonium peltatum (L.) ‘Pink Blizzard’ (ivy geranium) harvested just after the stigmatic lobes had separated. When flowering plants were first pretreated for 2 h with 1 m ll 1 1-MCP, ethylene-induced petal abscission was completely inhibited. However, the effect was transient, since percent abscission increased with time after 1-MCP treatment. Based on percent abscission from detached flowers after a 2-h ethylene exposure, the half-life of 1-MCP activity was about 2, 3 and 6 days after 1-MCP treatment at 25, 20.7, and 12°C, respectively, and there was no evidence for a residual effect after 4 or 5 days at 25 and 20.7°C, respectively. A second application of 1-MCP renewed the inhibitory effect. Following 1-MCP treatment, the force required to separate petals from the flower declined linearly with time. The time until complete loss of the inhibitory effect was strongly temperature dependent, e.g. : 1 day at 25°C versus 3‐4 days at 12°C. The usefulness of 1-MCP as a commercial treatment to prevent petal abscission from Pelargoniums will depend on shipping and storage temperature and application frequency.

Changes in ethylene production and 1-aminocyclopropane-1-carboxylic acid content of pollinated carnation flowers

Pollination of flowers of standard carnation (Dianthus caryophyllus L. cv. White Sim) with pollen from flowers of miniature carnations (D. caryophyllus L. cv. Exquisite) caused them to wilt irreversibly within 1 to 2 days. Pollination stimulated a sequential increase in ethylene production by stigmas, ovaries, receptacles, and petals of the flowers. The ACC content of the stigmas increased rapidly in the first few hours after pollination. The possibility that subsequent production of ethylene by other parts of the flower is stimulated by translocated ACC is discussed. Ethylene production and ACC content of other parts of the flower reached their maximum 24 h after pollination. The petal tissues contributed the bulk of the ethylene productionper flower thereafter. There appears to be a qualitative difference between the enzyme in the stigmas converting ACC to ethylene and that in other parts of the flower.

Identification of genes associated with perianth senescence in Daffodil (Narcissus pseudonarcissus L. ‘Dutch Master’)

We used a PCR-based subtractive procedure to isolate genes that increased in abundance in the perianth of incipiently senescent daffodil (Narcissus pseudonarcissus L. ‘Dutch Master’) flowers. Using RNA gel blot analysis we confirmed that 18 of the transcripts were senescence-associated. A number of these have homologs in previously-studied senescing floral and leaf tissue systems (cysteine proteases, serine proteases, endonuclease, vacuole processing enzyme). Some showed homology to genes encoding proteins with transport functions (auxin efflux carrier protein, low affinity nitrate transporter), others to enzymes that may have a role in cellular signaling (RSH-like protein). A surprising number of the isolated genes either showed no homology with previously-reported sequences or were sequences with no known function. A strong association of some of the transcripts with senescence was also indicated by their enhanced accumulation in the perianth of detached daffodil flowers, which senesce earlier than attached flowers. # 2002 Elsevier Science Ireland Ltd. All rights reserved.