Many intercellular solute transport processes require an apoplasmic step, that is, efflux from one cell and subsequent uptake by an adjacent cell. Cellular uptake transporters have been identified ...for many solutes, including sucrose; however, efflux transporters have remained elusive for a long time. Cellular efflux of sugars plays essential roles in many processes, such as sugar efflux as the first step in phloem loading, sugar efflux for nectar secretion, and sugar efflux for supplying symbionts such as mycorrhiza, and maternal efflux for filial tissue development. Furthermore, sugar efflux systems can be hijacked by pathogens for access to nutrition from hosts. Mutations that block recruitment of the efflux mechanism by the pathogen thus cause pathogen resistance. Until recently, little was known regarding the underlying mechanism of sugar efflux. The identification of sugar efflux carriers, SWEETs (Sugars Will Eventually be Exported Transporters), has shed light on cellular sugar efflux. SWEETs appear to function as uniporters, facilitating diffusion of sugars across cell membranes. Indeed, SWEET sprobably mediate sucrose efflux from putative phloem parenchyma into the phloem apoplasm, a key step proceeding phloem loading. Engineering of SWEET mutants using transcriptional activator-like effector nuclease (TALEN)- based genomic editing allowed the engineering of pathogen resistance. The widespread expression of the SWEET family promises to provide insights into many other cellular efflux mechanisms.
Molecular genetic framework for protophloem formation Rodriguez-Villalon, Antia; Gujas, Bojan; Kang, Yeon Hee ...
Proceedings of the National Academy of Sciences - PNAS,
08/2014, Volume:
111, Issue:
31
Journal Article
Peer reviewed
Open access
The phloem performs essential systemic functions in tracheophytes, yet little is known about its molecular genetic specification. Here we show that application of the peptide ligand CLAVATA3/EMBRYO ...SURROUNDING REGION 45 (CLE45) specifically inhibits specification of protophloem in Arabidopsis roots by locking the sieve element precursor cell in its preceding developmental state. CLE45 treatment, as well as viable transgenic expression of a weak CLE45G6T variant, interferes not only with commitment to sieve element fate but also with the formative sieve element precursor cell division that creates protophloem and metaphloem cell files. However, the absence of this division appears to be a secondary effect of discontinuous sieve element files and subsequent systemically reduced auxin signaling in the root meristem. In the absence of the formative sieve element precursor cell division, metaphloem identity is seemingly adopted by the normally procambial cell file instead, pointing to possibly independent positional cues for metaphloem formation. The protophloem formation and differentiation defects in brevis radix (brx) and octopus (ops) mutants are similar to those observed in transgenic seedlings with increased CLE45 activity and can be rescued by loss of function of a putative CLE45 receptor, BARELY ANY MERISTEM 3 (BAM3). Conversely, a dominant gain-of-function ops allele or mild OPS dosage increase suppresses brx defects and confers CLE45 resistance. Thus, our data suggest that delicate quantitative interplay between the opposing activities of BAM3-mediated CLE45 signals and OPS-dependent signals determines cellular commitment to protophloem sieve element fate, with OPS acting as a positive, quantitative master regulator of phloem fate.
In the plant meristem, tissue-wide maturation gradients are coordinated with specialized cell networks to establish various developmental phases required for indeterminate growth. Here, we used ...single-cell transcriptomics to reconstruct the protophloem developmental trajectory from the birth of cell progenitors to terminal differentiation in the
root. PHLOEM EARLY DNA-BINDING-WITH-ONE-FINGER (PEAR) transcription factors mediate lineage bifurcation by activating guanosine triphosphatase signaling and prime a transcriptional differentiation program. This program is initially repressed by a meristem-wide gradient of PLETHORA transcription factors. Only the dissipation of PLETHORA gradient permits activation of the differentiation program that involves mutual inhibition of early versus late meristem regulators. Thus, for phloem development, broad maturation gradients interface with cell-type-specific transcriptional regulators to stage cellular differentiation.
Summary
The growing impact of phloem‐limited pathogens on high‐value crops has led to a renewed interest in understanding how they cause disease. Although these pathogens cause substantial crop ...losses, many are poorly characterized. In this review, we present examples of phloem‐limited pathogens that include intracellular bacteria with and without cell walls, and viruses. Phloem‐limited pathogens have small genomes and lack many genes required for core metabolic processes, which is, in part, an adaptation to the unique phloem environment. For each pathogen class, we present multiple case studies to highlight aspects of disease caused by phloem‐limited pathogens. The pathogens presented include Candidatus Liberibacter asiaticus (citrus greening), Arsenophonus bacteria, Serratia marcescens (cucurbit yellow vine disease), Candidatus Phytoplasma asteris (Aster Yellows Witches’ Broom), Spiroplasma kunkelii, Potato leafroll virus and Citrus tristeza virus. We focus on commonalities in the virulence strategies of these pathogens, and aim to stimulate new discussions in the hope that widely applicable disease management strategies can be found.
Currently, phloem transport in plants under field conditions is not well understood. This is largely the result of the lack of techniques suitable for the measurement of the physiological properties ...of phloem.
We present a model that interprets the changes in xylem diameter and live bark thickness and separates the components responsible for such changes. We test the predictions from this model on data from three mature Scots pine trees in Finland. The model separates the live bark thickness variations caused by bark water capacitance from a residual signal interpreted to indicate the turgor changes in the bark.
The predictions from the model are consistent with processes related to phloem transport. At the diurnal scale, this signal is related to patterns of photosynthetic activity and phloem loading. At the seasonal scale, bark turgor showed rapid changes during two droughts and after two rainfall events, consistent with physiological predictions. Daily cumulative totals of this turgor term were related to daily cumulative totals of canopy photosynthesis. Finally, the model parameter representing radial hydraulic conductance between phloem and xylem showed a temperature dependence consistent with the temperature-driven changes in water viscosity.
We propose that this model has potential for the continuous field monitoring of tree phloem function.
Summary
The phloem of the Cucurbitaceae has long been a subject of interest due to its complex nature and the economic importance of the family. As in a limited number of other families, cucurbit ...phloem is bicollateral, i.e. with sieve tubes on both sides of the xylem. To date little is known about the specialized functions of the internal phloem (IP) and external phloem (EP). Here, a combination of microscopy, fluorescent dye transport analysis, micro‐computed tomography, laser capture microdissection and RNA‐sequencing (RNA‐Seq) were used to study the functions of IP and EP in the vascular bundles (VBs) of cucumber fruit. There is one type of VB in the peduncle, but four in the fruit: peripheral (PeVB), main (MVB), carpel (CVB) and placental (PlVB). The VBs are bicollateral, except for the CVB and PlVB. Phloem mobile tracers and 14C applied to leaves are transported primarily in the EP, and to a lesser extent in the IP. RNA‐Seq data indicate preferential gene transcription in the IP related to differentiation/development, hormone transport, RNA or protein modification/processing/transport, and nitrogen compound metabolism and transport. The EP preferentially expresses genes for stimulus/stress, defense, ion transport and secondary metabolite biosynthesis. The MVB phloem is preferentially involved in photoassimilate transport, unloading and long‐distance signaling, while the PeVB plays a more substantial role in morphogenesis and/or development and defense response. CVB and PlVB transcripts are biased toward development of reproductive organs. These findings provide an integrated view of the differentiated structure and function of the vascular tissue in cucumber fruit.
Significance Statement
The phloem of bicollateral vascular bundles (VBs) in the Cucurbitaceae has long been a subject of interest. However, little is known about the specialized functions of the internal (IP) and external phloem (EP) in these plants. This study provides a framework for understanding differentiated roles of IP and EP, and gene expression profiles of four interrelated VB systems in cucumber fruit.
Summary
Salt tolerance quantitative trait loci analysis of rice has revealed that the SKC1 locus, which is involved in a higher K+/Na+ ratio in shoots, corresponds to the OsHKT1;5 gene encoding a ...Na+‐selective transporter. However, physiological roles of OsHKT1;5 in rice exposed to salt stress remain elusive, and no OsHKT1;5 gene disruption mutants have been characterized to date. In this study, we dissected two independent T‐DNA insertional OsHKT1;5 mutants. Measurements of ion contents in tissues and 22Na+ tracer imaging experiments showed that loss‐of‐function of OsHKT1;5 in salt‐stressed rice roots triggers massive Na+ accumulation in shoots. Salt stress‐induced increases in the OsHKT1;5 transcript were observed in roots and basal stems, including basal nodes. Immuno‐staining using an anti‐OsHKT1;5 peptide antibody indicated that OsHKT1;5 is localized in cells adjacent to the xylem in roots. Additionally, direct introduction of 22Na+ tracer to leaf sheaths also demonstrated the involvement of OsHKT1;5 in xylem Na+ unloading in leaf sheaths. Furthermore, OsHKT1;5 was indicated to be present in the plasma membrane and found to localize also in the phloem of diffuse vascular bundles in basal nodes. Together with the characteristic 22Na+ allocation in the blade of the developing immature leaf in the mutants, these results suggest a novel function of OsHKT1;5 in mediating Na+ exclusion in the phloem to prevent Na+ transfer to young leaf blades. Our findings further demonstrate that the function of OsHKT1;5 is crucial over growth stages of rice, including the protection of the next generation seeds as well as of vital leaf blades under salt stress.
Significance Statement
OsHKT1;5 has been strongly suggested to exclude Na+ from leaves under salt stress in rice. However, the detailed molecular physiological mechanisms have remained elusive and the knock out mutant has not been investigated in cereal crops. The present manuscript provides evidence that the OsHKT1;5‐mediated leaf blade protection is established not only by Na+ unloading from the xylem in roots and leaf sheaths, but also by Na+ exclusion in the phloem of basal nodes.
Plant stem cell niches, the meristems, require long-distance transport of energy metabolites and signaling molecules along the phloem tissue. However, currently it is unclear how specification of ...phloem cells is controlled. Here we show that the genes SUPPRESSOR OF MAX2 1-LIKE3 (SMXL3), SMXL4, and SMXL5 act as cell-autonomous key regulators of phloem formation in Arabidopsis thaliana. The three genes form an uncharacterized subclade of the SMXL gene family that mediates hormonal strigolactone and karrikin signaling. Strigolactones are endogenous signaling molecules regulating shoot and root branching 1 whereas exogenous karrikin molecules induce germination after wildfires 2. Both activities depend on the F-box protein and SCF (Skp, Cullin, F-box) complex component MORE AXILLARY GROWTH2 (MAX2) 3–5. Strigolactone and karrikin perception leads to MAX2-dependent degradation of distinct SMXL protein family members, which is key for mediating hormonal effects 6–12. However, the nature of events immediately downstream of SMXL protein degradation and whether all SMXL proteins mediate strigolactone or karrikin signaling is unknown. In this study we demonstrate that, within the SMXL gene family, specifically SMXL3/4/5 deficiency results in strong defects in phloem formation, altered sugar accumulation, and seedling lethality. By comparing protein stabilities, we show that SMXL3/4/5 proteins function differently to canonical strigolactone and karrikin signaling mediators, although being functionally interchangeable with those under low strigolactone/karrikin signaling conditions. Our observations reveal a fundamental mechanism of phloem formation and indicate that diversity of SMXL protein functions is essential for a steady fuelling of plant meristems.
•SMXL3/4/5 genes act as general promoters of phloem formation•SMXL3/4/5 proteins are expressed and function very early during phloem development•SMXL3/4/5 proteins do not mediate strigolactone or karrikin signaling•Strigolactone/karrkin-dependent SMXL proteins are able to replace SMXL5
Plants depend on long-distance transport of energy metabolites and signaling molecules along the phloem tissue. Wallner et al. show that phloem formation requires a family of proteins closely related to mediators of a hormonal signalling pathway. An hormone-independent action of the proteins analyzed is essential for robust phloem formation.
The development of sink organs such as fruits and seeds strongly depends on the amount of nitrogen that is moved within the phloem from photosynthetic‐active source leaves to the reproductive sinks. ...In many plant species nitrogen is transported as amino acids. In pea (Pisum sativum L.), source to sink partitioning of amino acids requires at least two active transport events mediated by plasma membrane‐localized proteins, and these are: (i) amino acid phloem loading; and (ii) import of amino acids into the seed cotyledons via epidermal transfer cells. As each of these transport steps might potentially be limiting to efficient nitrogen delivery to the pea embryo, we manipulated both simultaneously. Additional copies of the pea amino acid permease PsAAP1 were introduced into the pea genome and expression of the transporter was targeted to the sieve element‐companion cell complexes of the leaf phloem and to the epidermis of the seed cotyledons. The transgenic pea plants showed increased phloem loading and embryo loading of amino acids resulting in improved long distance transport of nitrogen, sink development and seed protein accumulation. Analyses of root and leaf tissues further revealed that genetic manipulation positively affected root nitrogen uptake, as well as primary source and sink metabolism. Overall, the results suggest that amino acid phloem loading exerts regulatory control over pea biomass production and seed yield, and that import of amino acids into the cotyledons limits seed protein levels.
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