Phloem Research Articles

Reduction in sucrose contents by downregulation of fructose-1,6-bisphosphatase 2 causes tiller outgrowth cessation in rice mutants lacking glutamine synthetase1;2

BackgroundOur previous transcriptomic analysis revealed that downregulation of nitrogen and carbon metabolism in the basal portions of the shoots inhibited cytosolic glutamine synthetase1;2 (GS1;2), which severely reduced rice tiller number. In the present study, we used rice mutants lacking GS1;2 (gs1;2 mutants) to determine the contribution of carbon metabolism to tiller growth.ResultsMetabolomic analysis indicated the effects of carbon metabolism disorder such as reductions in the levels of sugar metabolites (e.g., sucrose and glucose 6-phosphate) in the shoot basal portions of the gs1;2 mutant seedlings. Decrease in sucrose caused by the lack of GS1;2 was successfully restored to the wild-type levels by introducing OsGS1;2 cDNA into the mutants. In the basal portions of the shoots, the lack of GS1;2 caused low expression of cytosolic fructose 1,6-bisphosphatase2 (OscFBP2), which is a key cytosolic sucrose synthesis enzyme; it is especially important in the phloem companion cells of the nodal vascular anastomoses. NH4+ supply upregulated OscFBP2 expression in the shoot basal portions of the wild type but not in those of the gs1;2 mutants. Rice mutants lacking cFBPase2 presented with ~ 30% reduction in total cFBPase activity in the basal portions of their shoots. These mutants displayed reductions in sucrose levels of the basal portions of their shoots but not in their leaf blades. They also had relatively lower tiller numbers at the early growth stage.ConclusionsMetabolomic analysis revealed that the lack of GS1;2 reduced sucrose metabolism in the basal portions of the shoots. Our results indicated that sucrose reduction was caused by the downregulation of OscFBP2 expression in the basal portions of the gs1;2 mutant shoots. The reduction in sucrose content caused by the lack of cFBPase2 resulted in lower tiller number at the early growth stage. Therefore, adequate sucrose supply via cFBPase2 may be necessary for tiller growth in the basal portions of rice shoots.

Tissue-specific element profiles in Scots pine (Pinus sylvestris L.) needles

Key messageElement profile signatures of needle tissues differentiated four tissues: epidermis (main contributor: calcium), endodermis (main contributors: magnesium, sulphur and manganese), mesophyll (main contributor: potassium), and transfusion parenchyma (main contributor: zinc).Distribution of elements in cross-sections of Scots pine (Pinus sylvestris L.) needles was investigated using micro-proton-induced X-ray emission. Tissue-specific distributions of magnesium (Mg), sulphur (S), calcium (Ca), phosphorus (P), potassium (K), chlorine (Cl), manganese (Mn), iron (Fe), zinc (Zn), aluminium (Al) and silicon (Si) were resolved in a quantitative manner. Distribution maps and tissue-specific concentrations revealed the largest concentration of Ca in epidermis, of Mg, S and Mn in endodermis, of K in mesophyll and phloem and of Zn in transfusion parenchyma. Phosphorus, Cl, Fe, Al and Si did not exhibit apparent tissue-specific distribution. Inverse allocation of P and Ca was observed, a likely mechanism to prevent their precipitation. Taking the area of tissues into account, relative element distribution calculations indicated that mesophyll contained the majority of the elements studied, except Ca, which predominated in the epidermis (79% of total Ca concentration) and Mn, which predominated in the endodermis (40% of total Mn concentration). When considering a complete element profile of a particular tissue, four clusters were differentiated, which generally supported single-element observations. The first cluster differentiated mesophyll, xylem, phloem, transfusion tracheids and Strasburger cells with predominance of K, the second cluster differentiated epidermis on the basis of Ca, the third cluster differentiated endodermis with contributions from Mg, S and Mn, and the fourth cluster differentiated transfusion parenchyma with contribution from Zn. Information on tissue-specific-element allocations will complement structural and functional knowledge of needle tissues and advance our understanding of element/nutrient transfers in Scots pine.

Identification of sieve elements and companion cell protoplasts by a combination of brightfield and fluorescence microscopy.

Premise of the StudyPhloem‐limited diseases are becoming increasingly pervasive, threatening the existence of crops worldwide. Studies of phloem diseases are complicated by the inaccessibility of the phloem tissue. Phloem cells are located deep inside the plant body, are interspersed with other cell types, are among the smallest cells in the plant kingdom, and make up a small percentage of the total cell population in a plant. These properties make phloem cells difficult to investigate.MethodsWe used leaf midrib protoplasts and a combination of organelle‐specific dyes including Neutral Red (acidic compartments), MitoTracker Green (mitochondria), Hoechst 3342 (nucleus), and chloroplast autofluorescence. We examined the protoplasts under light and fluorescence microscopy.ResultsWhen observed using brightfield and fluorescence microscopy, sieve element protoplasts were distinguished by the lack of vacuole and a nucleus, and by containing various mitochondria. Companion cells showed a dense cytoplasm and various small vacuoles. They also revealed their characteristic large nucleus and abundant mitochondria.DiscussionWe present evidence that a combination of organelle‐specific dyes and autofluorescence allows for the identification of sieve elements and companion cell protoplasts from citrus leaf tissue.

Stem anatomy at various developmental stages of secondary growth in Turbina corymbosa (Convolvulaceae)

Background – Population growth of lianas in the tropical forest is credited to their ability of CO2 sequestration and efficiency of the narrow stems to supply water required for the amount of foliage it bears. Turbina corymbosa (L.) Raf. (Convolvulaceae Juss.) is one of the fast-growing invasive species of scrambling woody lianas. It covers trees entirely within a short period to compete with above-ground resources (particularly sunlight). However, no information is available on how it manages to cope up with an increasing demand of water supply and mineral nutrients. What are the structural and developmental patterns adapted by this species to expand the stem diameter for efficient supply of below-ground resources? Therefore, our aim was to investigate the secondary growth patterns and structure of secondary xylem and phloem in T. corymbosa.Methods – Several samples of the stem with various diameters were studied using a histological method. Morphological and anatomical analyses were carried out using light microscopy.Key results – With the initiation of secondary growth, stems lose their circular outline rapidly due to unequal deposition of secondary xylem and formation of successive cambia. New successive cambia initiate from parenchymatous cells as small crescent-shaped fragments on asymmetric/opposite sides and result in a different stem conformation. Though several segments of successive cambia are formed, very few stem samples form complete cambium rings. The secondary xylem formed by successive cambia is diffuse porous with indistinct growth rings and is composed of both wide and narrow (fibriform) vessels, tracheids, fibres, axial and ray parenchyma cells. The secondary phloem consists of sieve tube elements, companion cells, axial and ray parenchyma cells. In fully grown plants, cambial action (internal cambium) occurrs between the intraxylary phloem and protoxylem and produces secondary xylem and phloem near the pith region.Conclusion – Structural alterations and unequal deposition of conducting elements, occurrence of intraxylary phloem and flattening of the stem are suggested to facilitate rapid growth of the plants by providing required minerals and nutrients. Internal cambium formed at the periphery of the pith is bidirectional and produces secondary xylem externally and intraxylary phloem internally. Continued development of intraxylary phloem from the internal cambium provides an additional path for rapid and safe translocation of photosynthates.

Knock-Down of Arabidopsis PLC5 Reduces Primary Root Growth and Secondary Root Formation While Overexpression Improves Drought Tolerance and Causes Stunted Root Hair Growth.

Phospholipase C (PLC) is a well-known signaling enzyme in metazoans that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol 1,4,5-trisphosphate and diacylglycerol as second messengers involved in mutiple processes. Plants contain PLC too, but relatively little is known about its function there. The model system Arabidopsis thaliana contains nine PLC genes. Reversed genetics have implicated several roles for PLCs in plant development and stress signaling. Here, PLC5 is functionally addressed. Promoter-β-glucuronidase (GUS) analyses revealed expression in roots, leaves and flowers, predominantly in vascular tissue, most probably phloem companion cells, but also in guard cells, trichomes and root apical meristem. Only one plc5-1 knock-down mutant was obtained, which developed normally but grew more slowly and exhibited reduced primary root growth and decreased lateral root numbers. These phenotypes could be complemented by expressing the wild-type gene behind its own promoter. Overexpression of PLC5 (PLC5-OE) using the UBQ10 promoter resulted in reduced primary and secondary root growth, stunted root hairs, decreased stomatal aperture and improved drought tolerance. PLC5-OE lines exhibited strongly reduced phosphatidylinositol 4-monophosphate (PIP) and PIP2 levels and increased amounts of phosphatidic acid, indicating enhanced PLC activity in vivo. Reduced PIP2 levels and stunted root hair growth of PLC5-OE seedlings could be recovered by inducible overexpression of a root hair-specific PIP 5-kinase, PIP5K3. Our results show that PLC5 is involved in primary and secondary root growth and that its overexpression improves drought tolerance. Independently, we provide new evidence that PIP2 is essential for the polar tip growth of root hairs.

Identification and expression characterization of the Phloem Protein 2 (PP2) genes in ramie (Boehmeria nivea L. Gaudich)

Phloem protein 2 (PP2) is one of the most abundant and enigmatic proteins in sieve elements and companion cells, which play important roles in the maintenance of morphology, photoassimilate transportation and wound protection in higher plants, but to date, no PP2 (BnPP2) genes had been identified in ramie. Here, a total of 15 full-length BnPP2 genes were identified. These BnPP2 genes exhibited different responses to abiotic stresses. Interestingly, the BnPP2 genes are more sensitive to insect pests than to other stresses. A study of the BnPP2-15 promoter revealed that pBnPP2-15 could drive specific GUS expression in the petiole, root and stamen and could also be induced by mechanical wounding and aphid infection in transgenic Arabidopsis lines. The subcellular localization of six BnPP2 proteins showed that GFP-BnPP2-1, GFP-BnPP2-6, GFP-BnPP2-7, GFP-BnPP2-9, GFP-BnPP2-11 and GFP-BnPP2-12 were predominantly located in the cytoplasm. These results provide useful information elucidating the functions of BnPP2 genes in ramie.

Physicochemical, Phytochemical and Pharmacognostic study of Limonium stocksii, a halophyte from Gujarat

The halophyte Limonium stocksii (Boiss) Kuntze belongs to the family Plumbaginaceae. In the present work, pharmacognostic studies of this medicinal halophyte was attempted which included physicochemical, phytochemical, macroscopic, microscopic studies and organoleptic evaluation. The plant powder characteristics were also elucidated. The physicochemical analyses were done by using WHO recommended parameters such as loss on drying, ash values (total ash, water soluble ash, acid insoluble ash, sulphated ash, carbonated ash, nitrated ash), extractive values. The qualitative phytochemical analysis revealed the presence of saponins, steroids and coumarins in maximum amount in crude as well as in various solvent extracts. The morphological studies exhibited the organoleptic and surface characteristics of the plant and its parts. The microscopic study showed the presence of various characteristics of whole plant like palisade tissue, metaxylem, protoxylem, phloem showed companion cells, bicollateral type of vascular bundles, parenchyma, rosette calcium oxalate crystals and paracytic stomata. The pharmacognostic characters enlisted in this study will help in identification of the crude drug; the standardization parameters laid down will ensure the efficacy of drug and also distinguish the drug from its adulterants. The distinguishing characters will also be helpful for the preparation of monograph of this halophytic plant.

Three cytosolic glutamine synthetase isoforms localized in different-order veins act together for N remobilization and seed filling in Arabidopsis.

Glutamine synthetase (GS) is central for ammonium assimilation and consists of cytosolic (GS1) and chloroplastic (GS2) isoenzymes. During plant ageing, GS2 protein decreases due to chloroplast degradation, and GS1 activity increases to support glutamine biosynthesis and N remobilization from senescing leaves. The role of the different Arabidopsis GS1 isoforms in nitrogen remobilization was examined using 15N tracing experiments. Only the gln1;1-gln1;2-gln1;3 triple-mutation affecting the three GLN1;1, GLN1;2, and GLN1;3 genes significantly reduced N remobilization, total seed yield, individual seed weight, harvest index, and vegetative biomass. The triple-mutant accumulated a large amount of ammonium that could not be assimilated by GS1. Alternative ammonium assimilation through asparagine biosynthesis was increased and was related to higher ASN2 asparagine synthetase transcript levels. The GS2 transcript, protein, and activity levels were also increased to compensate for the lack of GS1-related glutamine biosynthesis. Localization of the different GLN1 genes showed that they were all expressed in the phloem companion cells but in veins of different order. Our results demonstrate that glutamine biosynthesis for N-remobilization occurs in veins of all orders (major and minor) in leaves, it is mainly catalysed by the three major GS1 isoforms (GLN1;1, GLN1;2, and GLN1;3), and it is alternatively supported by AS2 in the veins and GS2 in the mesophyll cells.

Agronomical, biochemical and histological response of resistant and susceptible wheat and barley under BYDV stress.

Barley yellow dwarf virus-PAV (BYDV-PAV) is one of the major viruses causing a widespread and serious viral disease affecting cereal crops. To gain a better understanding of plant defence mechanisms of BYDV resistance genes (Bdv2 and RYd2) against BYDV-PAV infection, the differences in agronomical, biochemical and histological changes between susceptible and resistant wheat and barley cultivars were investigated. We found that root growth and total dry matter of susceptible cultivars showed greater reduction than that of resistant ones after infection. BYDV infected leaves in susceptible wheat and barley cultivars showed a significant reduction in photosynthetic pigments, an increase in the concentration of reducing sugar. The protein levels were also low in infected leaves. There was a significant increase in total phenol contents in resistant cultivars, which might reflect a protective mechanism of plants against virus infection. In phloem tissue, sieve elements (SE) and companion cells (CC) were severely damaged in susceptible cultivars after infection. It is suggested that restriction of viral movement in the phloem tissue and increased production of phenolic compounds may play a role in the resistance and defensive mechanisms of both Bdv2 and RYd2 against virus infection.

Outgrowth of Rice Tillers Requires Availability of Glutamine in the Basal Portions of Shoots

BackgroundOur previous studies concluded that metabolic disorder in the basal portions of rice shoots caused by a lack of cytosolic glutamine synthetase1;2 (GS1;2) resulted in a severe reduction in the outgrowth of tillers. Rice mutants lacking GS1;2 (gs1;2 mutants) showed a remarkable reduction in the contents of both glutamine and asparagine in the basal portions of shoots. In the current study, we attempted to reveal the mechanisms for this decrease in asparagine content using rice mutants lacking either GS1;2 or asparagine synthetase 1 (AS1). The contributions of the availability of glutamine and asparagine to the outgrowth of rice tillers were investigated.ResultsRice has two AS genes, and the enzymes catalyse asparagine synthesis from glutamine. In the basal portions of rice shoots, expression of OsAS1, the major species in this tissue, was reduced in gs1;2 mutants, whereas OsAS2 expression was relatively constant. OsAS1 was expressed in phloem companion cells of the nodal vascular anastomoses connected to the axillary bud vasculatures in the basal portions of wild-type shoots, whereas cell-specific expression was markedly reduced in gs1;2 mutants. OsAS1 was up-regulated significantly by NH4+ supply in the wild type but not in gs1;2 mutants. When GS reactions were inhibited by methionine sulfoximine, OsAS1 was up-regulated by glutamine but not by NH4+. The rice mutants lacking AS1 (as1 mutants) showed a decrease in asparagine content in the basal portions of shoots. However, glutamine content and tiller number were less affected by the lack of AS1.ConclusionThese results indicate that in phloem companion cells of the nodal vascular anastomoses, asparagine synthesis is largely dependent on glutamine or its related metabolite-responsive AS1. Thus, the decrease in glutamine content caused by a lack of GS1;2 is suggested to result in low expression of OsAS1, decreasing asparagine content. However, the availability of asparagine generated from AS1 reactions is apparently less effective for the outgrowth of tillers. With respect to the tiller number and the contents of glutamine and asparagine in gs1;2 and as1 mutants, the availability of glutamine rather than asparagine in basal portions of rice shoots may be required for the outgrowth of rice tillers.

Mesophyll Cells Are the Main Site of Abscisic Acid Biosynthesis in Water-Stressed Leaves.

The hormone abscisic acid (ABA) plays a critical role in enhancing plant survival during water deficit. Recent molecular evidence suggests that ABA is synthesized in the phloem companion cells and guard cells. However, the nature of cell turgor and water status in these two cell types cannot easily account for the rapid, water status-triggered ABA biosynthesis observed in leaves. Here, we utilize the unique foliar anatomies of an angiosperm (Hakea lissosperma) and four conifer species (Saxegothaea conspicua, Podocarpus latifolius, Cephalotaxus harringtonii, and Amentotaxus formosana) in which the mesophyll can be isolated from the vascular tissue to identify the main site of ABA biosynthesis in water-stressed leaves. In all five species tested, considerable ABA biosynthesis occurred in mesophyll tissue that had been separated from vascular tissue. In addition, the removal of the epidermis from the mesophyll in two conifer species had no impact on the observed increase in ABA levels under water deficit. Our results suggest that mesophyll cells are the predominant location of water deficit-triggered ABA biosynthesis in the leaf.

Elucidation of the Mechanisms of Long-Distance mRNA Movement in a Nicotiana benthamiana/Tomato Heterograft System.

Recent heterograft analyses showed that large-scale messenger RNA (mRNA) movement takes place in the phloem, but the number of mobile transcripts reported varies widely. However, our knowledge of the mechanisms underlying large-scale mRNA movement remains limited. In this study, using a Nicotiana benthamiana/tomato (Solanum lycopersicum) heterograft system and a transgenic approach involving potato (Solanum tuberosum), we found that: (1) the overall mRNA abundance in the leaf is not a good indicator of transcript mobility to the root; (2) increasing the expression levels of nonmobile mRNAs in the companion cells does not promote their mobility; (3) mobile mRNAs undergo degradation during their movement; and (4) some mRNAs arriving in roots move back to shoots. These results indicate that mRNA movement has both regulated and unregulated components. The cellular origins of mobile mRNAs may differ between herbaceous and woody species. Taken together, these findings suggest that the long-distance movement of mRNAs is a complex process and that elucidating the physiological roles associated with this movement is challenging but remains an important task for future research.

Morphology, structure and ultrastructure of staminal nectary in Lamprocapnos (Fumarioideae, Papaveraceae)

Nectaries show high a degree of variations in terms of location, morphology and structure; these characteristics are critical for systematic information. In Papaveraceae, staminal nectaries at the stamen base are only found in bisymmetric and zygomorphic flowers, but with little description on the ultra-structure of nectary available. Here we investigate structural details of flowers and nectaries of Lamprocapnos spectabilis using LM, SEM and TEM. The nectaries of L. spectabilis are initiated at the base of stamen filaments in the young bud. The oval/ball-shaped staminal nectaries are composed of four major types of tissues in general. First and outmost layers are epidermis and hypodermis with large and elliptical cells. Beneath the epidermal layers are the second type of tissue consisting of 20–30 layers of parenchyma cells which are further distinguished by two kinds of cells: smaller secretory parenchyma cells having dense cytoplasm, large nucleus and abundant cellular organelles, and larger sub-nectary parenchyma cells containing a relatively thin cytoplasm. The third type of tissue is the darkly-stained elongated cells and embedded at 8- to 12-cell-layer deep in the nectaries, and is associated with sieve tube elements and cells resembling companion cells, and they presumably belong to underdeveloped sieve tube elements or “protophloem” based on ultrastructure features such as conspicuous cell wall ingrowths and plentiful plasmodesmata. The forth type of tissue in the nectary is the simplified vascular bundle forming with only phloem. It is reasonable to suggest that secretions in the spacious intercellular spaces between secretory parenchyma cells are exuded to the exterior via micro-channel or epidermis/cuticle pathway. In conclusion, the results show that epidermis, hypodermis, secretory parenchyma, “Arber suggestive tissue (protophloem)”, and phloem act coordinately in nectar production, secretion and exudation in Lamprocapnos. The structure of the staminal nectaries are compared within Papaveraceae and with other related groups in Ranunculales.

Re-Evaluation of Florigen Transport Kinetics with Separation of Functions by Mutations That Uncouple Flowering Initiation and Long-Distance Transport.

In many plants, timing of flowering is regulated by day length. In Arabidopsis, florigen, FLOWERING LOCUS T (FT) protein, is synthesized in leaf phloem companion cells in response to long days and is transported to the shoot apical meristem (SAM) through the phloem. The temporal aspects of florigen transportation have been studied in various plants by physiological experiments. Nevertheless, little is known about how FT protein transportation is regulated in Arabidopsis. In this study, we performed heat shock-based transient FT induction in a single leaf blade and detected the FT protein in the shoot apex by 2D-PAGE. We demonstrated that detectable amounts of FT were transported from the leaf to the shoot apex within 8 h, and subsequent FT-induced target gene expression was detected within 8-12 h. Furthermore, we identified three amino acid residues (V70, S76 and R83) where missense mutations led to reduced mobility. Interestingly, these FT variants lost only their transportation ability, but retained their flowering promotion capacity, suggesting that discrete amino acids are involved in flowering regulation and transport regulation. Since the interaction with FT-INTERACTING PROTEIN 1 (FTIP1) was not affected in these FT variants, we hypothesize that the three amino acid residues are not involved in the FTIP1-mediated pathway of uploading, but rather in the subsequent step(s) of FT transport.

The long and winding road: transport pathways for amino acids in Arabidopsis seeds.

Pathways for assimilates. During their life cycle, plants alternate between a haploid stage, the gametophyte, and a diploid stage, the sporophyte. In higher plants, meiosis generates the gametophyte deeply embedded in the maternal tissue of the flower. The megaspore mother cell undergoes meiosis, and then, the surviving megaspore of the four megaspores produced undergoes mitotic divisions and finally gives rise to the female gametophyte, consisting of the egg cell, two synergids, the central cell, which due to the fusion of two nuclei is diploid (double haploid) in Arabidopsis and most angiosperms and the antipods, whose number is not fixed and varies significantly between species (Yadegari and Drews in Plant Cell 16(Suppl):S133-S141, 2004). The maternal tissues that harbor the female gametophyte and the female gametophyte are referred to as the ovule (Fig.1). Double fertilization of the egg cell and the central cell by the two generative nuclei of the pollen leads to the diploid embryo and the endosperm, respectively (Hamamura et al. in Curr Opin Plant Biol 15:70-77, 2012). Upon fertilization, the ovule is referred to as the seed. Seeds combine two purposes: to harbor storage compounds for use by the embryo upon germination and to protect the embryo until the correct conditions for germination are encountered. As a consequence, seeds are the plant tissue that is of highest nutritional value and the human diet, by a considerable amount, consists of seeds or seed-derived products. Amino acids are of special interest, because plants serve as the main source for the so-called essential amino acids, that animals cannot synthesize de novo and are therefore often a limiting factor for human growth and development (WHO in Protein and amino acid requirements in human nutrition. WHO technical report series, WHO, Geneva, 2007). The plant embryo needs amino acids for general protein synthesis, and additionally they are used to synthesize storage proteins in the seeds of certain plants, e.g., legumes as a resource to support the growth of the seedling after germination. The support of the embryo depends on transport processes that occur between the mother plant and the seed tissues including the embryo. In this review, we will focus on the processes of unloading amino acids from the phloem and their post-phloem transport. We will further highlight similarities between amino acid transport and the transport of the main assimilate and osmolyte, sucrose. Finally, we will discuss similarities and differences between different plant species in terms of structural aspects but for the molecular aspects we are almost exclusively focusing on Arabidopsis. Fig.1 Vascularization of the Arabidopsis ovule and seed. Plants expressing ER-localized mCherry under control of the companion cell-specific SUC2 promoter and ER-localized GFP under control of the sieve element marker PD1 as described (Müller et al. 2015) are shown to visualize the phloem in the funiculus and the chalazal regions. a Overview over an ovule. FG: female gametophyte. b A magnification of the region marked by a square in panel a. c Overview over a seed. ES: endosperm; E: embryo. d A magnification of the region marked by a square in panel c. The arrows in b and d point to the terminal companion cell and arrowheads to terminal sieve elements.

Improved water use efficiency and shorter life cycle of Nicotiana tabacum due to modification of guard and vascular companion cells

Severe droughts are predicted for the twenty-first century, which contrast with the increased demand for plant materials. Thus, to sustain future generations, a great challenge is to improve crop yield and water use efficiency (WUE), which is the carbon gained per water lost. Here, expression of maize NADP-malic enzyme (NADP-ME) in the guard and vascular companion cells of Nicotiana tabacum results in enhanced WUE, earlier flowering and shorter life cycle. Transgenic lines exhibit reduced stomatal aperture than wild-type (WT). Nevertheless, an increased net CO2 fixation rate is observed, which results in less water consumption and more biomass production per water used. Transgenic lines export sugars to the phloem at higher rate than WT, which leads to higher sugars levels in phloem exudates and veins. Leaf quantitative proteomic profiling revealed drastic differences in proteins related to cell cycle, flowering, hormone signaling and carbon metabolism between transgenic lines and WT. We propose that the increased sugar export from leaves in the transgenic lines alleviates sugar negative feedback on photosynthesis and thus, stomatal closure takes place without a penalty in CO2 assimilation rate. This results in improved WUE and accelerated overall life cycle, key traits for plant productivity in the near future world.

Differentiation of vascular elements in haustoria of Cuscuta japonica

ABSTRACTParasitic plants establish vascular-conducting cells in an intrusive organ called haustorium. In haustoria of a stem parasitic plant, Cuscuta japonica, the presence of cells expressing cell-type-specific genes of phloem companion cell, phloem sieve element, procambial cell and xylem vessel has recently been demonstrated. Differentiation of these vascular cells is regulated in a manner similar to that in conventional vascular tissues. However, the initiation of procambial cells occurs concomitantly with the differentiation of vascular-conducting cells. The differentiation process of phloem also differed from that of conventional vascular tissues because enucleation of sieve elements appeared to be impeded. These results collectively imply that the vascular differentiation process in haustoria of parasitic plants may be different from that in conventional vascular tissues.

FLOWERING LOCUS T mRNA is synthesized in specialized companion cells in Arabidopsis and Maryland Mammoth tobacco leaf veins

Flowering is triggered by the transmission of a mobile protein, FLOWERING LOCUS T (FT), from leaves to the shoot apex. FT originates in the phloem of leaf veins. However, the identity of the FT-synthesizing cells in the phloem is not known. As a result, it has not been possible to determine whether the complex regulatory networks that control FT synthesis involve intercellular communication, as is the case in many aspects of plant development. We demonstrate here that FT in Arabidopsis thaliana and FT orthologs in Maryland Mammoth tobacco (Nicotiana tabacum) are produced in two unique files of phloem companion cells. These FT-activating cells, visualized by fluorescent proteins, also activate the GALACTINOL SYNTHASE (CmGAS1) promoter from melon (Cucumis melo). Ablating the cells by expression of the diphtheria toxin gene driven by the CmGAS1 promoter delays flowering in both Arabidopsis and Maryland Mammoth tobacco. In Arabidopsis, toxin expression reduces expression of FT and flowering-associated genes downstream, but not upstream, of FT Our results indicate that specific companion cells mediate the essential flowering function. Since the identified cells are present in the minor veins of two unrelated dicotyledonous species, this may be a widespread phenomenon.

Autophagy controls resource allocation and protein storage accumulation in Arabidopsis seeds.

Autophagy is essential for nutrient recycling and plays a fundamental role in seed production and grain filling in plants. Autophagy participates in nitrogen remobilization at the whole-plant level, and the seeds of autophagy mutants present abnormal C and N contents relative to wild-type (WT) plants. It is well known that autophagy (ATG) genes are induced in leaves during senescence; however, expression of such genes in seeds has not yet been reported. In this study we show that most of the ATG genes are induced during seed maturation in Arabidopsis siliques. Promoter-ATG8f::UIDA and promoter-ATG8f::GFP fusions showed the strong expression of ATG8f in the phloem companion cells of pericarps and the funiculus, and in the embryo. Expression was especially strong at the late stages of development. The presence of many GFP-ATG8 pre-autophagosomal structures and autophagosomes confirmed the presence of autophagic activity in WT seed embryos. Seeds of atg5 and WT plants grown under low- or high-nitrate conditions were analysed. Nitrate-independent phenotypes were found with higher seed abortion in atg5 and early browing, higher total protein concentrations in the viable seeds of this mutant as compared to the WT. The higher total protein accumulation in atg5 viable seeds was significant from early developmental stages onwards. In addition, relatively low and early accumulation of 12S globulins were found in atg5 seeds. These features led us to the conclusion that atg5 seed development is accelerated and that the protein storage deposition pathway is somehow abnormal or incomplete.

A novel rejuvenation approach to induce endohormones and improve rhizogenesis in mature Juglans tree

BackgroundJuglans is a difficult-to-root tree. In the present study, we successfully rejuvenated stock plants by grafting and then burying them horizontally.ResultsRooting rates of rejuvenated shoots were 98.1% 20 days after cutting. We recorded spatial and temporal variation in endogenous indole-3-acetic acid (IAA), abscisic acid (ABA), gibberellin A3 (GA3) and zeatin-riboside (ZR) under root induction. The four types of endohormones were mainly confined to the phloem sieve and companion cells (S&Cs) at the base of either rejuvenated or mature soft shoots. IAA and ABA levels were higher in rejuvenated shoots than in mature shoots, whereas the opposite was true for GA3 and ZR. During rooting induction, GA3 was the first hormone to be observed outside phloem S&Cs, followed by IAA, ABA and ZR. In rejuvenating soft shoots, IAA accumulated in the cross-sectional areas of the cambium and phloem, where root primordia were evident.ConclusionsThe improvement in the rooting ability that was evident after rejuvenation most likely results a transformation of the plant to a juvenile form, from elevated IAA levels in phloem S&Cs and from a promotion of all four endohormones outside phloem S&Cs, in particular, from an accumulation of IAA in the cross-sectional areas of the cambium and phloem.