Turgor, solute import and growth in maize roots treated with galactose

Galactose induces formation of cell wall stubs and cell death in Arabidopsis roots

Arabidopsis seedlings growing on low concentration of galactose stop regular root growth. Incomplete cell division with cell wall stubs, binuclear and giant cells and lignified root tips are observed. Galactose is a sugar abundant in root cell walls of Arabidopsis. Nevertheless, we found that the germination of Arabidopsis seedlings on galactose containing media causes a strong modification of the root development, as shown by analysing the root with microscopy methods ranging from the bright field over confocal to transmission electron microscopy. At concentrations of about 1 mM, the growth of the primary root stops after a few days though stem cell markers like WOX5 are still expressed. The root tip swells and forms a slightly opaque, partially lignified structure in parts of the cortex and the central cylinder. The formation of the cell plate after mitosis is impaired, often leading to cell wall stubs and binuclear cells. Some cells in the cortex and the central cylinder degenerate, while some rhizodermal and cortical cells increase massively in size. The galactose toxicity phenotype in Arabidopsis depends on the activity of galactokinase and is completely diminished in galactokinase knock-out lines. From the comparison of the galactose toxicity phenotype with those of cytokinesis mutants and plants treated with appropriate inhibitors we speculate that the toxicity syndrome of galactose is caused by interference with intracellular vesicle transport or cell wall biogenesis.

Sucrose Metabolism and Transport in Grapevines, with Emphasis on Berries and Leaves, and Insights Gained from a Cross-Species Comparison

In grapevines, as in other plants, sucrose and its constituents glucose and fructose are fundamentally important and carry out a multitude of roles. The aims of this review are three-fold. First, to provide a summary of the metabolism and transport of sucrose in grapevines, together with new insights and interpretations. Second, to stress the importance of considering the compartmentation of metabolism. Third, to outline the key role of acid invertase in osmoregulation associated with sucrose metabolism and transport in plants.

In vitro acclimatization of Curcuma longa under controlled iso-osmotic conditions

In vitro acclimatization has been validated as the successful key to harden the plantlets before transplanting to ex vitro conditions. In the present study, we investigated the potential of different sugar types (glucose, fructose, galactose, sucrose) in regulating morphological, physiological and biochemical strategies, survival percentage and growth performance, and rhizome traits of turmeric under iso-osmotic potential. Leaf greenness (SPAD value) in acclimatized plantlets (4% glucose; -1.355 MPa osmotic potential) of 'ST018' was retained and greater than in 'PB009' by 1.69-fold, leading to maintain high F_v/F_m (maximum quantum yield of PSII), Φ_PSII (photon yield of PSII) and P_n (net photosynthetic rate) levels, and retained shoot height, leaf length, leaf width, shoot fresh weight and shoot dry weight after one month upon transplanting to ex vitro conditions. In addition, P_n, C_i (intracellular CO₂), g_s (stomatal conductance) and E (transpiration rate) in acclimatized plantlets (6% sucrose; -1.355 MPa osmotic potential) of 'PB009' were stabilized as physiological adapted strategies, regulating the shoot and root growth and fresh and dry weights of mini-rhizome. Interestingly, the accumulation of total curcuminoids in mini-rhizome derived from 6% sucrose acclimatized plantlets of 'ST018' was greater than in 'PB009' by 3.76-fold. The study concludes that in vitro acclimation of turmeric 'PB009' and 'ST018' using 6% sucrose and 4% glucose, respectively, promoted percent survival, physiological adaptations, and overall growth performances under greenhouse conditions.

Leaf wounding or simulated herbivory in young N. attenuata plants reduces carbon delivery to roots and root tips

MAIN CONCLUSION

In Nicotiana attenuata seedlings, simulated herbivo ry by the specialist Manduca sexta decreases root growth and partitioning of recent photoassimilates to roots in contrast to increased partitioning reported for older plants. Root elongation rate in Nicotiana attenuata has been shown to decrease after leaf herbivory, despite reports of an increased proportion of recently mobilized photoassimilate being delivered towards the root system in many species after similar treatments. To study this apparent contradiction, we measured the distribution of recent photoassimilate within root tissues after wounding or simulated herbivory of N. attenuata leaves. We found no contradiction: herbivory reduced carbon delivery to root tips. However, the speed of phloem transport in both shoot and root, and the delivery of recently assimilated carbon to the entire root system, declined after wounding or simulated herbivory, in contrast with the often-reported increase in root partitioning. We conclude that the herbivory response in N. attenuata seedlings is to favor the shoot and not bunker carbon in the root system.

Using 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) to study carbon allocation in plants after herbivore attack

BACKGROUND

Although leaf herbivory-induced changes in allocation of recently assimilated carbon between the shoot and below-ground tissues have been described in several species, it is still unclear which part of the root system is affected by resource allocation changes and which signalling pathways are involved. We investigated carbon partitioning in root tissues following wounding and simulated leaf herbivory in young Nicotiana attenuata plants.

RESULTS

Using 2-deoxy-2-[(18)F]fluoro-D-glucose ([(18)F]FDG), which was incorporated into disaccharides in planta, we found that simulated herbivory reduced carbon partitioning specifically to the root tips in wild type plants. In jasmonate (JA) signalling-deficient COI1 plants, the wound-induced allocation of [(18)F]FDG to the roots was decreased, while more [(18)F]FDG was transported to young leaves, demonstrating an important role of the JA pathway in regulating the wound-induced carbon partitioning between shoots and roots.

CONCLUSIONS

Our data highlight the use of [(18)F]FDG to study stress-induced carbon allocation responses in plants and indicate an important role of the JA pathway in regulating wound-induced shoot to root signalling.

Radiosynthesis of 3-indolyl[1-¹¹C]acetic acid for phyto-PET-imaging: an improved production procedure and formulation method

An improved production procedure and formulation method for the carbon-11 radiolabeled phytohormone, 3-indolyl-[l-(11)C]acetic acid ([(11)C]IAA), was developed by modifying selected original reaction parameters. This updated procedure both doubled the yield (from 25.9±6.7% (n=12) to 61.0±0.3% (n=10)) and increased the concentration (0.2-0.4 GBq/0.15-0.3 mL), enabling us to provide the radiotracer [(11)C]IAA suitable for in vivo phyto-PET-imaging studies. The specific activity was improved by more than a factor of three (26.7±5.6 GBq/µmol to 82.5±36.1 GBq/µmol). The total synthesis time for both production and formulation was 81.8±3.0 min (n=10). In addition, a streamlined semi-remote controlled production system, containing five processing modules, was designed and built for routine [(11)C]IAA production. This integrated system facilitated routine high radiation level production of [(11)C]IAA while minimizing radiation exposure to the production chemists.

Does Don Fisher's high-pressure manifold model account for phloem transport and resource partitioning?

The pressure flow model of phloem transport envisaged by Münch (1930) has gained wide acceptance. Recently, however, the model has been questioned on structural and physiological grounds. For instance, sub-structures of sieve elements may reduce their hydraulic conductances to levels that impede flow rates of phloem sap and observed magnitudes of pressure gradients to drive flow along sieve tubes could be inadequate in tall trees. A variant of the Münch pressure flow model, the high-pressure manifold model of phloem transport introduced by Donald Fisher may serve to reconcile at least some of these questions. To this end, key predicted features of the high-pressure manifold model of phloem transport are evaluated against current knowledge of the physiology of phloem transport. These features include: (1) An absence of significant gradients in axial hydrostatic pressure in sieve elements from collection to release phloem accompanied by transport properties of sieve elements that underpin this outcome; (2) Symplasmic pathways of phloem unloading into sink organs impose a major constraint over bulk flow rates of resources translocated through the source-path-sink system; (3) Hydraulic conductances of plasmodesmata, linking sieve elements with surrounding phloem parenchyma cells, are sufficient to support and also regulate bulk flow rates exiting from sieve elements of release phloem. The review identifies strong circumstantial evidence that resource transport through the source-path-sink system is consistent with the high-pressure manifold model of phloem transport. The analysis then moves to exploring mechanisms that may link demand for resources, by cells of meristematic and expansion/storage sinks, with plasmodesmal conductances of release phloem. The review concludes with a brief discussion of how these mechanisms may offer novel opportunities to enhance crop biomass yields.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

The RootChip: an integrated microfluidic chip for plant science

Studying development and physiology of growing roots is challenging due to limitations regarding cellular and subcellular analysis under controlled environmental conditions. We describe a microfluidic chip platform, called RootChip, that integrates live-cell imaging of growth and metabolism of Arabidopsis thaliana roots with rapid modulation of environmental conditions. The RootChip has separate chambers for individual regulation of the microenvironment of multiple roots from multiple seedlings in parallel. We demonstrate the utility of The RootChip by monitoring time-resolved growth and cytosolic sugar levels at subcellular resolution in plants by a genetically encoded fluorescence sensor for glucose and galactose. The RootChip can be modified for use with roots from other plant species by adapting the chamber geometry and facilitates the systematic analysis of root growth and metabolism from multiple seedlings, paving the way for large-scale phenotyping of root metabolism and signaling.

Variation in salinity tolerance and shoot sodium accumulation in Arabidopsis ecotypes linked to differences in the natural expression levels of transporters involved in sodium transport

Salinity tolerance can be attributed to three different mechanisms: Na+ exclusion from the shoot, Na+ tissue tolerance and osmotic tolerance. Although several key ion channels and transporters involved in these processes are known, the variation in expression profiles and the effects of these proteins on Na+ transport in different accessions of the same species are unknown. Here, expression profiles of the genes AtHKT1;1, AtSOS1, AtNHX1 and AtAVP1 are determined in four ecotypes of Arabidopsis thaliana. Not only are these genes differentially regulated between ecotypes, the expression levels of the genes can be linked to the concentration of Na+ in the plant. An inverse relationship was found between AtSOS1 expression in the root and total plant Na+ accumulation, supporting a role for AtSOS1 in Na+ efflux from the plant. Similarly, ecotypes with high expression levels of AtHKT1;1 in the root had lower shoot Na+ concentrations, due to the hypothesized role of AtHKT1;1 in retrieval of Na+ from the transpiration stream. The inverse relationship between shoot Na+ concentration and salinity tolerance typical of most cereal crop plants was not demonstrated, but a positive relationship was found between salt tolerance and levels of AtAVP1 expression, which may be related to tissue tolerance.

Glomus intraradices induces changes in root system architecture of rice independently of common symbiosis signaling

Arbuscular mycorrhizal fungi colonize the roots of most monocotyledons and dicotyledons despite their different root architecture and cell patterning. Among the cereal hosts of arbuscular mycorrhizal fungi, Oryza sativa (rice) possesses a peculiar root system composed of three different types of roots: crown roots; large lateral roots; and fine lateral roots. Characteristic is the constitutive formation of aerenchyma in crown roots and large lateral roots and the absence of cortex from fine lateral roots. Here, we assessed the distribution of colonization by Glomus intraradices within this root system and determined its effect on root system architecture. Large lateral roots are preferentially colonized, and fine lateral roots are immune to arbuscular mycorrhizal colonization. Fungal preference for large lateral roots also occurred in sym mutants that block colonization of the root beyond rhizodermal penetration. Initiation of large lateral roots is significantly induced by G. intraradices colonization and does not require a functional common symbiosis signaling pathway from which some components are known to be needed for symbiosis-mediated lateral root induction in Medicago truncatula. Our results suggest variation of symbiotic properties among the different rice root-types and induction of the preferred tissue by arbuscular mycorrhizal fungi. Furthermore, signaling for arbuscular mycorrhizal-elicited alterations of the root system differs between rice and M. truncatula.

Jasmonic acid treatment to part of the root system is consistent with simulated leaf herbivory, diverting recently assimilated carbon towards untreated roots within an hour

It is known that shoot application of jasmonic acid (JA) leads to an increased carbon export from leaves to stem and roots, and that root treatment with JA inhibits root growth. Using the radioisotope (11)C, we measured JA effects on carbon partitioning in sterile, split-root, barley plants. JA applied to one root half reduced carbon partitioning to the JA-treated tissue within minutes, whereas the untreated side showed a corresponding--but slower--increase. This response was not observed when instead of applying JA, the sink strength of one root half was reduced by cooling it: there was no enhanced partitioning to the untreated roots. The slower response in the JA-untreated roots, and the difference between the effect of JA and temperature, suggest that root JA treatment caused transduction of a signal from the treated roots to the shoot, leading to an increase in carbon allocation from the leaves to the untreated root tissue, as was indeed observed 10 min after the shoot application of JA. This supports the hypothesis that the response of some plant species to both leaf and root herbivores may be the diversion of resources to safer locations.