Plant responses to photoperiod

Photoperiod controls many developmental responses in animals, plants and even fungi. The response to photoperiod has evolved because daylength is a reliable indicator of the time of year, enabling developmental events to be scheduled to coincide with particular environmental conditions. Much progress has been made towards understanding the molecular mechanisms involved in the response to photoperiod in plants. These mechanisms include the detection of the light signal in the leaves, the entrainment of circadian rhythms, and the production of a mobile signal which is transmitted throughout the plant. Flowering, tuberization and bud set are just a few of the many different responses in plants that are under photoperiodic control. Comparison of what is known of the molecular mechanisms controlling these responses shows that, whilst common components exist, significant differences in the regulatory mechanisms have evolved between these responses

Arabidopsis SUC1 loads the phloem in suc2 mutants when expressed from the SUC2 promoter

Active loading of sucrose into phloem companion cells (CCs) is an essential process in apoplastic loaders, such as Arabidopsis or tobacco (Nicotiana sp.), and is even used by symplastic loaders such as melon (Cucumis melo) under certain stress conditions. Reduction of the amount or complete removal of the transporters catalysing this transport step results in severe developmental defects. Here we present analyses of two Arabidopsis lines, suc2-4 and suc2-5, that carry a null allele of the SUC2 gene which encodes the Arabidopsis phloem loader. These lines were complemented with constructs expressing either the Arabidopsis SUC1 or the Ustilago maydis srt1 cDNA from the SUC2 promoter. Both SUC1 and Srt1 are energy-dependent sucrose/H+ symporters and differ in specific kinetic properties from the SUC2 protein. Transgene expression was confirmed by RT-PCRs, the subcellular localization of Srt1 in planta with an Srt1-RFP fusion, and the correct CC-specific localization of the recombinant proteins by immunolocalization with anti-Srt1 and anti-SUC1 antisera. The transport capacity of Srt1 was studied in Srt1-GFP expressing Arabidopsis protoplasts. Although both proteins were found exclusively in CCs, only SUC1 complemented the developmental defects of suc2-4 and suc2-5 mutants. As SUC1 and Srt1 are well characterized, this result provides an insight into the properties that are essential for sucrose transporters to load the phloem successfully

Metabolite transport and associated sugar signalling systems underpinning source/ sink interactions

Metabolite transport between organelles, cells and source and sink tissues not only enables pathway co-ordination but it also facilitates whole plant communication, particularly in the transmission of information concerning resource availability. Carbon assimilation is co-ordinated with nitrogen assimilation to ensure that the building blocks of biomass production, amino acids and carbon skeletons, are available at the required amounts and stoichiometry, with associated transport processes making certain that these essential resources are transported from their sites of synthesis to those of utilization. Of the many possible posttranslational mechanisms that might participate in efficient co-ordination of metabolism and transport only reversible thiol-disulphide exchange mechanisms have been described in detail. Sucrose and trehalose metabolism are intertwined in the signalling hub that ensures appropriate resource allocation to drive growth and development under optimal and stress conditions, with trehalose-6-phosphate acting as an important signal for sucrose availability. The formidable suite of plant metabolite transporters provides enormous flexibility and adaptability in inter-pathway coordination and source-sink interactions. Focussing on the carbon metabolism network, we highlight the functions of different transporter families, and the important of thioredoxins in the metabolic dialogue between source and sink tissues. In addition, we address how these systems can be tailored for crop improvement

Untersuchung der physiologischen Funktion des Saccarosetransporters SUT4 in ausgewählten Solanaceen

Saccharosetransporter sind Membranproteine, die von der SUT-Genfamilie kodiert werden. In Solanaceen wurden Subfamilien: SUT1, SUT2, und SUT4 identifiziert. Die Funktion von SUT4 wurde bisher nur fragmentarisch geklärt. Mittels real time PCR wurde eine Organ- und Entwicklungs-spezifische SUT4-Expression in Wildtyp Kartoffeln (Solanum tuberosum ssp. tuberosum) der Varietät Désirée gezeigt. Die Expression von SUT4, SUT2 und SUT1 zeigt eine Tagesrhythmik. Mit Hilfe der RNA Interferenz-Technik wurden transgene Pflanzen mit einer reduzierten SUT4-Expression hergestellt. StSUT4-RNAi Kartoffeln zeigten: reduzierte Stängelelongation, frühere Blühinduktion, frühzeitige Knollenbildung, Veränderungen des Kohlenhydratprofils in den Source- und Sink-Organen, sowie in den Phloemexudaten. Eine vermehrte Zuckertranslokation von Source-zu–Sink wurde postuliert. Bestimmte Aspekte des StSUT4-RNAi Phänotyps wurden früher bereits für Pflanzen mit Veränderungen der Gibberellin-Antwort, sowie für Pflanzen mit deregulierter photoperiodischen Signaltransduktion beschrieben. In den StSUT4-RNAi Blättern wurden verringerte Transkriptmengen eines GA-Biosynthese-Enzyms (GA20ox1) und eine erhöhte PhyB-mRNA-Akkumulation festgestellt. Der Versuch der Komplementation des StSUT4-RNAi Phänotyps durch GA3-Behandlung war nicht erfolgreich. Es wurde gezeigt, dass es unter Beschattung zu einer erhöhten SUT4-mRNA-Akkumulation kommt. In den StSUT4-RNAi Pflanzen wurden Veränderungen der Expression photoperiodisch regulierter Gene beobachtet. Eine Rolle des SUT4 als ein Integrator der pflanzlichen Antwort auf Licht, Hormone und Zucker wurde postuliert.Sucrose transporters are membrane proteins, which are encoded by the SUT gene family. In Solanaceen the SUT1, SUT2, and SUT4 subfamily have been identified. So far, the function of the sucrose transporters belonging to the SUT4 subfamily is only poorly understood. The expression of SUT4 in wild type of potato, Solanum tuberosum ssp. tuberosum var. Désirée analyzed via real time PCR shows to be sink and development specific. The expression of SUT4, SUT2 und SUT1 follow a diurnal rhythm. An RNA interference approach was used for the generation of transgenic plants with reduced SUT4 expression. The transgenic potato plants show a reduced internode elongation, early flowering, early tuberization, changes of the carbohydrate content in source as well as in sink organs of these plants, together with changes in phloem efflux from leaves. Increased translocation rate of soluble sugars was postulated. Particular aspects of the StSUT4-RNAi phenotype were already described for plants with changes in the gibberellin response or changes in the photoperiodic signaling pathway. In the StSUT4-RNAi leaves was observed a reduction of the transcript level of the GA biosynthetic enzyme GA20ox1 and increased accumulation of PhyB transcripts. However, the complementation of the StSUT4-RNAi phenotype by GA3 treatment was not successful. Under shade conditions (or under far red light enrichement), the StSUT4 transcripts accumulated to higher levels. In the StSUT4-RNAi plants were observed changes in the expression of genes involved in the photoperiodic pathway. An integrative function of SUT4 in the coordination of the light signalling, the hormone signalling and the sugar signalling pathways of higher plants was postulated

Photoperiodic regulation of the sucrose transporter StSUT4 affects the expression of circadian-regulated genes and ethylene production

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Sucrose Transporter StSUT4 from Potato Affects Flowering, Tuberization, and Shade Avoidance Response1[W]

Sucrose (Suc) transporters belong to a large gene family. The physiological role of SUT1 proteins has been intensively investigated in higher plants, whereas that of SUT4 proteins is so far unknown. All three known Suc transporters from potato (Solanum tuberosum), SUT1, SUT2, and SUT4, are colocalized and their RNA levels not only follow a diurnal rhythm, but also oscillate in constant light. Here, we examined the physiological effects of transgenic potato plants on RNA interference (RNAi)-inactivated StSUT4 expression. The phenotype of StSUT4-RNAi plants includes early flowering, higher tuber production, and reduced sensitivity toward light enriched in far-red wavelength (i.e. in canopy shade). Inhibition of StSUT4 led to tuber production of the strict photoperiodic potato subsp. andigena even under noninductive long-day conditions. Accumulation of soluble sugars and Suc efflux from leaves of transgenic plants are modified in StSUT4-RNAi plants, leading to modified Suc levels in sink organs. StSUT4 expression of wild-type plants is induced by gibberellins and ethephon, and external supply of gibberellic acid leads to even more pronounced differences between wild-type and StSUT4-RNAi plants regarding tuber yield and internode elongation, indicating a reciprocal regulation of StSUT4 and gibberellins

The Potato Sucrose Transporter StSUT1 Interacts with a DRM-Associated Protein Disulfide Isomerase

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