The Long-Distance Transport of Some Plant Hormones and Possible Involvement of Lipid-Binding and Transfer Proteins in Hormonal Transport

Adaptation to changes in the environment depends, in part, on signaling between plant organs to integrate adaptive response at the level of the whole organism. Changes in the delivery of hormones from one organ to another through the vascular system strongly suggest that hormone transport is involved in the transmission of signals over long distances. However, there is evidence that, alternatively, systemic responses may be brought about by other kinds of signals (e.g., hydraulic or electrical) capable of inducing changes in hormone metabolism in distant organs. Long-distance transport of hormones is therefore a matter of debate. This review summarizes arguments for and against the involvement of the long-distance transport of cytokinins in signaling mineral nutrient availability from roots to the shoot. It also assesses the evidence for the role of abscisic acid (ABA) and jasmonates in long-distance signaling of water deficiency and the possibility that Lipid-Binding and Transfer Proteins (LBTPs) facilitate the long-distance transport of hormones. It is assumed that proteins of this type raise the solubility of hydrophobic substances such as ABA and jasmonates in hydrophilic spaces, thereby enabling their movement in solution throughout the plant. This review collates evidence that LBTPs bind to cytokinins, ABA, and jasmonates and that cytokinins, ABA, and LBTPs are present in xylem and phloem sap and co-localize at sites of loading into vascular tissues and at sites of unloading from the phloem. The available evidence indicates a functional interaction between LBTPs and these hormones.

Effect of source and journey on physiological variables in calves transported by road and ferry between Ireland and the Netherlands

This study aimed to establish baseline variables for calves transported by road and ferry from Ireland to the Netherlands and to investigate the effect of journey [two comparable journeys in April (J1) and May (J2) 2022] and source [source farm or mart (SF/MA)] on these variables. A total of 66 calves from the SF/MA were transported from Ireland to commercial veal farms in the Netherlands. Blood samples were collected at the SF/MA, assembly center (Ireland), lairage (France), and on arrival on the veal farm (Netherlands). They were analyzed for indicator variables related to energy balance, hydration/electrolytes, physical/muscular stress, immunity, and inflammation [glucose, beta-hydroxybutyrate (BHB), non-esterified fatty acids (NEFA), potassium, sodium, magnesium, chloride, urea, haematocrit, total protein, creatine kinase, L-lactate, cortisol, white blood cell, neutrophil, lymphocyte and monocyte counts, serum amyloid-A, and haptoglobin]. Health variables eye and nose discharge, skin tent (a measure of dehydration), and navel inflammation were scored by a trained observer, and calves were weighed at every blood-sampling time point. All blood variables and body weight changed significantly (P < 0.05) during transport, most notably between the assembly center and lairage. Reference ranges were available for 18 variables; 11 of these variables exceeded the reference ranges at the lairage, whilst 10 variables exceeded the reference ranges on arrival at the veal farm. However, health variables did not change during transport. A journey-to-journey comparison indicated much variation; 18 out of 25 variables differed significantly on at least one time point. In total, J1 calves experienced a more severe change in BHB, potassium, strong-ion-difference, L-lactate, and eye and nose discharge than J2 calves. The source of calves also affected their physiology; 12 out of 25 variables studied differed significantly, all of which were confined to the first time point. Specifically, MA calves had elevated levels of NEFA, urea, haematocrit, L-lactate, cortisol, white blood cell, neutrophil, and monocyte counts and lower levels of corrected chloride and lymphocyte count. Overall, calves in this study showed a generalized physiological disturbance beyond reference limits during long-distance transport, but no animal died during transport or for 3 weeks post-arrival.

Arabidopsis ABCG14 forms a homodimeric transporter for multiple cytokinins and mediates long-distance transport of isopentenyladenine-type cytokinins

Cytokinins (CKs), primarily trans-zeatin (tZ) and isopentenyladenine (iP) types, play critical roles in plant growth, development, and various stress responses. Long-distance transport of tZ-type CKs meidated by Arabidopsis ATP-binding cassette transporter subfamily G14 (AtABCG14) has been well studied; however, less is known about the biochemical properties of AtABCG14 and its transporter activity toward iP-type CKs. Here we reveal the biochemical properties of AtABCG14 and provide evidence that it is also required for long-distance transport of iP-type CKs. AtABCG14 formed homodimers in human (Homo sapiens) HEK293T, tobacco (Nicotiana tabacum), and Arabidopsis cells. Transporter activity assays of AtABCG14 in Arabidopsis, tobacco, and yeast (Saccharomyces cerevisiae) showed that AtABCG14 may directly transport multiple CKs, including iP- and tZ-type species. AtABCG14 expression was induced by iP in a tZ-type CK-deficient double mutant (cypDM) of CYP735A1 and CYP735A2. The atabcg14 cypDM triple mutant exhibited stronger CK-deficiency phenotypes than cypDM. Hormone profiling, reciprocal grafting, and ²H₆-iP isotope tracer experiments showed that root-to-shoot and shoot-to-root long-distance transport of iP-type CKs were suppressed in atabcg14 cypDM and atabcg14. These results suggest that AtABCG14 participates in three steps of the circular long-distance transport of iP-type CKs: xylem loading in the root for shootward transport, phloem unloading in the shoot for shoot distribution, and phloem unloading in the root for root distribution. We found that AtABCG14 displays transporter activity toward multiple CK species and revealed its versatile roles in circular long-distance transport of iP-type CKs. These findings provide new insights into the transport mechanisms of CKs and other plant hormones.

Noncell-autonomous HSC70.1 chaperone displays homeostatic feedback regulation by binding its own mRNA

The HSC70/HSP70 family of heat shock proteins are evolutionarily conserved chaperones involved in protein folding, protein transport, and RNA binding. Arabidopsis HSC70 chaperones are thought to act as housekeeping chaperones and as such are involved in many growth-related pathways. Whether Arabidopsis HSC70 binds RNA and whether this interaction is functional has remained an open question. We provide evidence that the HSC70.1 chaperone binds its own mRNA via its C-terminal short variable region (SVR) and inhibits its own translation. The SVR encoding mRNA region is necessary for HSC70.1 transcript mobility to distant tissues and that HSC70.1 transcript and not protein mobility is required to rescue root growth and flowering time of hsc70 mutants. We propose that this negative protein-transcript feedback loop may establish an on-demand chaperone pool that allows for a rapid response to stress. In summary, our data suggest that the Arabidopsis HSC70.1 chaperone can form a complex with its own transcript to regulate its translation and that both protein and transcript can act in a noncell-autonomous manner, potentially maintaining chaperone homeostasis between tissues.

Molecular identification and physiological functional analysis of NtNRT1.1B that mediated nitrate long-distance transport and improved plant growth when overexpressed in tobacco

Although recent physiological studies demonstrate that flue-cured tobacco preferentially utilizes nitrate ( NO 3 - ) or ammonium nitrate (NH₄NO₃), and possesses both high- and low-affinity uptake systems for NO 3 - , little is known about the molecular component(s) responsible for acquisition and translocation in this crop. Here we provide experimental data showing that NtNRT1.1B with a 1,785-bp coding sequence exhibited a function in mediating NO 3 - transport associated with tobacco growth on NO 3 - nutrition. Heterologous expression of NtNRT1.1B in the NO 3 - uptake-defective yeast Hp△ynt1 enabled a growth recovery of the mutant on 0.5 mM NO 3 - , suggesting a possible molecular function of NtNRT1.1B in the import of NO 3 - into cells. Transient expression of NtNRT1.1B::green fluorescent protein (GFP) in tobacco leaf cells revealed that NtNRT1.1B targeted mainly the plasma membrane, indicating the possibility of NO 3 - permeation across cell membranes via NtNRT1.1B. Furthermore, promoter activity assays using a GFP marker clearly indicated that NtNRT1.1B transcription in roots may be down-regulated by N starvation and induced by N resupply, including NO 3 - , after 3 days' N depletion. Significantly, constitutive overexpression of NtNRT1.1B could remarkably enhance tobacco growth by showing a higher accumulation of biomass and total N, NO 3 - , and even NH 4 + in plants supplied with NO 3 - ; this NtNRT1.1B-facilitated N acquisition/accumulation could be strengthened by short-term ¹⁵N- NO 3 - root influx assays, which showed 15%-20% higher NO 3 - deposition in NtNRT1.1B-overexpressors as well as a high affinity of NtNRT1.1B for NO 3 - at a K _m of around 30-45 µM. Together with the detection of NtNRT1.1B promoter activity in the root stele and shoot-stem vascular tissues, and higher NO 3 - in both xylem exudate and the apoplastic washing fluid of NtNRT1.1B-transgenic lines, NtNRT1.1B could be considered as a valuable molecular breeding target aiming at improving crop N-use efficiency by manipulating the absorption and long-distance distribution/transport of nitrate, thus adding a new functional homolog as a nitrate permease to the plant NRT1 family.

Exact Bayesian inference for the detection of graft-mobile transcripts from sequencing data

The long-distance transport of messenger RNAs (mRNAs) has been shown to be important for several developmental processes in plants. A popular method for identifying travelling mRNAs is to perform RNA-Seq on grafted plants. This approach depends on the ability to correctly assign sequenced mRNAs to the genetic background from which they originated. The assignment is often based on the identification of single-nucleotide polymorphisms (SNPs) between otherwise identical sequences. A major challenge is therefore to distinguish SNPs from sequencing errors. Here, we show how Bayes factors can be computed analytically using RNA-Seq data over all the SNPs in an mRNA. We used simulations to evaluate the performance of the proposed framework and demonstrate how Bayes factors accurately identify graft-mobile transcripts. The comparison with other detection methods using simulated data shows how not taking the variability in read depth, error rates and multiple SNPs per transcript into account can lead to incorrect classification. Our results suggest experimental design criteria for successful graft-mobile mRNA detection and show the pitfalls of filtering for sequencing errors or focusing on single SNPs within an mRNA.

Long-Distance Transported RNAs: From Identity to Function

There is now a wealth of data, from different plants and labs and spanning more than two decades, which unequivocally demonstrates that RNAs can be transported over long distances, from the cell where they are transcribed to distal cells in other tissues. Different types of RNA molecules are transported, including micro- and messenger RNAs. Whether these RNAs are selected for transport and, if so, how they are selected and transported remain, in general, open questions. This aspect is likely not independent of the biological function and relevance of the transported RNAs, which are in most cases still unclear. In this review, we summarize the experimental data supporting selectivity or nonselectivity of RNA translocation and review the evidence for biological functions. After discussing potential issues regarding the comparability between experiments, we propose criteria that need to be critically evaluated to identify important signaling RNAs.

HKT1;1 and HKT1;2 Na+ Transporters from Solanum galapagense Play Different Roles in the Plant Na+ Distribution under Salinity

Salt tolerance is a target trait in plant science and tomato breeding programs. Wild tomato accessions have been often explored for this purpose. Since shoot Na⁺/K⁺ is a key component of salt tolerance, RNAi-mediated knockdown isogenic lines obtained for Solanum galapagense alleles encoding both class I Na⁺ transporters HKT1;1 and HKT1;2 were used to investigate the silencing effects on the Na and K contents of the xylem sap, and source and sink organs of the scion, and their contribution to salt tolerance in all 16 rootstock/scion combinations of non-silenced and silenced lines, under two salinity treatments. The results show that SgHKT1;1 is operating differently from SgHKT1;2 regarding Na circulation in the tomato vascular system under salinity. A model was built to show that using silenced SgHKT1;1 line as rootstock would improve salt tolerance and fruit quality of varieties carrying the wild type SgHKT1;2 allele. Moreover, this increasing effect on both yield and fruit soluble solids content of silencing SgHKT1;1 could explain that a low expressing HKT1;1 variant was fixed in S. lycopersicum during domestication, and the paradox of increasing agronomic salt tolerance through silencing the HKT1;1 allele from S. galapagense, a salt adapted species.

Four plasma membrane-localized MGR transporters mediate xylem Mg2+ loading for root-to-shoot Mg2+ translocation in Arabidopsis

Magnesium (Mg²⁺), an essential structural component of chlorophyll, is absorbed from the soil by roots and transported to shoots to support photosynthesis in plants. However, the molecular mechanisms underlying root-to-shoot Mg²⁺ translocation remain largely unknown. We describe here the identification of four plasma membrane (PM)-localized transporters, named Mg²⁺ release transporters (MGRs), that are critical for root-to-shoot Mg transport in Arabidopsis. Functional complementation assays in a Mg²⁺-uptake-deficient bacterial strain confirmed that these MGRs conduct Mg²⁺ transport. PM-localized MGRs (MGR4, MGR5, MGR6, and MGR7) were expressed primarily in root stellar cells and participated in the xylem loading step of the long-distance Mg²⁺ transport process. In particular, MGR4 and MGR6 played a major role in shoot Mg homeostasis, as their loss-of-function mutants were hypersensitive to low Mg²⁺ but tolerant to high Mg²⁺ conditions. Reciprocal grafting analysis further demonstrated that MGR4 functions in the root to determine shoot Mg²⁺ accumulation and physiological phenotypes caused by both low- and high-Mg²⁺ stress. Taken together, our study has identified the long-sought transporters responsible for root-to-shoot Mg²⁺ translocation in plants.

Potential Risks of PM2.5-Bound Polycyclic Aromatic Hydrocarbons and Heavy Metals from Inland and Marine Directions for a Marine Background Site in North China

Ambient PM_2.5-bound ions, OC, EC, heavy metals (HMs), 18 polycyclic aromatic hydrocarbons (PAHs), 7 hopanes, and 29 n-alkanes were detected at Tuoji Island (TI), the only marine background atmospheric monitoring station in North China. The annual PM_2.5 average concentration was 47 ± 31 μg m^-3, and the average concentrations of the compositions in PM_2.5 were higher in cold seasons than in warm seasons. The cancer and non-cancer risks of HMs and PAHs in cold seasons were also higher than in warm seasons. BaP, Ni, and As dominated the ∑HQ (hazard quotient) in cold seasons, while the non-carcinogenic risk in warm seasons was mainly dominated by Ni, Mn, and As. The ILCR (incremental lifetime cancer risk) values associated with Cr and As were higher in the cold season, while ILCR-Ni values were higher in the warm season. The backward trajectory was calculated to identify the potential directions of air mass at TI. Through the diagnostic ratios of organic and inorganic tracers, the sources of particulate matter in different directions were judged. It was found that ship emissions and sea salt were the main sources from marine directions, while coal combustion, vehicles emissions, industrial process, and secondary aerosols were the main source categories for inland directions. In addition, potential HM and PAH risks from inland and marine directions were explored. The non-cancerous effects of TI were mainly affected by inland transport, especially from the southeast, northwest, and west-northwest. The cancerous effects of TI were mainly simultaneously affected by the inland direction and marine direction of transport.

Root-to-Shoot Long-Distance Mobile miRNAs Identified from Nicotiana Rootstocks

Root-derived mobile signals play critical roles in coordinating a shoot's response to underground conditions. However, the identification of root-to-shoot long-distance mobile signals has been scant. In this study, we aimed to characterize root-to-shoot endogenous mobile miRNAs by using an Arabidopsis/Nicotiana interfamilial heterograft in which these two taxonomically distant species with clear genetic backgrounds had sufficient diversity in differentiating miRNA sources. Small RNA deep sequencing analysis revealed that 82 miRNAs from the Arabidopsis scion could travel through the graft union to reach the rootstock, whereas only a very small subset of miRNA (6 miRNAs) preferred the root-to-shoot movement. We demonstrated in an ex vivo RNA imaging experiment that the root-to-shoot mobile Nb-miR164, Nb-miR395 and Nb-miR397 were targeted to plasmodesmata using the bacteriophage coat protein MS2 system. Furthermore, the Nb-miR164 was shown to move from the roots to the shoots to induce phenotypic changes when its overexpressing line was used as rootstock, strongly supporting that root-derived Nb-miR164 was able to modify the scion trait via its long-distance movement.

A Narrowband Spintronic Terahertz Emitter Based on Magnetoelastic Heterostructures

Narrowband terahertz (THz) radiation is crucial for high-resolution spectral identification, but a narrowband THz source driven by a femtosecond (fs) laser has remained scarce. Here, it is computationally predicted that a metal/dielectric/magnetoelastic heterostructure enables converting a fs laser pulse into a multicycle THz pulse with a narrow linewidth down to ∼1.5 GHz, which is in contrast to the single-cycle, broadband THz pulse from the existing fs-laser-excited emitters. It is shown that such narrowband THz pulse originates from the excitation and long-distance transport of THz spin waves in the magnetoelastic film, which can be enabled by a short strain pulse obtained from fs laser irradiation of the metal film when the thicknesses of the metal and magnetoelastic films both fall into a specific range. These results therefore reveal an approach to achieving optical generation of narrowband THz pulse based on heterostructure design, which also has implications in the design of THz magnonic devices.

Long-Distance Transport of Finisher Pigs in the Iberian Peninsula: Effects of Season on Thermal and Enthalpy Conditions, Welfare Indicators and Meat pH

Current legislation in the European Union places limits on live pig transport according to outside temperature, but less is known about the effects of sudden changes in the thermal microenvironment in trailers, particularly during long-distance transport. In this study, we measured the temperature and relative humidity inside livestock vehicles carrying 1920 Spanish finisher pigs (live weight 100 kg and 240 animals per journey) during eight long-distance (>15 h) commercial journeys to slaughter from northern Spain to Portugal in the summer and winter. Here, we report the rate of change in the air temperature (°C × min^-1) and air enthalpies in the transport vehicle (kg water kg dry air-1). At sticking, blood samples were taken for to measure cortisol, glucose, and creatine kinase (CK) as stress response indicators, and the meat pH after 45 min and the pH after 24 h were also determined. The rate of change in the air temperature and enthalpy was higher inside the livestock vehicle during the winter months and was positively related with higher cortisol and glucose levels and lower pH after 45 min (p < 0.05). It is proposed that the rate of temperature change and air enthalpy represent useful integrated indices of thermal stress for pigs during transport.

Asparagine accumulation in chicory storage roots is controlled by translocation and feedback regulation of asparagine biosynthesis in leaves

The presence of acrylamide (AA), a potentially carcinogenic and neurotoxic compound, in food has become a major concern for public health. AA in plant-derived food mainly arises from the reaction of the amino acid asparagine (Asn) and reducing sugars during processing of foodstuffs at high temperature. Using a selection of genotypes from the chicory (Cichorium intybus L.) germplasm, we performed Asn measurements in storage roots and leaves to identify genotypes contrasting for Asn accumulation. We combined molecular analysis and grafting experiments to show that leaf to root translocation controls Asn biosynthesis and accumulation in chicory storage roots. We could demonstrate that Asn accumulation in storage roots depends on Asn biosynthesis and transport from the leaf, and that a negative feedback loop by Asn on CiASN1 expression impacts Asn biosynthesis in leaves. Our results provide a new model for Asn biosynthesis in root crop species and highlight the importance of characterizing and manipulating Asn transport to reduce AA content in processed plant-based foodstuffs.

It's in the bag: mobile containers in human evolution and child development

Mobile containers are a keystone human innovation. Ethnographic data indicate that all human groups use containers such as bags, quivers and baskets, ensuring that individuals have important resources at the ready and are prepared for opportunities and threats before they materialize. Although there is speculation surrounding the invention of carrying devices, the current hard archaeological evidence only reaches back some 100,000 years. The dearth of ancient evidence may reflect not only taphonomic processes, but also a lack of attention to these devices. To begin investigating the origins of carrying devices we focus on exploring the basic cognitive processes involved in mobile container use and report an initial study on young children's understanding and deployment of such devices. We gave 3- to 7-year-old children (N = 106) the opportunity to spontaneously identify and use a basket to increase their own carrying capacity and thereby obtain more resources in the future. Performance improved linearly with age, as did the likelihood of recognizing that adults use mobile carrying devices to increase carrying capacity. We argue that the evolutionary and developmental origins of mobile containers reflect foundational cognitive processes that enable humans to think about their own limits and compensate for them.

Long-Distance Movement of Mineral Deficiency-Responsive mRNAs in Nicotiana Benthamiana/Tomato Heterografts

Deficiencies in essential mineral nutrients such as nitrogen (N), phosphorus (P), and iron (Fe) severely limit plant growth and crop yield. It has been discovered that both the local sensing system in roots and shoot-to-root systemic signaling via the phloem are involved in the regulation of the adaptive alterations in roots, in response to mineral deficiency. mRNAs are one group of molecules with systemic signaling functions in response to intrinsic and environmental cues; however, the importance of shoot-to-root mobile mRNAs stimulated by low mineral levels is not fully understood. In this study, we established a Nicotiana benthamiana/tomato heterograft system to identify shoot-to-root mobile mRNAs that are produced in response to low N, P or Fe. Multiple long-distance mobile mRNAs were identified to be associated with low mineral levels and a few of them may play important roles in hormonal metabolism and root architecture alteration. A comparison of the mobile mRNAs from our study with those identified from previous studies showed that very few transcripts are conserved among different species.

Coordinated Transport of Nitrate, Potassium, and Sodium

Potassium (K+) and nitrogen (N) are essential nutrients, and their absorption and distribution within the plant must be coordinated for optimal growth and development. Potassium is involved in charge balance of inorganic and organic anions and macromolecules, control of membrane electrical potential, pH homeostasis and the regulation of cell osmotic pressure, whereas nitrogen is an essential component of amino acids, proteins, and nucleic acids. Nitrate (NO3 -) is often the primary nitrogen source, but it also serves as a signaling molecule to the plant. Nitrate regulates root architecture, stimulates shoot growth, delays flowering, regulates abscisic acid-independent stomata opening, and relieves seed dormancy. Plants can sense K+/NO3 - levels in soils and adjust accordingly the uptake and root-to-shoot transport to balance the distribution of these ions between organs. On the other hand, in small amounts sodium (Na+) is categorized as a "beneficial element" for plants, mainly as a "cheap" osmolyte. However, at high concentrations in the soil, Na+ can inhibit various physiological processes impairing plant growth. Hence, plants have developed specific mechanisms to transport, sense, and respond to a variety of Na+ conditions. Sodium is taken up by many K+ transporters, and a large proportion of Na+ ions accumulated in shoots appear to be loaded into the xylem by systems that show nitrate dependence. Thus, an adequate supply of mineral nutrients is paramount to reduce the noxious effects of salts and to sustain crop productivity under salt stress. In this review, we will focus on recent research unraveling the mechanisms that coordinate the K+-NO3 -; Na+-NO3 -, and K+-Na+ transports, and the regulators controlling their uptake and allocation.

Theanine transporters identified in tea plants (Camellia sinensis L.)

Theanine, a unique non-proteinogenic amino acid, is an important component of tea, as it confers the umami taste and relaxation effect of tea as a beverage. Theanine is primarily synthesized in tea roots and is subsequently transported to young shoots, which are harvested for tea production. Currently, the mechanism for theanine transport in the tea plant remains unknown. Here, by screening a yeast mutant library, followed by functional analyses, we identified the glutamine permease, GNP1 as a specific transporter for theanine in yeast. Although there is no GNP1 homolog in the tea plant, we assessed the theanine transport ability of nine tea plant amino acid permease (AAP) family members, with six exhibiting transport activity. We further determined that CsAAP1, CsAAP2, CsAAP4, CsAAP5, CsAAP6, and CsAAP8 exhibited moderate theanine affinities and transport was H⁺ -dependent. The tissue-specific expression of these six CsAAPs in leaves, vascular tissues, and the root suggested their broad roles in theanine loading and unloading from the vascular system, and in targeting to sink tissues. Furthermore, expression of these CsAAPs was shown to be seasonally regulated, coincident with theanine transport within the tea plant. Finally, CsAAP1 expression in the root was highly correlated with root-to-bud transport of theanine, in seven tea plant cultivars. Taken together, these findings support the hypothesis that members of the CsAAP family transport theanine and participate in its root-to-shoot delivery in the tea plant.

Mechanisms for minimizing height-related stomatal conductance declines in tall vines

The ability to transport water through tall stems hydraulically limits stomatal conductance (g_s ), thereby constraining photosynthesis and growth. However, some plants are able to minimize this height-related decrease in g_s , regardless of path length. We hypothesized that kudzu (Pueraria lobata) prevents strong declines in g_s with height through appreciable structural and hydraulic compensative alterations. We observed only a 12% decline in maximum g_s along 15-m-long stems and were able to model this empirical trend. Increasing resistance with transport distance was not compensated by increasing sapwood-to-leaf-area ratio. Compensating for increasing leaf area by adjusting the driving force would require water potential reaching -1.9 MPa, far below the wilting point (-1.2 MPa). The negative effect of stem length was compensated for by decreasing petiole hydraulic resistance and by increasing stem sapwood area and water storage, with capacitive discharge representing 8-12% of the water flux. In addition, large lateral (petiole, leaves) relative to axial hydraulic resistance helped improve water flow distribution to top leaves. These results indicate that g_s of distal leaves can be similar to that of basal leaves, provided that resistance is highest in petioles, and sufficient amounts of water storage can be used to subsidize the transpiration stream.

Quinolizidine alkaloids are transported to seeds of bitter narrow-leafed lupin

Narrow-leafed lupin (NLL, Lupinus angustifolius) is a promising legume crop that produces seeds with very high protein content. However, NLL accumulates toxic quinolizidine alkaloids (QAs) in most of its tissues, including the seeds. To determine the level of in situ biosynthesis in the seeds, we compared the accumulation of QAs with the expression of the biosynthetic gene lysine decarboxylase (LDC) in developing seeds and pods of a bitter (high-QA) variety of NLL. While QAs accumulated steadily in seeds until the drying phase, LDC expression was comparatively very low throughout seed development. In contrast, both QA accumulation and LDC expression peaked early in pods and decreased subsequently, reaching background levels at the onset of drying. We complemented these studies with MS imaging, which revealed the distribution patterns of individual QAs in cross-sections of pods and seeds. Finally, we show that a paternal bitter genotype does not influence the QA levels of F1 seeds grown on a maternal, low-QA genotype. We conclude that the accumulation of QAs in seeds of bitter NLL is mostly, if not exclusively, transported from other tissues. These results open the possibility of using transport engineering to generate herbivore-resistant bitter NLL varieties that produce QA-free seeds.

m5C Methylation Guides Systemic Transport of Messenger RNA over Graft Junctions in Plants

In plants, transcripts move to distant body parts to potentially act as systemic signals regulating development and growth. Thousands of messenger RNAs (mRNAs) are transported across graft junctions via the phloem to distinct plant parts. Little is known regarding features, structural motifs, and potential base modifications of transported transcripts and how these may affect their mobility. We identified Arabidopsis thaliana mRNAs harboring the modified base 5-methylcytosine (m⁵C) and found that these are significantly enriched in mRNAs previously described as mobile, moving over graft junctions to distinct plant parts. We confirm this finding with graft-mobile methylated mRNAs TRANSLATIONALLY CONTROLLED TUMOR PROTEIN 1 (TCTP1) and HEAT SHOCK COGNATE PROTEIN 70.1 (HSC70.1), whose mRNA transport is diminished in mutants deficient in m⁵C mRNA methylation. Together, our results point toward an essential role of cytosine methylation in systemic mRNA mobility in plants and that TCTP1 mRNA mobility is required for its signaling function.

Regulation of K+ Nutrition in Plants

Modern agriculture relies on mineral fertilization. Unlike other major macronutrients, potassium (K+) is not incorporated into organic matter but remains as soluble ion in the cell sap contributing up to 10% of the dry organic matter. Consequently, K+ constitutes a chief osmoticum to drive cellular expansion and organ movements, such as stomata aperture. Moreover, K+ transport is critical for the control of cytoplasmic and luminal pH in endosomes, regulation of membrane potential, and enzyme activity. Not surprisingly, plants have evolved a large ensemble of K+ transporters with defined functions in nutrient uptake by roots, storage in vacuoles, and ion translocation between tissues and organs. This review describes critical transport proteins governing K+ nutrition, their regulation, and coordinated activity, and summarizes our current understanding of signaling pathways activated by K+ starvation.

Drought impacts on tree phloem: from cell-level responses to ecological significance

On-going climate change is increasing the risk of drought stress across large areas worldwide. Such drought events decrease ecosystem productivity and have been increasingly linked to tree mortality. Understanding how trees respond to water shortage is key to predicting the future of ecosystem functions. Phloem is at the core of the tree functions, moving resources such as non-structural carbohydrates, nutrients, and defence and information molecules across the whole plant. Phloem function and ability to transport resources is tightly controlled by the balance of carbon and water fluxes within the tree. As such, drought is expected to impact phloem function by decreasing the amount of available water and new photoassimilates. Yet, the effect of drought on the phloem has received surprisingly little attention in the last decades. Here we review existing knowledge on drought impacts on phloem transport from loading and unloading processes at cellular level to possible effects on long-distance transport and consequences to ecosystems via ecophysiological feedbacks. We also point to new research frontiers that need to be explored to improve our understanding of phloem function under drought. In particular, we show how phloem transport is affected differently by increasing drought intensity, from no response to a slowdown, and explore how severe drought might actually disrupt the phloem transport enough to threaten tree survival. Because transport of resources affects other organisms interacting with the tree, we also review the ecological consequences of phloem response to drought and especially predatory, mutualistic and competitive relations. Finally, as phloem is the main path for carbon from sources to sink, we show how drought can affect biogeochemical cycles through changes in phloem transport. Overall, existing knowledge is consistent with the hypotheses that phloem response to drought matters for understanding tree and ecosystem function. However, future research on a large range of species and ecosystems is urgently needed to gain a comprehensive understanding of the question.

Regulation of K+ Nutrition in Plants

Modern agriculture relies on mineral fertilization. Unlike other major macronutrients, potassium (K⁺) is not incorporated into organic matter but remains as soluble ion in the cell sap contributing up to 10% of the dry organic matter. Consequently, K⁺ constitutes a chief osmoticum to drive cellular expansion and organ movements, such as stomata aperture. Moreover, K⁺ transport is critical for the control of cytoplasmic and luminal pH in endosomes, regulation of membrane potential, and enzyme activity. Not surprisingly, plants have evolved a large ensemble of K⁺ transporters with defined functions in nutrient uptake by roots, storage in vacuoles, and ion translocation between tissues and organs. This review describes critical transport proteins governing K⁺ nutrition, their regulation, and coordinated activity, and summarizes our current understanding of signaling pathways activated by K⁺ starvation.

Nitrogen and phosphate metabolism in ectomycorrhizas

1047 I. Introduction 1047 II. Mobilization of soil N/P by ECM fungi 1048 III. N/P uptake 1048 IV. N/P assimilation 1049 V. N/P storage and remobilization 1049 VI. Hyphal N/P efflux at the plant-fungus interface 1052 VII. Conclusion and research needs 1054 Acknowledgements 1055 References 1055 SUMMARY: Nutrient homeostasis is essential for fungal cells and thus tightly adapted to the local demand in a mycelium with hyphal specialization. Based on selected ectomycorrhizal (ECM) fungal models, we outlined current concepts of nitrogen and phosphate nutrition and their limitations, and included knowledge from Baker's yeast when major gaps had to be filled. We covered the entire pathway from nutrient mobilization, import and local storage, distribution within the mycelium and export at the plant-fungus interface. Even when nutrient import and assimilation were broad issues for ECM fungi, we focused mainly on nitrate and organic phosphorus uptake, as other nitrogen/phosphorus (N/P) sources have been covered by recent reviews. Vacuolar N/P storage and mobilization represented another focus point of this review. Vacuoles are integrated into cellular homeostasis and central for an ECM mycelium at two locations: soil-growing hyphae and hyphae of the plant-fungus interface. Vacuoles are also involved in long-distance transport. We further discussed potential mechanisms of bidirectional long-distance nutrient transport (distances from millimetres to metres). A final focus of the review was N/P export at the plant-fungus interface, where we compared potential efflux mechanisms and pathways, and discussed their prerequisites.

MeNA, Controlled by Reversible Methylation of Nicotinate, Is an NAD Precursor that Undergoes Long-Distance Transport in Arabidopsis

Nicotinamide adenine dinucleotide (NAD) biosynthesis, including synthesis from aspartate via the de novo pathway and from nicotinate (NA) via the Preiss-Handler pathway, is conserved in land plants. Diverse species of NA conjugates, which are mainly involved in NA detoxification, were also found in all tested land plants. Among these conjugates, MeNA (NA methyl ester) has been widely detected in angiosperm plants, although its physiological function and the underlying mechanism for its production in planta remain largely unknown. Here, we show that MeNA is an NAD precursor undergoing more efficient long-distance transport between organs than NA and nicotinamide in Arabidopsis. We found that Arabidopsis has one methyltransferase (designated AtNaMT1) capable of catalyzing carboxyl methylation of NA to yield MeNA and one methyl esterase (MES2) predominantly hydrolyzing MeNA back to NA. We further uncovered that the transfer of [¹⁴C]MeNA from the root to leaf was significantly increased in both MES2 knockdown and NaMT1-overexpressing lines, suggesting that both NaMT1 and MES2 fine-tune the long-distance transport of MeNA, which is ultimately utilized for NAD production. Abiotic stress (salt, abscisic acid, and mannitol) treatments, which are known to exacerbate NAD degradation, induce the expression of NaMT1 but suppress MES2 expression, suggesting that MeNA may play a role in stress adaption. Collectively, our study indicates that reversible methylation of NA controls the biosynthesis of MeNA in Arabidopsis, which presumably functions as a detoxification form of free NA for efficient long-distance transport and eventually NAD production especially under abiotic stress, providing new insights into the relationship between NAD biosynthesis and NA conjugation in plants.

Transpiration directly regulates the emissions of water-soluble short-chained OVOCs

Most plant-based emissions of volatile organic compounds are considered mainly temperature dependent. However, certain oxygenated volatile organic compounds (OVOCs) have high water solubility; thus, also stomatal conductance could regulate their emissions from shoots. Due to their water solubility and sources in stem and roots, it has also been suggested that their emissions could be affected by transport in the xylem sap. Yet further understanding on the role of transport has been lacking until present. We used shoot-scale long-term dynamic flux data from Scots pines (Pinus sylvestris) to analyse the effects of transpiration and transport in xylem sap flow on emissions of 3 water-soluble OVOCs: methanol, acetone, and acetaldehyde. We found a direct effect of transpiration on the shoot emissions of the 3 OVOCs. The emissions were best explained by a regression model that combined linear transpiration and exponential temperature effects. In addition, a structural equation model indicated that stomatal conductance affects emissions mainly indirectly, by regulating transpiration. A part of the temperature's effect is also indirect. The tight coupling of shoot emissions to transpiration clearly evidences that these OVOCs are transported in the xylem sap from their sources in roots and stem to leaves and to ambient air.

Long-Distance Transport of Prosystemin Messenger RNA in Tomato

Main conclusion: The transcripts of transgenic prosystemin (PS) gene are mobile and the PS mRNA can be translated into protein in tomato and tobacco plants. Systemin (SYS) and its precursor protein, prosystemin (PS), are upstream components of the wound-induced signaling pathway in tomato. Although the mobile signal(s) for wound responses has been the subject of considerable research, its identity remains controversial. Intensive studies have revealed the essential role of mRNA on plant systemic signaling. We hypothesize that PS mRNA can act as a transmissible signal in tomato. Herein, we demonstrated that transgenic PS mRNA occurs in leaves located at considerable distances from the initial site of its generation by a transient Agrobacterium-infiltration assay system. We also showed that PS protein is present in the vascular bundle of the distant leaves. Our results indicate that transgenic PS mRNA may be functional as a long-distance signal to modulate systemic defense responses in tomato, providing novel insights into the multifaceted systems by which SYS signaling transports.

Chloride on the Move

Chloride (Cl^-) is an essential plant nutrient but under saline conditions it can accumulate to toxic levels in leaves; limiting this accumulation improves the salt tolerance of some crops. The rate-limiting step for this process - the transfer of Cl^- from root symplast to xylem apoplast, which can antagonize delivery of the macronutrient nitrate (NO₃^-) to shoots - is regulated by abscisic acid (ABA) and is multigenic. Until recently the molecular mechanisms underpinning this salt-tolerance trait were poorly defined. We discuss here how recent advances highlight the role of newly identified transport proteins, some that directly transfer Cl^- into the xylem, and others that act on endomembranes in 'gatekeeper' cell types in the root stele to control root-to-shoot delivery of Cl^-.

SLAH1, a homologue of the slow type anion channel SLAC1, modulates shoot Cl- accumulation and salt tolerance in Arabidopsis thaliana

Salinity tolerance is correlated with shoot chloride (Cl(-)) exclusion in multiple crops, but the molecular mechanisms of long-distance Cl(-) transport are poorly defined. Here, we characterize the in planta role of AtSLAH1 (a homologue of the slow type anion channel-associated 1 (SLAC1)). This protein, localized to the plasma membrane of root stelar cells, has its expression reduced by salt or ABA, which are key predictions for a protein involved with loading Cl(-) into the root xylem. Artificial microRNA knockdown mutants of AtSLAH1 had significantly reduced shoot Cl(-) accumulation when grown under low Cl(-), whereas shoot Cl(-) increased and the shoot nitrate/chloride ratio decreased following AtSLAH1 constitutive or stelar-specific overexpression when grown in high Cl(-) In both sets of overexpression lines a significant reduction in shoot biomass over the null segregants was observed under high Cl(-) supply, but not low Cl(-) supply. Further in planta data showed AtSLAH3 overexpression increased the shoot nitrate/chloride ratio, consistent with AtSLAH3 favouring nitrate transport. Heterologous expression of AtSLAH1 in Xenopus laevis oocytes led to no detectible transport, suggesting the need for post-translational modifications for AtSLAH1 to be active. Our in planta data are consistent with AtSLAH1 having a role in controlling root-to-shoot Cl(-) transport.

The gelatinous extracellular matrix facilitates transport studies in kelp: visualization of pressure-induced flow reversal across sieve plates

BACKGROUND AND AIMS

In vascular plants, important questions regarding phloem function remain unanswered due to problems with invasive experimental procedures in this highly sensitive tissue. Certain brown algae (kelps; Laminariales) also possess sieve tubes for photoassimilate transport, but these are embedded in large volumes of a gelatinous extracellular matrix which isolates them from neighbouring cells. Therefore, we hypothesized that kelp sieve tubes might tolerate invasive experimentation better than their analogues in higher plants, and sought to establish Nereocystis luetkeana as an experimental system.

METHODS

The predominant localization of cellulose and the gelatinous extracellular matrix in N. luetkeana was verified using specific fluorescent markers and confocal laser scanning microscopy. Sieve tubes in intact specimens were loaded with fluorescent dyes, either passively (carboxyfluorescein diacetate; CFDA) or by microinjection (rhodamine B), and the movement of the dyes was monitored by fluorescence microscopy.

KEY RESULTS

Application of CFDA demonstrated source to sink bulk flow in N. luetkeana sieve tubes, and revealed the complexity of sieve tube structure, with branches, junctions and lateral connections. Microinjection into sieve elements proved comparatively easy. Pulsed rhodamine B injection enabled the determination of flow velocity in individual sieve elements, and the direct visualization of pressure-induced reversals of flow direction across sieve plates.

CONCLUSIONS

The reversal of flow direction across sieve plates by pressurizing the downstream sieve element conclusively demonstrates that a critical requirement of the Münch theory is satisfied in kelp; no such evidence exists for tracheophytes. Because of the high tolerance of its sieve elements to experimental manipulation, N. luetkeana is a promising alternative to vascular plants for studying the fluid mechanics of sieve tube networks.

FE, a phloem-specific Myb-related protein, promotes flowering through transcriptional activation of FLOWERING LOCUS T and FLOWERING LOCUS T INTERACTING PROTEIN 1

In many flowering plants, the transition to flowering is primarily affected by seasonal changes in day length (photoperiod). An inductive photoperiod promotes flowering via synthesis of a floral stimulus, called florigen. In Arabidopsis thaliana, the FLOWERING LOCUS T (FT) protein is an essential component of florigen, which is synthesized in leaf phloem companion cells and is transported through phloem tissue to the shoot apical meristem where floral morphogenesis is initiated. However, the molecular mechanism involved in the long-distance transport of FT protein remains elusive. In this study, we characterized the classic Arabidopsis mutant fe, which is involved in the photoperiodic induction of flowering, and showed that FE encodes a phloem-specific Myb-related protein that was previously reported as ALTERED PHLOEM DEVELOPMENT. Phenotypic analyses of the fe mutant showed that FT expression is reduced in leaf phloem companion cells. In addition, the transport of FT protein from leaves to the shoot apex is impaired in the fe mutant. Expression analyses further demonstrated that FE is also required for transcriptional activation of FLOWERING LOCUS T INTERACTING PROTEIN 1 (FTIP1), an essential regulator for selective trafficking of the FT protein from companion cells to sieve elements. These findings indicate that FE plays a dual role in the photoperiodic induction of flowering: as a transcriptional activator of FT on the one hand, and its transport machinery component, FTIP1, on the other hand. Thus, FE is likely to play a role in regulating FT by coordinating FT synthesis and FT transport in phloem companion cells.

ZxNHX controls Na⁺ and K⁺ homeostasis at the whole-plant level in Zygophyllum xanthoxylum through feedback regulation of the expression of genes involved in their transport

BACKGROUND AND AIMS

In order to cope with arid environments, the xerohalophyte Zygophyllum xanthoxylum efficiently compartmentalizes Na(+) into vacuoles, mediated by ZxNHX, and maintains stability of K(+) in its leaves. However, the function of ZxNHX in controlling Na(+) and K(+) homeostasis at the whole-plant level remains unclear. In this study, the role of ZxNHX in regulating the expression of genes involved in Na(+) and K(+) transport and spatial distribution was investigated.

METHODS

The role of ZxNHX in maintaining Na(+) and K(+) homeostasis in Z. xanthoxylum was studied using post-transcriptional gene silencing via  Agrobacterium-mediated transformation. Transformed plants were grown with or without 50 mm NaCl, and expression levels and physiological parameters were measured.

KEY RESULTS

It was found that 50 mm NaCl induced a 620 % increase in transcripts of ZxSOS1 but only an 80 % increase in transcripts of ZxHKT1;1 in roots of wild-type (WT) plants. Consequently, the ability of ZxSOS1 to transport Na(+) exceeded that of ZxHKT1;1, and Na(+) was loaded into the xylem by ZxSOS1 and delivered to the shoots. However, in a ZxNHX-silenced line (L7), the capacity to sequester Na(+) into vacuoles of leaves was weakened, which in turn regulated long-distance Na(+) transport from roots to shoots. In roots of L7, NaCl (50 mm) increased transcripts of ZxSOS1 by only 10 %, whereas transcripts of ZxHKT1;1 increased by 53 %. Thus, in L7, the transport ability of ZxHKT1;1 for Na(+) outweighed that of ZxSOS1. Na(+) was unloaded from the xylem stream, consequently reducing Na(+) accumulation and relative distribution in leaves, but increasing the relative distribution of Na(+) in roots and the net selective transport capacity for K(+) over Na(+) from roots to shoots compared with the WT. Silencing of ZxNHX also triggered a downregulation of  ZxAKT1 and ZxSKOR in roots, resulting in a significant decrease in K(+) accumulation in all the tissues in plants grown in 50 mm NaCl. These changes led to a significant reduction in osmotic adjustment, and thus an inhibition of growth in ZxNHX-silenced lines.

CONCLUSIONS

The results suggest that ZxNHX is essential for controlling Na(+), K(+) uptake, long-distance transport and their homeostasis at whole-plant level via feedback regulation of the expression of genes involved in Na(+), K(+) transport. The net result is the maintenance of the characteristic salt accumulation observed in Z. xanthoxylum and the regulation of its normal growth. A model is proposed for the role of ZxNHX in regulating the Na(+) transport system in Z. xanthoxylum under saline conditions.

Dynamics of leaf gas exchange, xylem and phloem transport, water potential and carbohydrate concentration in a realistic 3-D model tree crown

BACKGROUND AND AIMS

Tree models simulate productivity using general gas exchange responses and structural relationships, but they rarely check whether leaf gas exchange and resulting water and assimilate transport and driving pressure gradients remain within acceptable physical boundaries. This study presents an implementation of the cohesion-tension theory of xylem transport and the Münch hypothesis of phloem transport in a realistic 3-D tree structure and assesses the gas exchange and transport dynamics.

METHODS

A mechanistic model of xylem and phloem transport was used, together with a tested leaf assimilation and transpiration model in a realistic tree architecture to simulate leaf gas exchange and water and carbohydrate transport within an 8-year-old Scots pine tree. The model solved the dynamics of the amounts of water and sucrose solute in the xylem, cambium and phloem using a fine-grained mesh with a system of coupled ordinary differential equations.

KEY RESULTS

The simulations predicted the observed patterns of pressure gradients and sugar concentration. Diurnal variation of environmental conditions influenced tree-level gradients in turgor pressure and sugar concentration, which are important drivers of carbon allocation. The results and between-shoot variation were sensitive to structural and functional parameters such as tree-level scaling of conduit size and phloem unloading.

CONCLUSIONS

Linking whole-tree-level water and assimilate transport, gas exchange and sink activity opens a new avenue for plant studies, as features that are difficult to measure can be studied dynamically with the model. Tree-level responses to local and external conditions can be tested, thus making the approach described here a good test-bench for studies of whole-tree physiology.

Abscisic acid alleviates iron deficiency by promoting root iron reutilization and transport from root to shoot in Arabidopsis

Abscisic acid (ABA) has been demonstrated to be involved in iron (Fe) homeostasis, but the underlying mechanism is largely unknown. Here, we found that Fe deficiency induced ABA accumulation rapidly (within 6 h) in the roots of Arabidopsis. Exogenous ABA at 0.5 μM decreased the amount of root apoplastic Fe bound to pectin and hemicellulose, and increased the shoot Fe content significantly, thus alleviating Fe deficiency-induced chlorosis. Exogenous ABA promoted the secretion of phenolics to release apoplastic Fe and up-regulated the expression of AtNRAMP3 to enhance reutilization of Fe stored in the vacuoles, leading to a higher level of soluble Fe and lower ferric-chelate reductase (FCR) activity in roots. Treatment with ABA also led to increased Fe concentrations in the xylem sap, partially because of the up-regulation of AtFRD3, AtYSL2 and AtNAS1, genes related to long-distance transport of Fe. Exogenous ABA could not alleviate the chlorosis of abi5 mutant resulting from the significantly low expression of AtYSL2 and low transport of Fe from root to shoot. Taken together, our data support the conclusion that ABA is involved in the reutilization and transport of Fe from root to shoot under Fe deficiency conditions in Arabidopsis.

Characteristics of Loads of Cattle Stopping for Feed, Water and Rest during Long-Distance Transport in Canada

This study is the first comprehensive examination of long-haul cattle being transported across Canada and off-loaded for feed, water and rest. A total of 129 truckloads were observed at one of two commercial rest stations near Thunder Bay, Ontario. Data collected included information regarding the truck driver, the trailer, the trip, the animals and animal handling. The majority of the loads stopping were feeder calves (60.94%) while 21.09% were weaned calves, and the remaining 14.84% were market weight cattle. The truck loads surveyed were in transit for, on average, 28.2 ± 5.0 hours before stopping and cattle were rested for an average of 11.2 ± 2.8 hours. These data suggest that loads stopping at the rest station were adhering to the regulations stated in the Health of Animals Act, which outline a maximum of 48 hours in transit before a mandatory stop of at least 5 hours for feed, water and rest. There was a large amount of variability around how well recommendations, such as stocking density were followed. Further research is required to assess how well cattle are coping with long-distance transport under current regulations and industry practices.

Holocene dynamics of the Florida Everglades with respect to climate, dustfall, and tropical storms

Aeolian dust is rarely considered an important source for nutrients in large peatlands, which generally develop in moist regions far from the major centers of dust production. As a result, past studies assumed that the Everglades provides a classic example of an originally oligotrophic, P-limited wetland that was subsequently degraded by anthropogenic activities. However, a multiproxy sedimentary record indicates that changes in atmospheric circulation patterns produced an abrupt shift in the hydrology and dust deposition in the Everglades over the past 4,600 y. A wet climatic period with high loadings of aeolian dust prevailed before 2800 cal BP (calibrated years before present) when vegetation typical of a deep slough dominated the principal drainage outlet of the Everglades. This dust was apparently transported from distant source areas, such as the Sahara Desert, by tropical storms according to its elemental chemistry and mineralogy. A drier climatic regime with a steep decline in dustfall persisted after 2800 cal BP maintaining sawgrass vegetation at the coring site as tree islands developed nearby (and pine forests covered adjacent uplands). The marked decline in dustfall was related to corresponding declines in sedimentary phosphorus, organic nitrogen, and organic carbon, suggesting that a close relationship existed between dustfall, primary production, and possibly, vegetation patterning before the 20th century. The climatic change after 2800 cal BP was probably produced by a shift in the Bermuda High to the southeast, shunting tropical storms to the south of Florida into the Gulf of Mexico.

Phloem long-distance delivery of FLOWERING LOCUS T (FT) to the apex

Cucurbita moschata FLOWERING LOCUS T-LIKE 2 (hereafter FTL2) and Arabidopsis thaliana (Arabidopsis) FLOWERING LOCUS T (FT), components of the plant florigenic signaling system, move long-distance through the phloem from source leaves to the vegetative apex where they mediate floral induction. The mechanisms involved in long-distance trafficking of FT/FTL2 remain to be elucidated. In this study, we identified the critical motifs on both FT and FTL2 required for cell-to-cell trafficking through mutant analyses using a zucchini yellow mosaic virus expression vector. Western blot analysis, performed on phloem sap collected from just beneath the vegetative apex of C. moschata plants, established that all mutant proteins tested retained the ability to enter the phloem translocation stream. However, immunolocalization studies revealed that a number of these FTL2/FT mutants were defective in the post-phloem zone, suggesting that a regulation mechanism for FT trafficking exists in the post-phloem unloading step. The selective movements of FT/FTL2 were further observed by microinjection and trichome rescue studies, which revealed that FT/FTL2 has the ability to dilate plasmodesmata microchannels during the process of cell-to-cell trafficking, and various mutants were compromised in their capacity to traffic through plasmodesmata. Based on these findings, a model is presented to account for the mechanism by which FT/FTL2 enters the phloem translocation stream and subsequently exits the phloem and enters the apical tissue, where it initiates the vegetative to floral transition.

Identification of lipids and lipid-binding proteins in phloem exudates from Arabidopsis thaliana

The phloem plays a crucial role in assimilate and nutrient transport, pathogen response, and plant growth and development. Yet, few species have yielded pure phloem exudate and, if proteins need to be analysed, those species may not have sequenced genomes, making identification difficult. The enrichment of Arabidopsis thaliana phloem exudate in amounts large enough to allow for metabolite and protein analysis is described. Using this method, it was possible to identify 65 proteins present in the Arabidopsis phloem exudate. The majority of these proteins could be grouped by response to pathogens, stress, or hormones, carbon metabolism, protein interaction, modification, and turnover, and transcription factors. It was also possible to detect 11 proteins that play a role in lipid/fatty acid metabolism (aspartic protease, putative 3-β-hydroxysteroid dehydrogenase, UDP-sulphoquinovose synthase/SQD1, lipase, PIG-P-like protein: phosphatidylinositol-N-acetylglucosaminyltransferase), storage (glycine-rich protein), binding (annexin, lipid-associated family protein, GRP17/oleosin), and/or signalling (annexin, putative lipase, PIG-P-like protein). Along with putative lipid-binding proteins, several lipids and fatty acids could be identified. Only a few examples exist of lipids (jasmonic acid, oxylipins) or lipid-binding proteins (DIR1, acyl-CoA-binding protein) in the phloem. Finding hydrophobic compounds in an aqueous environment is not without precedence in biological systems: human blood contains a variety of lipids, many of which play a significant role in human health. In blood, lipids are transported while bound to proteins. The present findings of lipids and lipid-binding proteins in phloem exudates suggest that a similar long-distance lipid signalling exists in plants and may play an important role in plant growth and development.

A role for thiol-peptides in the long-distance transport of cadmium and the effect of cadmium on iron translocation

Phytochelatins (PCs) are glutathione-derived peptides that function in heavy metal detoxification in plants and certain fungi. Recent research in Arabidopsis has shown that PCs undergo long-distance transport between roots and shoots. However, it remains unknown which tissues or vascular systems, xylem or phloem, mediate PC translocation and whether PC transport contributes to physiologically relevant long-distance transport of cadmium (Cd) between shoots and roots. To address these questions, xylem and phloem sap were obtained from Brassica napus to quantitatively analyze which thiol species are present in response to Cd exposure. High levels of PCs were identified in the phloem sap within 24 h of Cd exposure using combined mass spectrometry and fluorescence HPLC analyses. Unexpectedly, the concentration of Cd was more than four-fold higher in phloem sap compared to xylem sap. Cadmium exposure dramatically decreased iron levels in xylem and phloem sap whereas other essential heavy metals such as zinc and manganese remained unchanged. Data suggest that Cd inhibits vascular loading of iron but not nicotianamine. The high ratios [PCs]/[Cd] and [glutathione]/[Cd] in the phloem sap suggest that PCs and glutathione (GSH) can function as long-distance carriers of Cd. In contrast, only traces of PCs were detected in xylem sap. Our results suggest that, in addition to directional xylem Cd transport, the phloem is a major vascular system for long-distance source to sink transport of Cd as PC-Cd and glutathione-Cd complexes.