Sucrose transport through phloem to the developing grains play a pivotal role in determining productivity hence its measurement under abiotic stress could be a vital tool to determine productivity vis-a-vis abiotic stress tolerance. Sucrose transport to the developing grains is a measure of sum of current photosynthesis, availability of stored Non structural carbohydrates (NSCs), and ability of plants to remobilize NSCs, and sink metabolic strength.
The overall sink-source dynamics determines how much, where and when carbohydrates will be allocated. Abiotic stress may alter this sink-source dynamics depending on the severity of the stress and crop growth stage. Cereals have the ability to buffer sink-source interaction by transiently storing carbohydrates in stem tissues when production from the source is greater than whole-plant demand. These reserves improve yielding ability by providing an alternative source when photosynthetic capacity is reduced during the later phase of grain filling or during period of abiotic stress. The overall rate of sucrose transport to the panicles is determined by rate of current photosynthesis, remobilization of stored stem reserves and the sink metabolic activity.
At present there are various methods to determine abiotic stress tolerance in crop plants, such as, canopy temperature depression, panicle/leaf water potential and cell membrane stability which are indirect and often difficult to measure on large number of genotypes. If the volume and rate of flow of sucrose to the panicle during grain filling under abiotic stress condition is measured, it could be far more reliable tool for measuring abiotic stress tolerance than the currently used tools. To develop such a probe for the measurement of abiotic stress tolerance in crop plants, we need to generate knowledge regarding measurement of the rate of phloem sap flow and sucrose concentration to the developing grains in crops and extent of genetic variation for remobilization of stem reserve under these abiotic stress conditions. Moreover, the relationship between sucrose flow rate and crop yield under these stress have to be understood. For developing a probe will require basic knowledge on sucrose translocation in the plant. After studying the sucrose transporters and phloem anatomy suitable designing of the device will be required. For taking out the phloem sap for sucrose measurement, stylet of sap sucking insects such as aphids/plant hoppers should be mimicked to draw 1-2 micro liters of phloem sap. The stylet of the inset is served after the insect insets it's stylet into phloem to collect sap or introduce marker dyes (Fujimaki et al. 2000). To develop such as microprobe, a complete understanding of the phloem architecture is required. For developing a probe, we need to combine the devise of the measurement of sucrose flow rate and sucrose concentration. Currently large size sap flow probes available commercially to measure sap flow but these probes are suited to the large diameter of stem such as maize and sugarcane, and the large tree trunk, and also the throughput is very low. Therefore, we need to develop a device for the measurement of sap flow rate by devising a method suitable for other cereal crops stem. After recording phloem sap flow rate, sucrose concentration should be determined at the basal node of the peduncle by developing a bioprobe. Finally high throughput probe could be devised by combining these two components with the help of analytical/optical/conductometric methods or any suitable technique. The probe should be a non-destructive method for the measurement of sucrose transport to the peduncle and ultimately to the developing grain to determine productivity. Higher the sucrose flow rate under these abiotic stresses, higher should be the efficiency to remobilize stem reserve and yield.
It is very clear that the transport of sucrose is mainly through the phloem tissues which play a vital role in transport of photoasimilates from site of primary production to the heterotropic tissues. In the literature basic transport mechanism including sucrose mobilization from the stem reserve is well mentioned. High concentrations of sucrose in the sieve elements of source tissues raise turgor pressure, resulting in hydrostatic pressure-driven mass flow of sugars to the sieve elements of sink tissues, where sugars are unloaded and turgor pressure drops. Three different strategies for loading sugars into the phloem have been described (Slewinski and Braun, 2010) which vary in routes that sugars take to enter the phloem and the energetics of accumulation. The symplastic mechanism, termed polymer trapping, represents thermodynamically active accumulation of these raffinose sugars, because energy is used to create a high concentration in the phloem versus the surrounding mesophyll (Turgeon and Wolf, 2009). Increased photoassimilate efflux acts to maintain a constant apoplastic turgor in spite of enhanced flux through the apoplasm. Consequently, the increased potential for photoassimilate could be fully realized by the developing grain (Itoh et al. 2005). Sucrose transporter proteins (STPs) function as electrogenic Suc-H+ symporters with 1:1 stoichiometry. Sucrose transport occurs through sucrose /H+ symporters and the process is tightly linked to shaker-like K+ channels. Sucrose is transported into phloem companion cell/sieve element complexes through these transporters. Typically, apoplastic sucrose is on the order of 20 mM, while sucrose concentration in the phloem is close to 1M. Apoplastic loaders generally have few plasmodesmata connecting the minor vein phloem to surrounding cells, although this is not a conserved anatomical feature of apoplastic loaders. It is suggested that phloem loading is regulated by protein phosphorylation cascade that controls sucrose symporter transcriptional activity, which ultimately determines symporter protein abundance, and therefore phloem-loading capacity. Studies conducted so far on sucrose/H+ symporters and K+ channels in plants are based on the heterologous expression systems (Xenopous oocytes and yeast).
Thus measuring sucrose flow rate and its concentration could be an excellent tool for the assessment of abiotic stress tolerance in cereal crops. Development of portable probe for high throughput phenotyping of sucrose transport will unravel the genetic variability, and could be used in genetic improvement programmes aimed at enhancing abiotic stress tolerance. In addition, this will unravel the molecular and genetic basis of carbohydrate partitioning and harvest index which is essential to break the yield barriers and to improve seed quality in crop plants.
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