Spines and thorns deter most animals, except for mammals with thick fur; some birds have specialized beaks to get past such defenses. Plant defenses from herbivory : a Spines and b thorns are examples of plant defenses. Herbivory has been used by seed plants for their own benefit in a display of mutualistic relationships.
The dispersal of fruit by animals is the most striking example. An extreme example of collaboration between an animal and a plant is the case of acacia trees and ants.
The trees support the insects with shelter and food. In return, ants discourage herbivores, both invertebrates and vertebrates, by stinging and attacking leaf-eating organisms. Grasses are a successful group of flowering plants that are wind pollinated. They produce large amounts of powdery pollen carried over large distances by the wind. The flowers are small and wisp-like. Large trees such as oaks, maples, and birches are also wind pollinated.
More than 80 percent of angiosperms depend on animals for pollination: the transfer of pollen from the anther to the stigma. Consequently, plants have developed many adaptations to attract pollinators.
The specificity of specialized plant structures that target animals can be very surprising. Many bird or insect-pollinated flowers secrete nectar, a sugary liquid. They also produce both fertile pollen for reproduction and sterile pollen rich in nutrients for birds and insects. Butterflies and bees can detect ultraviolet light. Large, red flowers with little smell and a long funnel shape are preferred by hummingbirds who have good color perception, a poor sense of smell, and need a strong perch.
White flowers that open at night attract moths. Other animals such as bats, lemurs, and lizards can also act as pollinating agents. Any disruption to these interactions, such as the disappearance of bees as a consequence of colony collapse disorders, can lead to disaster for agricultural industries that depend heavily on pollinated crops. Animal-aided pollination : As a bee collects nectar from a flower, it is dusted by pollen, which it then disperses to other flowers.
Human life has become dependent on plants for the qualities and developments that they provide, which include medicine and food production. Seed plants are cultivated for their beauty and smells, as well as their importance in the development of medicines. Plants are also the foundation of human diets across the world. Many societies eat, almost exclusively, vegetarian fare and depend solely on seed plants for their nutritional needs.
A few crops rice, wheat, and potatoes dominate the agricultural landscape. Many crops were developed during the agricultural revolution when human societies made the transition from nomadic hunter—gatherers to horticulture and agriculture. Cereals, rich in carbohydrates, provide the staple of many human diets. In addition, beans and nuts supply proteins. Fats are derived from crushed seeds, as is the case for peanut and rapeseed canola oils, or fruits such as olives.
Recent studies have also revealed that plasmodesmata and the phloem sieve tube system function cooperatively to mediate the long-distance delivery of proteins and a diverse array of RNA species. Delivery of these information macromolecules is discussed in terms of their roles in control over the vegetative-to-floral transition, tuberization in potato, stress-related signalling involving miRNAs, and genetic reprogramming through the delivery of nucleotide small RNAs that function in transcriptional gene silencing in recipient sink organs.
Finally, we discuss important future research areas that could contribute to developing agricultural crops with engineered performance characteristics for enhance yield potential.
The economic value of extreme deviations, such as losing all pollination services, cannot be soundly estimated Heal, If calculable, the economic value of keeping pollination services would be very high, similar to their intrinsic value.
California until fig wasps were imported in the s for pollination and growers learned to identify the proper species for pollination and determine overwintering requirements to synchronize wasp life cycles with the plants McGregor, ; Swingle, Although pollinators are in most cases managed for crop pollination, there are examples of pollinator management to achieve other goals. Hobby beekeepers often keep bees primarily for honey production or for personal satisfaction rather than for pollination.
Honey bees have been recruited. Bees are used to deliver Bacillus subtilis to blueberry flowers to suppress Monilinia vaccinicorymbosi , or mummy berry disease, a devastating fungus Dedej et al.
They also have been used to deliver Trichoderma harzianum , a commercially produced agent for control of the pathogenic fungus Botrytis cinerea on strawberries Kovach et al. There is a continuing effort to investigate the potential of honey bees as biological monitors for environmental contaminants Bromenshenk et al.
In addition to active management, pollinators can also be managed passively—that is, their activities can be manipulated by altering environmental conditions to promote their diversity and population growth or to influence particular behavior Shepherd et al.
Passive management includes farming to promote the growth of floral resources, providing artificial nest materials and nest sites, and protecting nesting habitat.
All of these practices are designed to increase the diversity of the pollinator community and the abundances of particular species Kevan et al. Pollination as a biotic process has both commercial and ecological value. In the context of agriculture, pollination provides a wide range of benefits to a broad diversity of commodities across North America.
In some cases, production of the commodity itself results directly from the act of pollination for example, fruit production. In other cases, although pollination does not result in production of the commodity itself, the process contributes to crop propagation for example, production of seeds used to grow a root crop such as carrots or quality for example, size of tomatoes has been linked to repeated pollination.
There are indirect benefits as well, through food-chain relationships. Although these indirect effects tend to exaggerate the economic value of pollination, they have been used in several widely cited studies see Table The annual value of honey bee pollination to U. Southwick and Southwick, b.
The lowest U. Table compares studies that include estimates of such willingness to pay for pollination services and it provides a breakdown of total reported values as direct benefits to crops, indirect benefits to crops, and indirect benefits to livestock. Values reported by Morse and Calderone and by Levin include indirect benefits of the honey bee pollination required for seed production in alfalfa hay, asparagus, broccoli, carrot, cauliflower, celery, onion, and sugar beet.
Levin included Attributing the full market value of such indirect effects to pollination exaggerates the economic value of pollination services, because indirect products like alfalfa hay or cattle require many production inputs besides alfalfa seed.
Even the alfalfa seed made possible by pollination requires that farmers provide other costly production inputs. These and other limitations of estimating economic values are discussed in Box Given the estimates currently available, consistent comparisons can be made across those economic values based on the direct effects of pollinators. The contributions of A. Some vertebrates also operate as pollinators of ecologically and economically important plants.
Tropical trees of the family Bombacaceae, which includes species used for timber, silk cotton, balsa wood, and other products, rely primarily on bats for pollination Bawa, ; Watson and Dallwitz, Many columnar cacti and agaves, which are important sources of alcoholic beverages tequila, mescal and other products sisal fibers , also depend on bats and birds for pollination Arizaga and Ezcurra, ; Arizaga et al. Globally, pollinators are fundamentally important for the production of roughly 30 percent of the human diet and most fibers cotton and flax , edible oils, alcoholic beverages, nutraceuticals, and medicines created from plants Buchmann and Nabhan, ; McGregor, ; Roubik, Estimating the value of pollinators and pollination in natural ecosystems and predicting the consequences of their losses are considerably more challenging than estimating their economic value in agriculture.
Such estimates are complicated by both the number of species involved globally, more than , and the relative paucity of information available for most of those species. For example, in their effort to calculate the economic value of ecological services provided by insects, Losey and Vaughan did not attempt to place a dollar value on the contributions of pollinators to maintenance of natural plant communities, although it is reasonable to assume that a significant proportion of plants in uncultivated terrestrial communities rely upon pollinators.
These plants, in turn, contribute to many ecosystem services of value to humans, such as water filtration, carbon sequestration, and flood and erosion control Daily et al. An added complication is that insect pollinators may contribute ecosystem services other than pollination in their larval stages.
The value of these services is. Dobson et al. In this system, pollination is considered a Type C or E service for most ecosystems, with species losses having significant impacts on trophic stability. Indeed, pollination is the only mutualistic association singled out by Dobson et al. The study of pollinator-plant interactions is a thriving, albeit small, area of inquiry. There are no professional societies dedicated to this pursuit, and publications in the field appear in a wide range of journals.
In Canada, pollination biology courses have been recently offered at the University of Guelph, Ontario, and at the University of Manitoba. Many pollination biologists, conservationists, and land managers benefit from an annual day class the Bee Course, now in its ninth year,. Concerns about the status of pollinators in North America over the last quarter-century have arisen in two different contexts.
The agricultural community has voiced concerns over fluctuations in the health and availability of A. Although the U. Although A. However, the actual contribution of this. Today, the primary concern among beekeepers is the varroa mite, which continues to cause major losses of managed hives Caron and Hubner, ; Finley et al. Infection with V. The high rate of mortality is the combined result of several factors, including low levels of natural resistance to mites in the honey bee population; inadequate stock development and production facilities; widespread use of pesticides, which helps to maintain mite-susceptible genotypes in the honey bee population; and widespread pesticide resistance in the mite population.
Few honey bee breeding programs artificial selection have successfully consolidated low levels of existing mite resistance into strains with significant levels of mite resistance Harbo and Harris, a. Honey bees have been widely regarded as having suffered under the weight of those stresses. Also, parasitic mites had, by all accounts, an even more serious and negative effect on the population of feral honey bee colonies Hoopingarner, ; Kraus and Page, ; Loper, , , These losses occurred as demand for agricultural pollination services was increasing dramatically, particularly for crops that depend completely on pollinators.
The almond-growing business presents a compelling example Figure Over the 25 years between and , U. Approximately 1. According to forecasts of the California Almond Board cited in Sumner and Boriss, , almond growing will expand to , acres by , eventually requiring the services of 2.
Other pollinator-dependent crops include squash Figure and muskmelon cantaloupe Figure ; domestic production of these crops has nearly doubled over the past two decades. The growth in demand for crop pollination is not restricted to field crops. Although USDA does not collect data on acreage of greenhouse tomatoes, other sources suggest a recent large increase.
Total greenhouse crop. In , NASS stopped counting beekeeping operations with fewer than five colonies. The result was an estimated reduction of 0. Almost all almonds are grown in California.
Insight into the operation of such a control system has been gained through a number of studies, including those on the regulation of the plant root-to-shoot ratio.
Here, environmental inputs, such as water or nutrient phosphate [Pi], nitrogen, etc. These stress conditions generally cause a change in root architecture—notably an increase in lateral root development—with a concomitant increase in root relative to shoot growth Nibau et al.
The underlying events that control this shift in resource allocation, towards the root system, ensure that the plant can adapt effectively to the ever-changing local environments within the soil to acquire the essential nutrients and water required for growth at the whole-plant level.
In split-root experiments, in which the root system was divided to allow exposure to Pi-sufficient and Pi-deficient conditions, lateral root development and gene expression associated with Pi acquisition remained unaffected in the region of the root exposed to Pi-deficient conditions provided that the other portion of the root was bathed in Pi-sufficient media Baldwin et al.
Similar studies involving localized application of jasmonic acid to one part of a split-root system resulted in a rapid reallocation of phloem delivery to the untreated part of the root system Henkes et al. Importantly, in this study it was established that this reallocation of resources was not directly related to a change in sink strength in the untreated portion of the root.
Results of this nature clearly reveal the presence of a signalling network, involving both the xylem and the phloem that can exert long-distance control over developmental and physiological processes. These findings support the hypothesis that a hierarchical control network operates within the body of the plant to regulate resource allocation between competing sinks Fig. The phloem translocation stream contains sugars, amino acids, mineral nutrients, hormones, and unique populations of proteins and small interfering si RNA, micro mi RNA, and mRNA Bostwick et al.
As phloem long-distance delivery of both proteins and mRNA has been well demonstrated Golecki et al. In this regard, it is noteworthy that grafting studies with Arabidopsis demonstrated that IAA18 and IAA28 transcripts, synthesized in the phloem of mature source leaves, are delivered via the phloem into the root system where they influence root architecture by negatively regulating the development of lateral roots Notaguchi et al. It will be interesting to determine whether environmental inputs control transcription of these auxin-related genes and if a regulatory system, at the level of the CC—SE PD, might control entry of the transcripts into the phloem translocation stream.
Based on split-root experiments involving Pi-deficiency treatments, it is clear that a signal s from the roots must travel through the xylem to the shoots where it then activates a response output that travels through the phloem down into the divided root system to prevent the upregulation of both lateral root development and Pi transport activity.
Recent studies have established that phloem long-distance delivery of miR plays an important role in regulating Pi transport across the root Buhtz et al. Furthermore, functional genomics studies are providing insights into the gene regulatory networks that are activated in shoot and root tissues in response to nutrient e.
Pi-deficiency stress treatment to the root system Misson et al. However, the nature of the phloem-borne signals involved in regulating root architecture in response to abiotic and biotic stresses remains as an important area for future studies. The phloem sieve tube system serves effectively as a pressure manifold, running throughout the body of the plant, in which the SE—CC, CC—phloem parenchyma, and SE—phloem parenchyma PD represent gated valves.
For those species that load photosynthate via an apoplasmic step Lucas et al. In practical terms, this means that the size exclusion limit of the cytoplasmic microchannels within these PD must be below the size of the sugars and amino acids being loaded for delivery to distant organs Fig. Plasmodesmata play a central role in controlling phloem delivery of nutrients to sink tissues.
A Plasmodesmata interconnect the cytoplasm of mesophyll, bundle sheath, and phloem parenchyma cells to that of the companion cell—sieve element SE complex. In the source region of apoplasmic loading plants, the size exclusion limit SEL of plasmodesmata connecting phloem parenchyma cells to companion cells must be downregulated to prevent the back diffusion of sucrose loaded across the plasma membrane into the companion cell—SE complex.
The same condition holds for the long-distance transport region of the phloem. However, here, at times, there is a need to adjust the plasmodesmata SEL to allow the release of sugars, either for local metabolism or for use in carbohydrate storage.
In sink regions, control over plasmodesmata permeability can act as an important regulatory site to control phloem unloading. B and C Control over plasmodesmata permeability through enzymic adjustments in the level of callose. A similar condition holds for the long-distance region of the pathway, except that here, release of some sugars and amino acids is needed to sustain metabolism within tissues such as the vascular cambium and establish starch reserves in parenchyma storage tissues.
Within the unloading regions located in the various sinks, many species use a PD symplasmic pathway for nutrient delivery into the surrounding tissues Lalonde et al. Thus, PD along the phloem can serve both as pathways for the trafficking of important information macromolecules, as described above, and as valves having the capacity to control local resource release allocation Fig. The important question is, how might a plant exert control over these PD valves?
Unfortunately, although it has been established that such movement protein-induced increases in PD size exclusion limit involve protein—protein interactions, which can be blocked by mutations engineered within the movement protein, the molecular mechanism s underlying these changes largely remain to be elucidated Benitez-Alfonso et al.
A number of endogenous proteins have also been shown to have the capacity to increase PD size exclusion limit Lucas et al. Furthermore, recent studies have identified PD-localized proteins that have the capacity to regulate PD size exclusion limit e. Another mechanism shown to alter PD size exclusion limit involves disruption to actin filaments. Treatment of cells with either cytochalasin D or profilin causes an increase in PD size exclusion limit Ding et al.
It has also been shown that the movement protein of Cucumber mosaic virus can interact with and sever actin filaments, which gives rise to an increase in PD size exclusion limit Su et al.
That NET1A, a member of a plant-specific super family of actin-binding proteins, has been localized to PD, both at the orifice and along the length of the PD plasma membrane Deeks et al. Collectively, these findings establish that plants have evolved an array of mechanisms by which to modulate PD permeability.
Studies have shown that such changes in PD, within source leaves, can influence the ability of plants to export sugars from these leaves, thereby leading to a decrease in root-to-shoot ratio Zavaliev et al. Here, it is interesting to note that the maize tdy2 mutant, which accumulates high levels of sugars and starch in specific regions of its source leaves, was recently cloned and shown to encode for a callose synthase Slewinski et al.
However, the mechanism underlying this loading defect in tdy2 has yet to be elucidated. A similar phenotype, in terms of accumulation of carbohydrates in source leaves and an associated increase in shoot biomass, has been described for transgenic plants overexpressing phloem-specific NHL26 Vilaine et al. Plasmodesmal function, including modulation of size exclusion limit, can also be regulated by a range of cellular processes and signalling molecules, including changes in redox state, reactive oxygen species ROS , etc.
Burch-Smith and Zambryski, In terms of PD function in controlling unloading of sugars and subsequent movement out into sink tissues, studies on the gat1 mutant, which encodes an allele of thioredoxin-m3 from plastids, established that restriction of GFP exit from the unloading phloem into root sink tissues was highly correlated with an increase in ROS and PD callose Benitez-Alfonso et al.
Further support for the role of ROS in regulating nutrient flux through root PD was provided by studies integrating mathematical modelling of the symplasmic pathway with fluorescence recovery after photobleaching experiments Rutschow et al. A possible link between PD permeability and plant metabolism was suggested based on genetic studies on KOBITO1 , which encodes a glycosyltransferase-like protein Kong et al.
Importantly, various mutants displayed an increase in protein movement cell to cell without any detectable changes in callose around the PD orifice. Based on phloem unloading studies, the observed change from primary to lateral root growth appeared to be accomplished through a shift in competition for resource allocation away from the primary root in favour of the lateral roots.
Potato tubers serve as an important food source, worldwide. Tubers develop from underground juvenile lateral shoots, called stolons, whose development is regulated by a short-day photoperiod, which is sensed by the aboveground vegetative tissues Fig.
Thus, although stolons are present during the long days of summer, they do not function as strong sinks until a short-day-induced signal, originating in the mature source leaves, moves through the phloem to upregulate the process of tuberization i.
Phloem delivery of tuberization signals controls potato yield. A Potato plants grown under long-day LD photoperiodic conditions do not develop tubers, whereas those grown under short-day SD conditions receive tuberization signals from the source leaves and subsequently produce tubers. These transcripts then move long distance through the phloem in the form of ribonucleoprotein complexes RNPc , which are delivered to underground stolons where they enter meristematic cells to initiate tuber formation.
The nature of the phloem-mobile tuber-inducing signal s has been the subject of intense study. Interestingly, although transcription of StBEL5 occurs in the phloem cells of mature source leaves under long-day photoperiodic conditions, mRNA entry into the long-distance sieve tube system is facilitated by a short-day photoperiod Banerjee et al. Hence, photoperiodic control over long-distance transport of StBEL5 transcripts may be achieved through short-day control over synthesis of the requisite RNA-binding protein.
Alternatively, as transgenic potato plants overexpressing StBEL5 under a leaf-specific promoter produce tubers under long-day conditions Banerjee et al.
In summary, experiments with a range of transgenic potato plant lines clearly established a direct relationship between phloem delivery of StBEL5 mRNA, its level of accumulation in stolons, and tuber yield Hannapel, Fig.
Additional phloem-mobile signalling agents can also contribute to tuber induction Jackson, Irrefutable proof for the ability of FT to serve as a phloem-mobile tuber-inducing signal was obtained from studies in which Hd3a , the FT orthologue in rice, was expressed in transgenic potato driven by a vascular-specific promoter Navarro et al.
Here, the wild-type control potato line only developed tubers under short-day conditions; however, the Hd3a:GFP transgenic lines formed tubers when grown under long-day photoperiodic conditions. Furthermore, Hd3a:GFP scions grafted onto wild-type stocks also resulted in tuber formation, when plants were grown under long-day conditions.
Finally, fluorescence associated with Hd3a-GFP was detected in the stolons of these heterografted plants. Tissue expression analysis of potato FT orthologues revealed that, for plants transferred to tuber-inducing short-day conditions, StSP6A transcript levels were first elevated in leaves and subsequently in stolons.
Confirmation that StSP6A is the potato paralogue of FT was obtained by studies in which overexpression of this gene StSP6Aox caused tuberization in plants grown under noninductive long-day conditions Navarro et al. The time-dependent appearance of StSP6A transcripts in stolons, following transition to a short-day photoperiod, was not due to transcript delivery from the source leaves. Rather, through an elegant series of experiments using grafting between wild-type, StSP6Aox , and StCOox plants, results were obtained consistent with phloem delivery of StSP6A into the stolon to cause the activation of an autoregulatory loop that upregulates StSP6A expression in the stolons to drive tuber induction Fig.
The control of tuberization by multiple phloem-mobile inputs likely underscores the acquisition of tubers as an important avenue for vegetative reproduction. The operation of these parallel signalling systems also provides an excellent example of the critical role played by the phloem in mediating between perception in the source leaves of an environmental input a change in day length for tuber induction and activation of an agriculturally important developmental programme in a distantly located plant organ the underground stolon.
Furthermore, this knowledge opens the door to an enhancement in production yield potential for potato plants being grown under a range of environmental conditions Kloosterman et al. The switch from the vegetative to reproductive state in plants is of paramount importance to agriculture, as seeds represent one of the major sources of food for both animals and humans. A central role for the phloem in exerting control over this developmental transition has long been known, largely due to pioneering grafting studies which revealed the presence of an unknown photoperiod-controlled signal, termed florigen, whose delivery to the apex caused the onset of floral induction Zeevaart, ; Lang et al.
Progress in the quest to identify the molecular nature of the phloem-mobile florigenic signal was made possible by molecular genetic studies performed with Arabidopsis.
In this plant, mutants in either CO or FT do not flower or are greatly delayed in flowering time. Interestingly, expression of both genes was detected in the phloem of leaves rather than within the plant apex Takada and Goto, ; An et al.
A similar result was obtained when CO production was restricted to the small minor veins in source leaves Ayre and Turgeon, In contrast, expression of FT under either a CC-specific source leaf promoter or a shoot apical meristem-specific promoter did result in floral induction. Florigen was deemed to be a universal signal capable of promoting flowering in all plants Zeevaart, The role of FT as this universal, graft-transmissible signal was tested by Lifschitz et al.
Grafting P 35S :SFT scions onto sft tomato stocks restored flowering and later removal of the scion returned the stock to the sft phenotype. Importantly, grafting tomato P 35S :SFT scions onto Maryland Mammoth tobacco stocks grown under long-day conditions similarly resulted in flowering in this short-day plant. Finally, transgenic Arabidopsis plants expressing SFT under a leaf-specific promoter exhibited a very early flowering phenotype.
Collectively, these findings provided strong support for the notion that FT protein, or an SFT-derived mobile signal, serves as the universal florigenic signal. Use of the CC-specific SUC2 promoter, which is active not only in source leaves but along the length of the phloem pathway Martens et al. As might have been expected by the absence of detectable FT-GFP signal in the apex proper, the capacity of this graft-transmitted signal in floral induction was weak.
Direct analysis of FT within the phloem sap was carried out on a cucurbit species, Cucurbita moschata squash that flowers only under short-day conditions Lin et al. Ectopic expression of FT or CmoFT , in mature source leaves was highly effective at floral induction of long-day-grown plants. In addition, heterografting studies demonstrated efficient transmission of the florigenic signal from day-neutral flowering C. Application of quantitative mass spectroscopy methods to phloem sap collected from these heterografted floral-induced C.
Considering that the phloem has been shown to mediate the long-distance trafficking of certain mRNA species Ruiz-Medrano et al. Consistent with these findings, transgenic plants in which artificial miRNAs that target FT mRNA are expressed in CCs have been shown to cause late flowering, but not when they were expressed in the meristem Mathieu et al.
Experiments based on viral vectors suggested that the first nucleotides of the FT RNA may act as a cis element to mediate its long-distance movement, as well as that of viral RNAs, in Nicotiana benthamiana Li et al. These observations were further investigated through the generation of a wide range of transgenic Arabidopsis plants carrying various FT permutations being driven either by the 35S or the SUC2 promoter Lu et al.
These transgenic lines were used as stocks onto which were grafted either WT or ft mutant recipient scions. As the RFP-FT protein does not move cell to cell, these results are consistent with the hypothesis that the RFP-FT transcripts detected in the scion were responsible for the observed formation of flowers on the scions.
In addition, it is unfortunate that more experiments were not carried out using the FT promoter, as concern can always be raised with results obtained employing the 35S promoter or the SUC2 promoter, due to their rather strong ubiquitous activity along the length of the phloem. To address this mode of FT movement, Yoo et al. Western blot analysis, performed on phloem sap collected from just beneath the vegetative apex of long-day-grown C.
Phloem-mediated control over flowering. B Antiflorigenic ATC signalling. In the apex, ATC interacts with FD to repress AP1 expression, thereby acting as an antiflorigenic signalling agent to preventing flowering. C FT-independent trehalosephosphate T6P signalling pathway. The linear relationship between sucrose and T6P concentrations in the apex right graft serves as an integrator between the capacity of the plant to produce photosynthate and flowering time.
Parallel floral induction and immunolocalization studies performed on these c-Myc-tagged FT mutants identified a number of amino acid residues that are essential for both cell-to-cell movement through the post-phloem zone and floral induction. A combination of microinjection and trichome rescue assays established that wild-type FT can interact with PD to mediate its trafficking cell to cell.
Furthermore, various FT mutants that were dysfunctional in moving out from the phloem into the apex were also unable to rescue trichome formation. Further evidence in support of this mode of trafficking by FT was provided by coimmunoprecipitation studies using c-Myc-FT and a C.
It will be interesting to perform mutant analysis of these putative FT-interacting proteins to test their role in FT delivery into the shoot apical meristem.
At first sight, a nonselective mode for FT entry into the sieve tube system seems inconsistent with the central role played by FT as an environmental agent that signals between mature leaves and the vegetative apex. However, along the phloem long-distance pathway the CC—SE complexes functions as a symplasmic domain in which molecular exchange with the surrounding cells is highly regulated Knoblauch and van Bel, ; Itaya et al.
It is also important to note that here also the SE conduit volume is much larger than that of the CCs. Hence, FT sequestered in CCs should have a minimal effect on the efficacy of the florigenic signalling process. Growth under long-day conditions of heterografted atc-2 knockout mutant scions and 35S:ATC transgenic stock plants resulted in a significant late-flowering phenotype in the scions.
Western analysis of protein samples extracted from atc-2 scions grafted onto either act-2 or wild-type stocks failed to detect ATC in the atc-2 : atc-2 homografts, but ATC was routinely detected in the atc-2 scions from WT : atc-2 heterografts. Of significant interest, parallel RNA analyses failed to detect ATC transcripts in the atc-2 : atc-2 homografts, but transcripts were detected in WT : atc-2 heterografts. Expression of ATC is strongly upregulated under short-day growth conditions, where flowering occurs only after a prolonged growth period.
Evidence for translation of phloem delivered ATC transcripts, within the receiving shoot apical meristem, would provide further support for this model.
Finally, as FT , TFL , and ATC are highly conserved, sequence swapping studies should provide insight into the cis- acting motif s in the ATC transcript that is presumably responsible for its observed phloem mobility. As the phloem carries sugars to the vegetative apex, a role for sugar signalling in floral induction has long been in contention Lejeune et al.
Studies on Arabidopsis grown under noninductive short-day conditions revealed the presence of an FT-independent pathway, as both ft-1 and wild-type plants flowered earlier when they were grown under a combination of short-days and high light intensity King et al. These transgenic plants had a significant reduction in the product of this enzyme, trehalosephosphate T6P , and a delay in flowering.
As sucrose levels increased in these silenced plants, this implicated a role for T6P in sugar sensing and regulation of flowering time.
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