Symplastic Uptake Research Articles

Root-zone regulation and longitudinal translocation cause intervarietal differences for phthalates accumulation in vegetables

Selecting and cultivating low-accumulating crop varieties (LACVs) is the most effective strategy for the safe utilization of di-(2-ethylhexyl) phthalate (DEHP)-contaminated soils, promoting cleaner agricultural production. However, the adsorption-absorption-translocation mechanisms of DEHP along the root-shoot axis remains a formidable challenge to be solved, especially for the research and application of LACV, which are rarely reported. Here, systematic analyses of the root surface ad/desorption, root apexes longitudinal allocation, uptake and translocation pathway of DEHP in LACV were investigated compared with those in a high-accumulating crop variety (HACV) in terms of the root-shoot axis. Results indicated that DEHP adsorption was enhanced in HACV by root properties, elemental composition and functional groups, but the desorption of DEHP was greater in LACV than HACV. The migration of DEHP across the root surface was controlled by the longitudinal partitioning process mediated by root tips, where more DEHP accumulated in the root cap and meristem of LACV due to greater cell proliferation. Furthermore, the longitudinal translocation of DEHP in LACV was reduced, as evidenced by an increased proportion of DEHP in the root apoplast. The symplastic uptake and xylem translocation of DEHP were suppressed more effectively in LACV than HACV, because DEHP translocation in LACV required more energy, binding sites and transpiration. These results revealed the multifaceted regulation of DEHP accumulation in different choysum (Brassica parachinensis L.) varieties and quantified the pivotal regulatory processes integral to LACV formation.

Exogenous auxin alters the polycyclic aromatic hydrocarbons apoplastic and symplastic uptake by wheat seedling roots

Polycyclic aromatic hydrocarbons (PAHs) are a category of organic pollutants known for their high carcinogenicity. Our previous research has illustrated that plant roots actively absorb PAHs through a co-transport mechanism with H+ ions. Because auxin can increase the H+-ATPase activity, the wheat roots were exposed to PAHs with/without auxins to study whether auxins facilitate the uptake of PAHs by plant roots and to gain insights into the underlying mechanisms of this process. In our study, indole acetic acid (100 μM) and α-naphthaleneacetic acid (10 μM) significantly increased the PAHs concentrations in apoplast and symplast, and the treating time and concentrations were positively correlated with PAHs accumulations. The time-dependent kinetics for 36 h followed the Elovich equation, and the concentration-dependent kinetics of apoplastic and symplastic uptake for 4 h could be described with the Freundlich and Michaelis-Menten equations, respectively. The proportion of PAHs accumulated in apoplast could be enhanced by auxins in most treatments. Our findings offer novel insights into the mechanisms of PAH uptake by plant roots under auxin exposure. Additionally, this research aids in refining strategies for ensuring crop safety and improving phytoremediation of PAH-contaminated soil and water.

Apoplastic and symplastic uptake of phenanthrene in wheat roots.

The contamination of agricultural crops by polycyclic aromatic hydrocarbons (PAHs) has drawn considerable attention due to their carcinogenicity, mutagenicity, and toxicity. However, the uptake process of PAHs in plant roots has not been clearly understood. In this work, we first study the radial uptake of phenanthrene in hydroculture wheat roots by vacuum-infiltration-centrifugation method. The concentration-dependent kinetics of apoplastic and symplastic uptake at phenanthrene concentrations of 0-6.72μM for 4h can be described with the Langmuir and Michaelis-Menten equations, respectively; whereas, their time-dependent kinetics at 5.60μM phenanthrene for 36h follow the Elovich equation. The apoplastic and symplastic uptake increases with temperature of 15-35°C. The apparent Arrhenius activation energies for apoplastic and symplastic uptake are 77.5 and 9.39KJmol-1, respectively. The symplastic uptake accounts for over 55% of total phenanthrene uptake, suggesting that symplast is the dominant pathway for wheat root phenanthrene uptake. Larger volume of symplast in roots and lower activation energy lead to the greater contribution of symplast to total uptake of phenanthrene. Our results provide not only novel insights into the mechanisms on the uptake of PAHs by plant roots, but also the help to optimize strategies for crop safety and phytoremediation of PAH-contaminated soil/water.

Response to lead pollution: mycorrhizal Pinus sylvestris forms the biomineral pyromorphite in roots and needles.

The development of mycorrhized pine seedlings grown in the presence of lead was assessed in order to investigate how higher plants can tolerate lead pollution in the environment. Examination with scanning electron microscopy (SEM) revealed that Pb uptake was prominent in the roots, while a smaller amount was found in pine needles, which requires symplastic uptake and root-to-shoot transfer. Lead was concentrated in nanocrystalline aggregates attached to the cell wall and, according to elemental microanalyses, is associated with phosphorus and chlorine. The identification of the nanocrystalline phase in roots and needles was performed by transmission electron microscopy (TEM) and synchrotron X-ray micro-diffraction (μ-XRD), revealing the presence of pyromorphite, Pb5[PO4]3(Cl, OH), in both roots and needles. The extracellular embedding of pyromorphite within plant cell walls, featuring an indented appearance of the cell wall due to a callus-like outcrop of minerals, suggests a biogenic origin. This biomineralization is interpreted as a defense mechanism of the plant against lead pollution.

How do glycine and histidine in nutrient solution affect zinc uptake and root-to-shoot translocation by wheat and triticale?

Understanding how complexes with amino acids in soil solution affect plant zinc (Zn) uptake may aid in optimising plant Zn nutrition. We investigated the influence of histidine and glycine in nutrient solution on apoplastic and symplastic uptake and root-to-shoot translocation of Zn in a triticale (×Triticosecale cv. Elinor) and a bread wheat cultivar (Triticum aestivum cv. Back Cross Rushan). Six-week-old seedlings of the two cultivars were transferred to a nutrient solution containing 20 µm Zn, to which 50 µm histidine, 50 µm glycine or no amino acids were added. Control plants were transplanted to nutrient solution with no Zn or amino acids. Higher concentrations of Zn were found in root and shoots of wheat and triticale plants supplied with Zn than in the control plants without Zn supply. Among the treatments with Zn supply, histidine enhanced, whereas glycine reduced, symplastic root Zn concentration in bread wheat. Both amino acids decreased the symplastic root Zn concentration but had no effect on apoplastic Zn in triticale. Both amino acid treatments also reduced Zn concentrations in the xylem sap of the two plant species compared with the treatment with Zn addition only. In bread wheat, the effect was stronger for glycine than for histidine; in triticale, it was the same for both amino acids. The concentration of Zn in xylem sap was always greater in wheat than in triticale. Addition of histidine to the nutrient solution increased the total amount of shoot Zn in triticale but not in bread wheat, compared with the treatment with Zn addition only, whereas glycine had no significant effect on total shoot Zn in either of the plant species. The results show that histidine, but not glycine, can enhance Zn uptake and translocation into the aboveground parts of triticale. We suggest that this ability of histidine was due to the formation of strong complexes with Zn.

Can ligand addition to soil enhance Cd phytoextraction? A mechanistic model study.

Phytoextraction is a potential method for cleaning Cd-polluted soils. Ligand addition to soil is expected to enhance Cd phytoextraction. However, experimental results show that this addition has contradictory effects on plant Cd uptake. A mechanistic model simulating the reaction kinetics (adsorption on solid phase, complexation in solution), transport (convection, diffusion) and root absorption (symplastic, apoplastic) of Cd and its complexes in soil was developed. This was used to calculate plant Cd uptake with and without ligand addition in a great number of combinations of soil, ligand and plant characteristics, varying the parameters within defined domains. Ligand addition generally strongly reduced hydrated Cd (Cd(2+)) concentration in soil solution through Cd complexation. Dissociation of Cd complex ([Formula: see text]) could not compensate for this reduction, which greatly lowered Cd(2+) symplastic uptake by roots. The apoplastic uptake of [Formula: see text] was not sufficient to compensate for the decrease in symplastic uptake. This explained why in the majority of the cases, ligand addition resulted in the reduction of the simulated Cd phytoextraction. A few results showed an enhanced phytoextraction in very particular conditions (strong plant transpiration with high apoplastic Cd uptake capacity), but this enhancement was very limited, making chelant-enhanced phytoextraction poorly efficient for Cd.

Symplastic and apoplastic uptake and root to shoot translocation of nickel in wheat as affected by exogenous amino acids

This study investigated the effect of exogenous amino acids on apoplastic and symplastic uptake and root to shoot translocation of nickel (Ni) in two wheat cultivars. Seedlings of a bread (Triticum aestivum cv. Back Cross) and a durum wheat cultivar (T. durum cv. Durum) were grown in a modified Johnson nutrient solution and exposed to two levels (50 and 100μM) of histidine, glycine, and glutamine. Application of amino acids resulted in increasing symplastic to apoplastic Ni ratio in roots of both wheat cultivars, although glutamine and glycine were more effective than histidine under our experimental conditions. The amino acid used in the present study generally increased the relative transport of Ni from the roots to shoots in both wheat cultivars. Higher amounts of Ni were translocated to wheat shoots in the presence of histidine than the other amino acids studied, which indicated that histidine was more effective in translocation of Ni from roots to shoots. Amino acids used in the present study largely increased root symplastic Ni, but shoot Ni accumulation was much lower than the total Ni accumulation in roots, indicating a large proportion of Ni was retained or immobilized in wheat roots (either in the apoplastic or symplastic space), with only a very small fraction of Ni being translocated from the root to the shoot. According to the results, glutamine and glycine were more effective than histidine in enhancing the symplastic to apoplastic Ni ratio in the roots, while more Ni was translocated from the roots to the shoots in the presence of histidine.

Influence of exposure solution composition and of plant cadmium content on root cadmium short-term uptake

The aim was to evaluate the influence of (i) cations potentially present in soil solution and (ii) of plant Cd content on apoplastic and symplastic root uptake of Cd. Hydroponically grown maize ( Zea mays) roots were exposed to two Cd concentrations together with a cation five times the strength of the Cd. The influence of three pH and four Ca levels was also assessed. In addition, maize and alpine penny-cress ( Noccaea caerulescens) were grown so their tissues contained Cd at three levels when their roots were exposed for 1 h to three Cd concentrations. Maize Cd uptake in the 5 μM Cd solution was reduced by the presence of Pb 2+, Cu 2+, Co 2+, Zn 2+, Mg 2+ or Mn 2+. At a 1 μM Cd concentration, Cu 2+, as well as Ca 2+ and H + reduced the Cd uptake. However, the Ca concentration had to be much higher than that of Cd to restrain its uptake. High plant Cd content was responsible for an increase in the apoplastic Cd uptake and a decrease in the symplastic absorption, for both species. Low plant Cd content neither affected the Cd apoplastic uptake whatever the exposure concentration, nor the symplastic uptake in 0.1 μM Cd exposure solution. Moreover, the hyperaccumulator symplastic uptake increased when exposed to 10 μM Cd, but not when exposed to 50 μM Cd. Maize roots showed a decrease in membrane net flux when exposed to 10 μM and 50 μM Cd. Finally, the results suggest that for plants exposed to usual soil solution concentrations, it is mainly calcium and protons which interfere with Cd internalization.

Cadmium distribution in the root tissues of solanaceous plants with contrasting root-to-shoot Cd translocation efficiencies

Root-to-shoot cadmium (Cd) translocation in Solanum torvum is lower than that of the eggplant Solanum melongena; therefore, grafting S. melongena onto S. torvum rootstock can effectively reduce the Cd concentration in eggplant fruits. We hypothesized that Cd transport in S. torvum roots is restricted in the path between the epidermis and xylem vessel; hence, we investigated the Cd distribution in the roots at the micron-scale. Elemental maps of Cd, Zn and Fe accumulation in S. melongena and S. torvum root sections were obtained by synchrotron micro X-ray fluorescence spectrometry. The Cd was localized in both the stele and the epidermis of the S. melongena root cross sections regardless of the distance from the root apex. In S. torvum root sections taken at 30 and 40mm above the root apex, a higher abundance of Cd was found within the cells of the endodermis and pericycle. The results suggested that the symplastic uptake and xylem loading of Cd in S. torvum roots were restricted, and thereby, the Cd that was unable to be loaded into the xylem accumulated in the endodermis and in the pericycle. Because symplastic uptake differs only slightly between the two species, the difference in xylem loading would explain the comparatively lower Cd concentration in S. torvum shoots.

Contribution of apoplast and symplast to short term nickel uptake by maize and Leptoplax emarginata roots

To characterize the complete Ni root sink over the short term, the contributions of apoplast and symplast to the metal root uptake were measured for both maize and the Ni hyperaccumulator Leptoplax emarginata, using a physico-chemical fractionation of root Ni after exposure. The apoplastic uptake accounted for 81.3–88.0% and for 90.6–95.5% of the total root uptake of maize and leptoplax, respectively. This could be modelled using exchange isotherms, such as the Langmuir or Freundlich isotherms. Root cell walls of both plant species showed similar affinity to Ni, although the hyperaccumulator had a higher apoplastic uptake. For maize as for leptoplax, the symplastic influx fitted a Michaelis–Menten function with similar K m and I max values, although the two plant species have contrasting abilities to accumulate Ni. This suggests that symplastic uptake is not the main factor of hyperaccumulation and that the Ni transport system might be similar in these two plants species. More research is needed to understand how the high level of Ni adsorption on root apoplast might affect the overall Ni uptake.

Plasmodesmos: transporte simplástico de herbicidas na planta

Plasmodesmos são canais responsáveis pela conexão citoplasmática entre células vizinhas, possibilitando a troca de moléculas de informação, funcionais, estruturais ou ainda de xenobióticos entre as células pertencentes a um mesmo grupo. Células pertencentes ao mesmo conjunto (domínio) constituem-se numa unidade funcional, e substâncias podem se mover entre estas células com velocidade muito superior à observada no transporte através de membranas. Os plasmodesmos podem atuar ainda no transporte a longa distância, tanto pela associação com o floema como pelo intercâmbio entre domínios simplásticos. Quando a planta se encontra sob estresse e as taxas de transporte via xilema e floema são mais reduzidas, os plasmodesmos podem ser mais efetivos no transporte a longa distância, das moléculas de herbicidas sistêmicos. Falta ainda esclarecer se existe afinidade entre moléculas de determinados herbicidas com as proteínas da superfície interna dos plasmodesmos, o que poderia facilitar o transporte desses herbicidas pela manipulação do Tamanho Limite de Exclusão do plasmodesmo, independentemente do tamanho da molécula, bem como determinar se a semelhança com algum composto natural da planta promove maior taxa de transporte simplástico. Pouca importância tem sido dada à participação dos plasmodesmos no transporte de herbicidas sistêmicos. No entanto, o avanço dos trabalhos com produtos marcados e a intensificação das pesquisas em fisiologia vegetal para melhor entendimento dos processos referentes à absorção, translocação, conjugação e/ou degradação de herbicidas podem esclarecer muitos aspectos ainda não definidos do transporte de herbicidas via xilema e floema e sua associação com o apoplasto e domínios simplásticos.