Comparison of the variations in the percentage of 14C partitioned into starch with changes in starch accumulation could indicate if there are additional regulatory mechanisms leading to turnover, i.e.simultaneous synthesis and degradation.Cold, mild osmotic and salinity stress triggered enhanced starch accumulation at ED in the source.Twelve hours later , only cold and mild salinity kept starch accumulation high relative to the non-stressed control.In comparison, the 14C that partitioned into starch decreased from ED to EN , thus, the increased starch content observed might be due to inhibited starch degradation early in the day.A similar pattern was found in the roots — higher starch accumulation even though there was no change in the percentage of 14C partitioned to starch over the same period.Because the starch-sugar inter conversion in source leaf was acutely regulated in response to environmental cues, we further examined if changes in starch metabolism and sugar export was accompanied by the regulation of the known T6P/SnRK1 stress signaling pathway genes.Te transcript level of five selected genes in source leaf exposed to 300mM mannitol and 200mM NaCl stress, which triggered the most dynamic changes in starch metabolism, were evaluated.These genes are involved in starch synthesis, sucrose transport, and are components of the T6P/SnRK1 stress signaling pathway.AtTPS1 encodes trehalose-6-phosphatesynthase, AtSnRK1.1 and AtSnRK1.1 encode two major isoforms of SnRK1,dutch bucket for tomatoes the central players of the T6P/SnRK1 pathway.AtAPL3 encodes the large sub-unit of ADP-glucose pyrophosphorylase that catalyzes the first committed step in starch biosynthesis.AtSWEET11 encodes a transporter that exports sucrose from leaf mesophyll cells into the phloem for transport to sinks.Our measurements showed that AtSnRK1.2 was up-regulated by osmotic and salinity stress after 6hours of treatment.
However, the transcription of AtTPS1 and AtSnRK1.1 did not change.AtSWEET11 was down regulated by severe osmotic stress at the end of day.AtAPL3 was up-regulated at MD and at EN by 300mM mannitol stress, and was up-regulated from ED to EN by 200mM NaCl.Our overall aim was to develop a comprehensive map of time-dependent changes in carbon allocation and partitioning, to see how these processes were affected under different stresses.In our study, the 14C partitioning in source and different types of sinks over the diurnal cycle was examined.Under control conditions, 14C distribution into different metabolic pools in source and sink This issue, followed expectation based on previous knowledge.Source leaf, sink leaves and roots This issues showed different carbon partitioning, with most dynamism in the source.Most carbon in source leaf flowed into storage compounds , and less flowed into structural compounds during the day.This result is similar to a previous study.Te roots also generally incorporated more carbon into RICs while sink leaves partitioned more into starch.This indicates a clear differentiation in carbon use between sink leaves and roots.Carbon allocation to the sinks was modulated by all abiotic stress conditions used in our study.Stress conditions should reduce photosynthetic capacity and carbon available for export.Knolling et al.showed that carbon export from the source to sink leaves was reduced in Arabidopsis experiencing dark-induced carbon-starvation.Our study included roots, which is a stronger sink than leaves.We found that the C-fluxes into the roots were more vulnerable to stresses than those into sink leaves.Furthermore, plants might regulate carbon allocation differently in response to long-term and short-term stresses requiring caution when making comparisons between studies.Durand et al.observed a higher percentage of 14C allocated into roots in the long-term water defcit stressed Arabidopsis.However, data from plants exposed to short-term stress in our study and plants exposed to a 16h night showed the opposite results: reduced percentage of 14C exported into roots.This underscores that timing, intensity, and type of stress regulate carbon allocation differently, even if some stresses show similar responses.
Osmotic, salinity, and cold stress all triggered complex changes in carbon partitioning and shared some commonalities.All stresses increased the carbon partitioned into sugars in both source and sink This issues.They also decreased the 14C partitioning into starch in the source leaf while increasing organic acids and amino acids.Each stress had a more dramatic impact on source leaf than the sink This issues, with most changes occurring within the first 12h of stress application.Te abiotic stresses used here all triggered decreased 14C flux into RICs in the roots.Among the major metabolites pools affected, changes of carbohydrates were most consistent.Kolling et al.observed an increase of 14C into sugars and a reduction of 14C flux into the RICs pool in both source and sink leaves.However, in our study, the increased 14C flux into sugars in the source leaf was due to the re-partitioning of 14C from storage compound , while the increase in the sink could be explained by the reduced 14C partitioning into structural compounds.Different abiotic stresses may uniquely regulate carbon use.Only cold stress caused a decrease in 14C in RICs in source leaf.Osmotic and cold stress, but not salinity stress, increased 14C flux into organic acids and amino acids in root This issues, and enhanced 14C into amino acids in sink leaves.Only cold and salinity stress, provoked changes in 14C in protein in source and sink leaves.Higher 14C in protein at the early stage of the stress progression may be due to the accumulation of stress-responsive proteins and enzymes.When stress continued, storage compound like the storage, cytosolic, and vacuolar proteins are degraded and recycled to provide energy and substrates for respiration.The regulation of starch accumulation by abiotic stress in Arabidopsis were mainly studied during the day and only focused on leaves.Mild-to-moderate mannitol stress triggered starch accumulation, whereas higher mannitol concentrations or severe drought led to decreased leaf starch.Moderate-to-severe salinity decreased starch in Arabidopsis leaves.Cold stress induced starch accumulation in leaves in some studies, while decreased starch accumulation in others.Our study differentiated between source and sink This issues, and starch content was regulated by abiotic stress in both.There was a lack of congruency in the starch accumulation and 14C-starch partitioning under cold, mild osmotic, and salt stress in source and roots.Higher starch content in sink under stress might be due to decreased starch utilization.In the roots, more 14C accumulated as sugars because of the decreased 14C partitioning into structural compounds.In this case, it might not be necessary to degrade starch into sugars.
Starch, as a sugar reservoir, regulates plant carbon balance to avoid potential famine.Maintaining sugar levels by cycles of synthesis and degradation of starch could permit metabolic fexibility with respect to starch-sugar interconversion.Te sugars so produced may act as Reactive Oxygen Species scavengers, osmoprotectants and be an immediate source of carbon and energy to mitigate against stress.Sugar conversion to starch in leaves may prevent feedback inhibition of photosynthesis, and higher starch in the roots could help gravitational response under stress, and enhance biomass for better foraging.Transcripts levels of T6P/ SnRK1 pathway genes were regulated by abiotic stress in this study.AtSWEET11, one of the sucrose transporters, is important in whole-plant carbon allocation.It is expressed when sucrose export is high and repressed during osmotic stress in Arabidopsis leaves, when presumably export is lower.In our study, AtSWEET11 was down regulated by osmotic stress at the end of day, which suggests that the export of sugar to the sinks was inhibited.Te repression was likely due to feedback inhibition by excess sugars, this is supported by our data, which showed more 14C in sugars in the source leaf at ED, and decreased 14C imported into roots.AtAPL3 was shown to be up-regulated by 150mM NaCl stress in Arabidopsis.Our study also observed the up-regulation of AtAPL3 by 200 mM NaCl,blueberry grow pot and 300 mM mannitol stress.Interestingly, the percentage of 14C partitioned into starch was reduced, and the end point starch content remained unchanged.Changes in the post-transcriptional regulation of AGPase rather than at the transcriptional level under stress may underscore starch contents assayed.SnRK1 has a pivotal role in regulating carbohydrate metabolism and resource partitioning under stress.In this study, AtSnRK1.2 was up-regulated by osmotic and salinity stress after 6 hours of stress treatment.However, the transcript of AtTPS1 and AtSnRK1.1 did not change, indicating a possible delayed response to stress compared with AtSnRK1.2.Te inconsistency in transcript changes of AtSnRK1.1 and AtSnRK1.2 might also be due to the specifcity of these isoforms in terms of spatial expression and function.In maize, salinity stress triggered more starch and sugar accumulation in both source and sink This issues and the transcripts of the ZmTPSI.1.1 and ZmTPSII.2.1 genes in the source leaf were down-regulated, while SnRK1 target genes AKINβ was affected mainly in the sink but not in the source.Large-scale metal contamination can result in severe environmental damage, and remediation efforts represent a substantial financial burden for industry, government and taxpayers.Anthropogenic metal inputs include spoil from metal mining operations, fallout from refinery emissions, waste disposal, electroplating, combustion of fossil fuels, and agricultural application of pesticides and bio-solids.Traditional remediation efforts are not feasible for large-scale impacts and therefore alternative remediation strategies are necessary when vast areas of land have been contaminated.Hyper accumulator plants concentrate trace metals in their harvestable biomass , thereby offering a sustainable treatment option for metal-contaminated sites and an opportunity to mine metal-rich soils.Cultivating nickel hyper accumulator plants on metal-enriched soils and ashing the harvestable biomass to produce Ni oreis an economically viable alternative for metal recovery.Soils suitable for Ni phytomining include serpentine soils and industrially contaminated soils.Serpentine soils develop from ultramafic parent material and thus contain appreciable quantities of Ni, cobalt , chromium , manganese , iron and zinc.The Ni : Co ratios in serpentine soils typically range from 5 to 10.Anthropogenic metal inputs generally involve discharge of a mixed-element waste stream.For instance, emissions from Ni smelters are typically enriched with other trace metals from the ore , Co, leadand Zn.Heavy metals are incorporated into enzymes and are thereby toxic to living organisms in excessive amounts.Cobalt contamination is an environmental concern, and the radionuclide 60Co is classified as a priority pollutant.Hyper accumulator plants used to extract Ni from metal-enriched soils must be tolerant of co-contaminants.Therefore, the effects of metal co-contaminants on the physiology and biochemistry of hyper accumulators, and ultimately on the efficiency of metal phytoextraction, is of concern for metal recovery efforts.Several first-row transition metals have important roles in biological systems as activators of enzymes or as key components of enzyme systems.Cobalt is essential for Rhizobium , free-living nitrogen-fixing bacteria and cyanobacteria.However, there is no evidence that Co has a direct role in the metabolism of higher plants.Nickel is the element most recently classified as an ‘essential’ plant nutrient and is a key component of the Ni-containing enzyme, urease.Transmembrane transport systems with specificity for Ni or Co have not been identified in higher plants.
The nickel hyperaccumulator, Alyssum murale, a herbaceous perennialnative to Mediterranean serpentine soils, has been developed as a commercial crop for phytoremediation/phytomining.Hyperaccumulator species of Alyssumaccumulate Co from Co-enriched soils ; Cobalt accumulation is most efficient in mildly acidic soils, whereas Ni is most effectively accumulated from neutral soils.Alyssum sequesters Ni via epidermal compartmentalization, a metal sequestration strategy exploiting leaf epidermal This issue as the sink for metal storage.Epidermal cell vacuoles are responsible for Ni sequestration in Alyssum , and vacuolar sequestration has been recognized as a key component of cellular-level metal tolerance tolerance for several hyper accumulator species.However, Co sequestration in Alyssum epidermal cell vacuoles has not been reported previously.Information regarding metal localization and elemental associations in accumulator plants is crucial to understanding the mechanisms of hyper accumulation and tolerance.Synchrotron-based techniques such as X-ray microfluorescence and computed microtomography can be used to image elements in hyper accumulator plants.SXRF imaging of an intact, transpiring thallium accumulator showed that Tl is distributed throughout the vascular network, and X-ray absorption spectroscopy identified aqueous Tlas the primary species in plant Thissue.X-ray CMT imaging techniques such as differential absorption and fluorescence microtomography resolve the three-dimensional distribution of elements within a sample, and hydrated biological specimens can often be analyzed with minimal or no sample preparation and alteration.DA-CMT and F-CMT were used to visualize Fe localization in seeds of mutant and wild-type Arabidopsis, revealing that Fe storage in seeds was mediated by the vacuolar Fe transporter.F-CMT and DA-CMT showed Ni enrichment in leaf epidermal This issue of A.murale grown in Ni-contaminated soils.Soils naturally enriched or industrially contaminated with Ni typically have co-contaminants present; however, the influence of common metal co-contaminants on Ni hyper accumulation remains poorly understood.In the present work, the effect of Co and Zn on Ni accumulation and localization in A.murale was examined.