Essentially, this would entail the manipulation of the genes associated with key enzymes involved in the sucrose accumulation process. Work on characterizing sucrose accumulation specifically for sugarcane began in the early 1960s. Specific attention will be paid to the occurrence of sucrose conversion in the culm tissue.
Sucrose accumulation in sugarcane
Source to sink movement of sucrose
At five hours after radiolabeling, the radiolabeled photosynthate in the fed intemode was 93% sucrose, 4% phosphates and amino acids, and 3% hexose sugars. In intemodes above the fed intemode, the amount of radiolabel in sucrose was found to decrease while the amount of radiolabel in hexose sugars increased. The reverse was found to be true in internodes below the fed intemode, with radiolabeled sucrose accounting for 100% of the radiolabel in certain intemodes.
The sugar accumulation cycle and associated enzymes
Initially, SAI activity was found to be high in the apoplast and vacuole of immature tissue and very low or absent in mature tissue (Hatch and Glasziou 1963; Sacher et al., 1963a). SAI activity was found to be low in mature internodes of high-sucrose members and relatively high in mature internodes of low-sucrose storage varieties (Hatch and Glasziou, 1963). In addition to the possible roles of invertases in regulation, sucrose synthases have also been implicated in the regulation of sucrose accumulation (Hatch, 1964).
Confirmation of the occurrence of the sugar accumulation cycle at the
Work at the tissue section level established the existence of a cyclic sucrose accumulation pathway in which sucrose storage is controlled by changes in. The relationship between growth, SAI activity and sucrose accumulation in batch cultures of sugarcane cell suspensions was found to be similar to that described in tissue slices (Goldner et al., 1991). However, cell suspension work shed some light on the possible roles of neutral invertase, SuSy, and cell wall-bound acid invertase in the sucrose accumulation process.
Developmental relationship between sucrose accumulation and carbon
The decreased distribution of carbon in respiration also coincided with a reduction in the flux of hexose monophosphates into the respiratory tract. It was proposed that underlying regulatory changes in the pathways of cell wall polysaccharide synthesis, glycolysis, and the OPP pathway may function to decrease the entry of hexose monophosphates into structural matter synthesis and respiration. Marked changes in the rate of sucrose accumulation were also observed during the growth cycle of suspension cultures of sugarcane cells (Wendler et al., 1990; Goldner et al., 1991).
Solute distribution between the vacuole and cytoplasm in storage
However, it is not clear whether the stored solutes are actively accumulated in the vacuole. This makes it possible to determine the degree of compartmentalization of sucrose in the sugar cane. In the sucrose storage phase, the concentration of sucrose in the vacuole and cytosol increased to the same extent, with no accumulation observed.
Sucrose accumulation in other plants
- Sucrose accumulation in Sugar beet
- Sugar accumulation in fleshy fruits
- Determination of radiolabel in the fed part of leaf 6
- Determination of total radiolabel per internode
- Determination of radiolabeled internodal tissue components
- Tissue extraction
- Determination of radioiabeled components in the leaf sheath
- Determination of sucrose, glucose and fructose content
- Determination of insoluble matter content
In addition, SPS activity was found to be higher in the sugar beet kernel, a region known to contain the highest sucrose content. SuSy activity was 60% greater in storage tissue than in peripheral tissues of the sugar bee (Silvius and S yder, 1979a). Based on these findings, SuSy is believed to play a key role in sucrose accumulation in the sugar bee.
The need for invertase activity to be reduced before sucrose accumulation can occur suggests that invertase activity is crucial in regulating photosynthate partitioning between growth and storage. Based on these experiments, it was concluded that sucrose is hydrolyzed before uptake from the apoplast and resynthesized in the cytoplasm of sugarcane storage tissue. These results showed that sucrose hydrolysis does not occur before or after accumulation in the sugar beet storage root under short-term conditions.
The low activities of SPS in the developing peach fruit suggest that SPS may not play a key role in sucrose accumulation in the peach. It was found that the SAI activity was significantly higher in the high-hexose sugar-storing variety than in the low-hexose sugar-storing variety. To produce labeled carbon dioxide, an equivalent amount of 0.1 mHCL was added to sodium bicarbonate in a glass chamber.
Data obtained were used to determine the amount of radiolabeled carbon dioxide in the cuvette at each time interval.
Dissemination of 14C from leaf tissue after uptake
Because the ultimate goal of feeding 14CO2 to leaf 6 was to create a radiolabeled sucrose pool in the culm tissue, it was necessary to determine that a sufficient amount of radiolabeled carbon left the leaf in the form of radiolabeled carbon.
Bq umol1 is very close to the observed value of 1.18 Bq umor1 (Table 4.7)
Prior to the present study, the majority of the detailed work on zinc metabolism in sugarcane was carried out using either tissue slices or cell suspension cultures. This raises questions as neither system has been conclusively proven to be representative of the entire plant system. By investigating zinc metabolism in the whole sugarcane plant, the present study aimed to determine whether conclusions drawn from the previous non-whole plant work apply to the whole sugarcane plant system.
Although these sink-related aspects may be considered the focal point of this discussion, results obtained from prior work on source leaf sucrose production, sucrose distribution within the apex, and phloem discharge also warrant discussion. From work on sugarcane and other C4 species, especially maize, it appears that the enzymes and metabolites of the C4 pathway and the enzymes and metabolites of the RPP pathway are spatially separated from each other. Carbon dioxide is fixed in KM cells by phosphoenolpyruvate carboxylase (PEP), with the initial product, oxaloacetate, converted to 4-carbon organic acids, malate and aspartate.
In the BS cells, CO2 released from the malate of Rubisco is fixed by the RPP pathway to 3-PGA. The byproduct of malate decarboxylation, pyruvate, is returned to the KM cells and converted to PEP in the KM cells (Hatch and Osmond, 1976; Furbank et al., 1985; Stitt and Heldt, 1985). Thus, the supplementary C4 pathway and the compartmentalization of photosynthetic enzymes serve to concentrate CO2 in the vicinity of Rubisco.
The adaptive significance of the CO2 concentration mechanism is that it enables very high photosynthetic rates by reducing the inhibitory effect of O2 on photosynthesis, reducing photoassimilate losses due to photorespiration, and by reducing water loss during high carbon fixation rates.
In C3 plants, triose phosphate is exported from the chloroplast and converted to sucrose in the cytosol. The presence of this phosphate in the cytosol inhibits the synthesis of the signaling metabolite fructose-2,6-bisphosphate (Stitt and Quick, 1989). Results obtained in the current study indicate that relatively little of the fixed carbon is stored.
In the case of a purely symplastic pathway, sucrose moves from the mesophyll cells to the sieve tube via the plasmodesmata. Thus, in the symplastic case, only the mesophyll cells determine the amount of sucrose to be loaded. Consequently, in the apoplastic case, mesophyll cells and phloem cells can influence the amount of sucrose loaded.
In the C3 species investigated, the retention of radiolabeled carbon was shown to be much higher. In the present study, leaf sheath analysis confirms that 98% of the carbon entering the sugarcane column is in the form of sucrose. The majority of the radiolabeled carbon is found in the mature intemode (internode 9), with relatively little found in the intermediate tissue (internode 6) and in the immature tissue (intemode 3).
Finally, during late ripening, a slowdown in the rate of sucrose accumulation is observed as intemode becomes 9.
In the current study, the total 14CO2 loss and carbon flux from the hexose sugar pool could not be determined. However, the determination of the percentage of total radiolabel in the ionic (anion and cation) pool in intermodes 3,6 and 9 confirms the findings of Whittaker and Botha (1997). Thus, in the immature tissue of the present study, a much larger percentage of the total radiolabel is found in the hexose sugar pool.
In intemode 9 of this study, the percentage of radiolabel in the hexose sugar group (11%) is very similar to that in the mature tissue of the previous study (10%). The exact reason for the difference between the amounts of radiolabeled carbon in the hexose sugar. The retention of a large percentage of the total radiolabel in the hexose sugar pool may also indicate that within the immature (intemode 3) and elongated (internode 6) storage parenchymal tissue, sucrose resynthesis is relatively low and sucrose degradation relatively high.
In a detailed review of work on sucrose accumulation in the sugarcane stalk, Glasziou and Gaylor (1972) presented a well-accepted model of the pathways for sucrose uptake and transformation by internode tissues. Thus, according to the model, there is both a cycle of sucrose synthesis and breakdown in the metabolic compartment and a cycle of sugar movement between all three compartments. A hydrolysis-resynthesis-rehydrolysis scheme has been proposed in the hexose-accumulating grape (Brown and Coombe, 1982).
Evidence for the involvement of sucrose phosphate synthase in the pathway of sucrose accumulation in sucrose accumulating tomato fruit. The purification and properties of sucrose-phosphate synthase from spinach leaves: The involvement of this enzyme and fructose bisphosphatase in the regulation of sucrose biosynthesis. Developmental changes in sugarcane stem anatomy in relation to phloem unloading and sucrose storage.
F (1968). The control and paltem of movement of carbohydrates in
Regulation of growth, sucrose storage and ion content in sugarcane cells measured with suspension cells in continuous culture grown under nitrogen, phosphorus or carbon limitation. Evidence for and consequences of a barrier to solute diffusion between the apoplast and vascular bundles in sugarcane stem tissue. Sucrose storage in cell suspension cultures of Saccharum sp. sugar cane) is regulated by a cycle of.
Pyrophosphate-dependent phosphofructokinase (PFP) activity and other aspects of sucrose metabolism in sugarcane internodal tissues Doctoral dissertation.
