Based on the results, we propose the presence of quantitative trait locus or loci for root traits in the distal 15% of the physical length of 1RS arm. In cereals, most of the gene rich regions for agronomic traits are concentrated in the distal ends of the chromosomes . Kim et al. conducted Weld studies for the agronomic performance of 1R from different sources of origin. They found 1RS increases the grain yield significantly, and interestingly, all the lines with 1RS did not show significant differences for shoot biomass. They did not look at the root traits, which could have also been useful. In a similar study, Waines et al. compared 1RS from different sources to study root biomass in hexaploid as well as tetraploid wheats. The translocated hexaploid wheats with 1RSAmigo and 1RSKavkaz showed 9 and 31% increase in root biomass than Pavon 76, respectively. Similar results were reported for the durum wheat ‘Aconchi’ versus Aconchi with the 1RS arm. These studies point toward the definite presence of gene for greater rooting ability on 1RS, and also the differential expression of alleles from different sources of 1RS in root traits. In a recent study on rice root anatomy, Uga et al. identified a QTL for metaxylem anatomy on the distal end of the long arm of chromosome 10. In another comparative study of rye DNA sequences with rice genome, the distal end of the long arm of chromosome 10 of rice was syntenic to 1RS . Both these studies provide evidence to support the general applicability of our mapping method to locate the probable region on 1RS,hydroponic grow system carrying gene/QTL for root traits. Our present finding on root studies prepares a platform to find gene/QTL for root traits on 1RS.
Future work will focus on use of a larger number of recombinant lines to narrow down the QTL region of 1RS responsible for increased root traits and find the molecular markers linked to these QTL. Ultimately, this would lead to our goal of physical mapping and then positional cloning of the root QTL.The increasing use of NPs in commercial products has led to NP-accumulation in the environment and within the food chain.Chronic exposure to NPs can lead to health issues as some inorganic NPs have biological activity at the cellular and sub-cellular level with an unknown cytotoxicity and genotoxicity.In particular, metal oxide NPs are the most abundant form of NP in the environment with the most potential toxic risks.Locating, quantifying, and imaging NPs in vivo can provide information on bio-distribution and fate of NPs in living systems.However, many challenges to quantitatively assess their bio-distributions under realistic environmental exposure concentrations remain.To date, the visualization of most NPs within plants has relied on the use of micro-X-ray fluorescence spectrometry , confocal microscopy, TEM, SEM, scanning transmission electron microscopy , or scanning transmission ion microscopy .The most prominent quantification techniques have been inductively coupled plasma spectroscopy utilizing either optically emission spectroscopy or mass spectrometry .These techniques required mineralization of the plant material generally with hydrogen peroxide and nitric acid, and may not be sufficiently sensitive enough to quantify small changes in the amount of a metal ion.Notably, high background concentrations of essential nutrients make detecting and quantifying the small variation in NP-related metal ion content an analytical challenge as the measurement is often of the same magnitude as noise or at the detection limit.
Optically tagged NPs have also been investigated, but the challenge of overcoming the plants’ own bioluminescence can make quantification difficult.Although used in medical imaging, radio labeled NPs for noninvasive tracking and quantification in plants have not been significantly explored.Prior radio labeling of NPs for medical imaging has utilized three main approaches: post radio labeling via attachment of chelator to NPs first, then reaction with the a metal radio nuclide; preradiolabeling, where a radioactive prosthetic group, a small molecule that the radioisotope is attached, followed by attachment to the NP; and direct radio labeling.Of these three main radio labeling approaches, the third approach has been the only method used to study NP distribution in plants where Zhang et al.reported the use of [141Ce]CeO2-NPs produced via neutron bombardment of CeO2-NPs to study distribution in cucumbers. The produced radionuclide 141Ce has a 32.51 day half-life, and the specific activity of the synthesized [141Ce]CeO2-NPs was 2.7 μCi/mg of NP.Despite numerous approaches to analyze NPs, a combination of technologies is required due to the low detection limits and high resolution needed to address the intact nature of NP-transport into plants. Thus, multiple tools must be utilized to quantify and determine the intact nature of NPs at a given location within biological environments .Previous studies have provided conflicting evidence about the intact nature of NP uptake and transport into plants: some studies indicated intact NP uptake and vascular transport,some observed NPs in plants due to dissolution events and reformation within the plant tissue,and still others have indicated that NPs cannot be transported into plants.These varied observations could be linked to inadequate techniques available to track NP movement in vivo. The main challenge in determining intact NP transport into plants is ruling out NP dissolution, as reductive precipitation and formation of NPs within plant tissue has been documented.Even natural formation of NPs within plants and fungi is known.
Further complicating the picture is the fact that many studies assessing NP uptake exposed plants over long periods of time from 2 to 130 days, with very large amounts of NPs per plant, which could make dissolution events more prevalent.Avoiding excessive exposure to the NPs and carefully analyzing the stability of the administered NPs are key to avoid erroneous conclusions due to NP-dissolution and subsequent reformation. In this study we evaluated an analytical method using a radioactive label to non-invasively track and quantify transport and accumulation of NPs in lettuce seedlings in vivo. This method studied NP-size dependent transport immediately upon exposure , an early time frame that has rarely been explored in plants.The visualization of NP transport and accumulation in lettuce seedlings was done by autoradiography and PET/CT imaging and further confirmed by gamma-counting, SEM, and TEM. Our study was designed to use highly uniform NPs of two size sets , which were theoretically too large for passive transport across plant tissues.To ensure a narrow size distribution with a uniform geometric shape and the ability to thoroughly investigate stability , a preradiolabeling method with the PET-radioisotope copper-64, the “clickable” chelator ADIBO-NOTA, and commercially available spherical Fe3O4- NPs containing azides was explored. This radio labeling approach yielded a high specific activity and allowed for size characterization of the NPs after the plant accumulation and imaging period, and avoided complication from fabricating radioactive NPs and production of less stable NP material with a larger size distribution. Rigorous stability studies were carried out at a variety of pHs to investigate possible dissolution of the 64Cu-radiolabeled NPs and substantiate the intact nature of NP-transport into lettuce seedlings.To date the uptake, bio-accumulation, bio-transformation, and risks of NPs in food crops is not well understood.Most studies on NP uptake in plants have focused on the effects of NPs on plants, and have not focused on the transport or entry of intact NPs. Several studies concluded that NPs do not gain entry into plants,vertical grow rack while those that do show NP uptake in plants have found the NP amounts to vary widely between 0.05 μg/g and 38983 μg/g of plant.In vivo tracking of NP transport in plants has traditionally relied on destructive analytical techniques to quantify NP-uptake and accumulation, requiring mineralization for metal quantification mostly by ICP.These analytical techniques face the challenge of being sensitive enough to reliably measure the small changes in metal concentrations caused by NP-uptake and accumulation within the plant. The extensive range of NP accumulation reported in plants suggests that NP-uptake is dependent on several parameters: quantity of NP administered per plant, plant species, NP-size, NP-composition, and duration of exposure.Further complicating the understanding of NP transport and accumulation in plants are the studies that have observed NP-uptake due to dissolution.Collectively the variations of parameters in every study on NP transport in plants have made direct comparison and accurate conclusions challenging. Thus, it was our goal to develop a noninvasive visualization approach to track and quantify the distribution of intact radiolabeled [ 64Cu]-NPs in lettuce seedlings. Using 64Curadioactively tagged NPs, we employed a range of complementary noninvasive analytical tools including autoradiography and PET/CT imaging to spatially and temporally visualize and quantify intact-NP uptake and accumulation in plants. The stability study described demonstrates dissolution of the [ 64Cu]-NPs did not occur. Ligand effects on NP mobility within the lettuce was minimized by modification of ≤5% of the NP surface with [64Cu]-ADIBO-NOTA as to negligibly change the NP surface properties.
No ligand detachment or leaching of 64Cu-ion from the NPs within the imaging time frame and at various pHs was observed, by both HPLC and gamma counting analysis, indicating that the radioactive signal in the lettuce seedlings was due to intact [64Cu]-NPs. Additionally, indirect evidence further supported that the observed uptake was from intact [64Cu]-NPs as control lettuce seedlings given only [64Cu]CuCl2 had much higher radioactivity with 4-fold higher concentrations in the cotyledons and 10-fold higher concentrations in the root . These control plants also had visibly higher amounts of radioactivity in each part of the plant by autoradiography images suggesting that if the observed radioactivity was due to [64Cu]CuCl2, then the uptake should be much higher. Furthermore, the use of covalently bound optical-tagged NPs also exhibiting high stability and illustrated the same NP movement from the root to the cotyledon . Thus, helping to substantiate that NP-uptake and transport to the cotyledon was from intact NPs. TEM-sectioning of the plant tissue also helped to corroborate the presence of NPs within the plant tissue . It should be noted that detection of NPs within plant tissue via TEM is challenging,but based on our stability studies of the 64Cu-radiolabeled NPs along with the short exposure time that the observed uptake was attributed to intact [64Cu]-NP transport through the roots and into the cotyledons. This study has shown that NPs were transported intact into plants and can be tracked non-invasively using a radioactive tag for in vivo imaging by autoradiography and PET/CT and quantification using a gamma counter. This method allows for a highly sensitive method capable of quantifying NP amounts in an individual seedling, a level that would be challenging by the traditional ICP quantification.However, different accumulation patterns for the cotyledons were observed for the two different sized [64Cu]- NPs , while the root and whole plant were similar . Most of the accumulation for the larger [ 64Cu]-NPs was within the first hour, where cotyledon NP amounts were ∼0.35 ± 0.15 μg/g with the only significant increase after 1 h between the 12 and 24 h time point in which accumulation plateaued at around ∼0.7 μg/g . The larger [64Cu]-NPs also had higher accumulation than the [64Cu]-NPs at the early time points up to the 4 h time period. The smaller [64Cu]-NPs had ∼8.8 fold increase in cotyledon accumulation from the 4 h time point to the 24 h time point with an increase of ∼1.6 fold between 12 and 24 h time period appearing to have a linear increase in absorption over time. The differences in cotyledon accumulation between the two sized [ 64Cu]-NPs maybe linked to NP size effects on the lettuce hydraulic conductivity. Our work suggests that [64Cu]-NPs around 20 nm in size appear to clog root cortical cell walls, or pit membrane preventing further uptake, explaining why 11 reaches a plateau, while the smaller [64Cu]- NPs continued to increase in amount over time. Initial studies with duckweed also illustrated [64Cu]-NP accumulation in regions of growth and at the node and apex of the cotyledons , suggesting that [64Cu]-NP transport to the cotyledons could occur via the phloem. The TEM images further shows the appearance of intact NPs in the lettuce tissue within the expected size range for the [64Cu]-NPs , but [64Cu]-NPs had a size that appeared smaller than those administered; suggesting that the plant may filter larger NPs and has a size-threshold for uptake , which may also explain the clogging phenomenon.In summary, the combined analysis of the imaging by autoradiography and PET/CT and TEM suggested that both sized [64Cu]-NPs are transported intact from the root to the cotyledons.