W.G. van Doorn

Is Petal Senescence Due to Sugar Starvation

Senescence occurs at every stage of plant development. Shriveling of the cotyledons in young plants and the seasonal recurrence of leaf yellowing are obvious examples. Similarly, after pollination, petals have fulfilled their biological role and become senescent. Despite this ubiquity, however, the molecular events that initiate senescence have thus far remained a mystery. Thimann et al. (1977) hypothesized that sugar starvation is the direct cause of leaf senescence. Later work with Arabidopsis plants, grown in the light, showed that leaves exhibit reduced expression of photosynthetic genes, after a fixed time span. The decrease in photosynthesis was followed by expression of senescence-associated genes, apparently induced by sugar starvation. If Arabidopsis leaves were held in darkness, the ensuing low carbohydrate levels also induced expression of senescenceassociated genes (Hensel et al., 1993; Quirino et al., 2000; Lam et al., 2001). However, other arguments may favor the opposite hypothesis: An increase rather than a decrease in sugar levels induces leaf senescence. Arabidopsis and tomato (Lycopersicon esculentum) plants in which hexokinase (which acts as a sugar sensor) was overexpressed, exhibited accelerated leaf senescence, and transgenic Arabidopsis plants expressing antisense hexokinase showed delayed senescence. Additionally, sugar levels were highest in tobacco (Nicotiana tabacum) leaves that were about to senesce, compared with younger and older leaves on the same plant, and sugar treatment hastened senescence of tobacco leaf discs (Masclaux et al., 2000; Yoshida, 2003). Petal senescence may also be due to sugar starvation or sugar accumulation. Sugar starvation may be involved because application of sugars to cut flowers generally delays visible senescence. A role for sugar starvation is also suggested by the similarities between starvation-induced changes in cell physiology and those observed before cell death during senescence. However, sugar concentrations are still high when petals show the first visible senescence symptoms. What then is the role of sugars, if any, in petal senescence? Three alternative models about the cause of petal senescence are shown in Figure 1. Degradation of polysaccharides, proteins, lipids, and nucleic acids results in mobilization of sugars and nitrogenous compounds, before visible senescence. These mobile molecules are transported, through the phloem, to other plant parts. Mobilization is common to all three models. There are at least three conceivable signals for mobilization: maturation, starvation, and sugar accumulation. According to the standard model (Fig. 1A), the maturation and starvation signals act independently. According to Thimann’s model (Fig. 1B), the maturation signal results in starvation, and starvation causes expression of genes involved in mobilization. Finally, if sugar accumulation were a signal for senescence (Fig. 1C), it may also act on genes that induce mobilization. A maturation signal may precede sugar accumulation. Advanced senescence symptoms are accompanied by cell death. At least two types of cell death are described in plants. The first, of which the hypersensitive response to invading micro-organisms is an example, is limited to a relatively low number of cells and exhibits a short time between external stimulus and death. In the hypersensitive response, rapid cell death is required as the dead cells pose a barrier to the intruding organism. The second type, of which leaf and petal senescence are examples, is characterized by export of valuable materials and takes considerably more time. This Update article discusses how mobilization and cell death are related and examines the possible role of sugars as a signal for petal cell death. It will emphasize the close similarities between starvationinduced changes in cell physiology and those observed upon senescence. It will nonetheless be concluded that there is, at present, no good reason to accept the hypothesis for either sugar starvation or sugar accumulation as a general signal for petal cell death, in flowers of intact plants. The discussion is mostly relevant to petal senescence but also draws some parallels with leaf senescence and may have a bearing to senescence-related cell death in general.

Gene expression during anthesis and senescence in Iris flowers.

We investigated changes in gene expression in Irishollandicaflowers by microarray technology. Flag tepals were sampled daily, from three days prior to flower opening to the onset of visible senescence symptoms. Gene expression profiles were compared with biochemical data including lipid and protein degradation and DNA coiling, and with morphological data. Plasmodesmata of mesophyll cells closed about two days before flower opening, while in the epidermis they closed concomitant with opening. Similarly, the onset of visible senescence in the epidermis cells occurred about two days later than in the mesophyll. About 1400 PCR-amplified clones, derived from a subtractive cDNA library enriched for tepal-specific genes, were spotted and about 240 clones, including 200 that were expressed most differentially, were sequenced. The expression patterns showed three main clusters. One exhibited high expression during tepal growth (cluster A). These genes were putatively associated with pigmentation, cell wall synthesis and metabolism of lipids and proteins. The second cluster (B) was highly expressed during flower opening. The third cluster (C) related to the final stages of senescence, with genes putatively involved in signal transduction, and the remobilization of phospholipids, proteins, and cell wall compounds. Throughout the sampling period, numerous plant defence genes were highly expressed. We identified an ion channel protein putatively involved in senescence, and some putative regulators of transcription and translation, including a MADS-domain factor.

Evidence for a wounding-induced xylem occlusion in stems of cut chrysanthemum flowers

A temperature-dependent xylem occlusion was found in cut chrysanthemum stems (Dendranthema grandiflora, cv. Viking) which were placed for 24 h in air at 5°C prior to vase life evaluation. The response was inhibited by a 5-h treatment, prior to placement in air, with aqueous solutions at low initial pH or solutions containing near-neutral antioxidants (n-propylgallate, phloroglucinol, butylated hydroxytoluene). Bacteria are known to occlude stems, but the occlusion was not related to bacterial counts in the stem ends. The number of cavitations in the xylem conduits, detected by ultrasonic acoustic emission, remained low during the storage treatment at high ambient relative humidity. The uptake of air into the stem ends ceased within 20 min whereas the occlusion developed only after several hours, showing that aspired air was not the sole cause. A xylem blockage was also found in stems placed in water directly after cutting. In these flowers, treatments with anti-oxidants delayed the occlusion, but did not affect the number of bacteria in the stem ends. The onset of xylem cavitation occurred after the occlusion. The results suggest that the stem forms a xylem blockage both during dry storage and in stems directly placed in water. The blockage apparently involves oxidative reactions.

Wounding-induced xylem occlusion in stems of cut chrysanthemum flowers: roles of peroxidase and cathechol oxidase

A wounding-induced xylem occlusion, resulting in severe leaf wilting, occurs in stems of cut chrysanthemum flowers (Dendranthema grandiflora), cv. Vyking. The blockage develops after about 1 h in flowers held in air at 20 °C. It is initially located in the lowermost 2 cm of the stem and upon prolonged exposure to air it is also found above 2 cm. We tested the possible role of peroxidase (EC 1.11.1.7) and phenoloxidases in the blockage. Some peroxidase inhibitors (copper ions and 3-amino-1,2,4-triazole) delayed the occlusion, and treatment with compounds that inhibit peroxidase but stimulate phenoloxidase (catechol, hydroquinone, p-phenylene diamine) had the same effect. Some inhibitors of phenoloxidase (p-nitrophenol, p-chlorophenol, p-nitrocatechol, and sodium metabisulfite) also delayed the occlusion. Phenoloxidase activity in plants comprises catechol oxidase (EC 1.10.3.1) and laccase (EC 1.10.3.2). The blockage was considerably delayed by catechol oxidase inhibitors (tropolone and 4-hexylresorcinol), even more so than with general phenoloxidase inhibitors. The results indicate that the occlusion is mainly due to a physiological (oxidative) process, requiring both peroxidase and catechol oxidase activity.

Hydrogel Regulation of Xylem Water Flow: An Alternative Hypothesis

The concentration of cations in the xylem sap influences the rate of xylem water flow in angiosperm plants. It has been speculated that this is due to the shrinking and swelling of pectins in the pit membranes. However, there is as yet minimal evidence for the presence of pectin in pit membranes of angiosperms. The little pectin that has been found at the pit membrane edges of some species might not be adequate to explain the swelling and shrinking phenomena. The presence of hemicelluloses is also not certain. Lignin, by contrast, seems to be sometimes present, apart from cellulose, which is the main component. An alternative hypothesis is formulated, which involves the shrinking of any polyelectrolyte polymers in the pit membrane and a change in volume of the mobile phase in the pit pores. These phenomena are the result of electrostatic events. Some pit membrane polymers are negatively charged because of proton dissociation from functional groups. This charge is compensated by cations in the aqueous phase, which form a diffuse double layer (DDL). Inside the pit pores, an increase of the electrolyte concentration in the xylem sap will reduce the extent of the DDL. This will result in an increase in water flow. Additional flow enhancement, upon increase of the cation concentration, can be due to shrinkage of all membrane polymers. This contraction will also lead to an increase of the pit pore diameter. These processes will only be partly counteracted by forces that decrease the diameter of the pit pore due to relaxation.

Quality loss in packed rose flowers due to Botrytis cinerea infection as related to temperature regimes and packaging design

Abstract The effects of package design and temperature treatment (cooling and rewarming) on the quality of rose flowers (cv. Sweet Promise) packed in five types of boxes were investigated, with special regard to fungus ( Botrytis cinerea ) infection. A significant increase of B. cinerea spotting was observed on flowers which had experienced both cooling and slow rewarming in the box. The infection rate of rewarmed flowers ranged from 42 up to 95%, whereas the infection rate of untreated flowers was 30%. The flowers, which were taken out of the box directly after the cooling period and immediately placed in vases, in most cases had the same level of spotting as the untreated flowers. A significant effect of package design (size and location of ventilation holes) on the proportion of flowers with spotting was found. Boxes with large ventilation holes and effective air ventilation around the buds improved the dispersion of the condensed water on the packed flowers and thus lowered the chance of germination of spores. This resulted in 42% infected flowers, whereas a commercially used box resulted in 62% infected flowers, which is significantly different from the infection shown by the untreated flowers. These results indicate that package design can be used as a practical tool for controlling fungus infection.

Effect of surface coating on ripening and early peel spotting in ‘Sucrier’ banana (Musa acuminata)

Abstract ‘Sucrier’ bananas (Musa acuminata, AA Group) show peel spotting when the peel is just about as yellow as green, which coincides with optimum eating quality. As consumers might relate the spotting to overripe fruit, early spotting is considered undesirable, especially for export markets. Fruit were left uncoated (controls) or coated with polyethylene parafilm wax at concentrations of 20%, 25%, and 30% (v/v) and then held at 29–30°C for 5 days. Compared with controls, each of the concentrations delayed early peel spotting. Eating quality was not affected by the 20% coating, but was negatively affected by the higher concentrations. Further tests, using 20% coating, showed that the delay of peel spotting was not associated with a change in peel total free phenolics or with polyphenol oxidase (PPO) activity, but it was accompanied by reduced in vitro phenylalanine ammonia lyase (PAL) activity. Results suggest that the delay in peel spotting, after surface coating, is a result, at least in part, of reduced PAL activity. Low rate of oxygen diffusion through the coating might be the factor that limits the last step to blackening.