Source and sink tissues therefore partitioned carbon into starch differently in response to abiotic stress . Source leaf showed reduced 14C partitioning into starch at MD under both mild and severe salinity and osmotic stress, and at ED under severe stress only.Since the plants were labeled 5 h after the stress treatment, the 14C flux into starch cannot provide a whole picture of starch metabolism changes during the entire stress period. It only informs on percentage change in partitioning and allocation. Therefore, absolute starch content measurements were made in source leaf and roots , to determine changes in accumulation over the time-course. 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. Therefore stress may have reduced the rate of starch turnover in the roots.
Because the starch-sugar inter conversion in source leaf was acutely regulated in response to environmental cues,dutch bucket for tomatoes 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-phosphate synthase, AtSnRK1.1 and AtSnRK1.1 encode two major isoforms of SnRK1, the central players of the T6P/SnRK1 pathway. AtAPL3 encodes the large subunit 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 tissue, followed expectation based on previous knowledge.
Source leaf, sink leaves and roots tissues 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 deficit 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 tissues. 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 tissues, 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 tissues, 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,blueberry grow pot and vacuolar proteins are degraded and recycled to provide energy and substrates for respiration.Te 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 tissues, 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 flexibility with respect to starch-sugar inter conversion. 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, 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 specificity 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 tissues 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.The vascular system is essential for information exchange and resource allocation throughout the plant, from roots to aerial tissues. It is composed of two vascular tissue types: phloem and xylem. The phloem sap contains photo assimilates and other macromolecules that move throughout the plant from areas of synthesis or excess to areas of use and storage. The xylem sap transports water and nutrients from roots to aerial tissues, driven by a difference in water potential due to transpiration. Xylem sap can also contain a wide range of proteins involved in growth regulation, protection against environmental stress, and plant defense against pathogens. These biological processes depend on vesicular trafficking of proteins to the extracellular space, which can follow either conventional or unconventional secretion routes in plant cells. Conventional secretion requires N-terminal signal peptides or other recognition signals to direct them to the endomembrane system pathway, while proteins that follow the unconventional secretion route lack these signals. Proteins that follow unconventional secretion can allow plants to respond to a wider range of extracellular stresses and stimuli, facilitating defense responses under stress. Despite the biological importance of secreted proteins in the extracellular space to plant survival and development, proteome studies are scarce because of technological challenges. Vascular sap studies have advanced our understanding of plant responses to vascular plant diseases. The Gram-negative gammaproteobacterium Xylella fastidiosais a xylem-limited pathogen that colonizes several economically important crops worldwide causing diseases such as Pierce’s disease in grapevines, citrus variegated chlorosis, and most recently olive quick decline syndrome in Europe. Because of its significant economic impact on citrus production in Brazil, Xf was the first plant pathogen to have its genome sequence determined. The genomic landscape provided an initial description of potential virulence factors and revealed the absence of a type III secretion system commonly employed by plant pathogens to deliver virulence effectors inside plant cells. Subsequent molecular and cellular studies proposed that the mechanism of disease symptoms would be associated with biofilm formation and xylem blockage triggering the observed disease symptoms. Additionally, genomics and proteomics showed the importance of virulence factors secreted by the type II secretion system and outer membrane vesicles for symptom development. These studies highlighted the molecular complexity of the plant-pathogen interaction that takes place in the vascular system. The importance of proteins in the plant response to Xf was detailed in several proteome studies comparing infected and uninfected grapevine stems and the infection responses of different cultivars. These studies identified more than 200 proteins associated with disease resistance, energy metabolism, protein processing and degradation, biosynthesis, stress-related functions, cell wall biogenesis, signal transduction, and ROS detoxification among others.