Starch, or the proportion of the amylose fraction of starch, is used as a common ripening biomarker for apple, banana, and pear. This marker relies on the ability of amylose to physically interact with iodide to form a triiodide blue-black complex. Starch can also influence the quality of fruit juice. Although starch is degraded to sugars when fruit ripens, this conversion is not complete. Ripe fruit processed for juice therefore contains starch, which is treated with amylases for clarifcation. Further, the amylose content of the remanant starch in some fruit processed for juice, may alter juice viscosity.Prepackaged leafy greens are convenient and healthy, and are popular options for salads in western countries. Metabolism in this horticultural product can be considered over distinct phases in its lifecycle: pre- and post harvest. In developing spinach, the photosynthetic organ, i.e., the leaf, fixes carbon, and partitions a large portion ~20% to starch biosynthesis during the light period under lab conditions . During the night, the leaf starch is degraded into sugar, to maintain plant metabolism, resulting in an empty polysaccharide reserve before the next light period. In Arabidopsis, the expression of SBEs and the changes of amylopectin and amylose show a similar trend, but there is variation in when SBE transcripts peak. Although there is no information on SBE transcriptional levels in spinach during the diel,nursery grow bag there may be some similarities with Arabidopsis because the pattern of leaf starch accumulation is comparable in spinach and Arabidopsis. The preharvest starch reserve may alter the postharvest quality of leafy greens.
Harvested green produce are stored in optimized packaging under limited light exposure conditions which restricts new energy and carbon input from photosynthesis. However, respiratory activity, which is the carbon skeleton generation process for cellular metabolites, although reduced, does not stop. In detached leaves, the starch can be broken down to glucose, and sugars become the main source of fuel for cellular metabolism and ATP generation in the early stage of respiration . In the late stage of the respiratory process, the depleted sugars will be replaced by proteins, lipids, and membranes, triggering leaf senescence and cell death. Tis results in undesirable produce quality and ultimately, in produce loss. Preharvest and post harvest starch content may determine post harvest energy reserves and influence the time span that buffers the onset of senescence, thus influencing shelf-life of harvested green leaves. Correlations between leaf starch content and post harvest longevity have been found. For example, lettuce and red chard harvested at the end of the day, when leaf starch content was highest, had a longer extended shelf-life than organs harvested at other times of day. This may not be true of all varieties e.g., salad roquette. Starch also correlated with improved shelf-life quality after light exposure to detached leaves in vegetables such as Chinese kale and lettuce. The accessibility of sugars from the degraded starch may relate to leafy-green quality, and the upregulation of SBEs would convert amylose to the more catabolically available amylopectin, providing a more readily available source of sugar.The amylose-to-amylopectin ratio in Arabidopsis influences flowering time and reproductive growth, key markers of development, and fitness. Whether starch molecular structure and composition influences the preharvest growth of leafy greens in a similar way, remains unknown, but it seems likely.
Potato, sweet potato, and cassava are generally considered as high glycemic index foods because the starch in their storage organs is easily digested to sugars when consumed, leading to a rapid increase in blood sugar level. It is established that high GI food exacerbate metabolic disorders such as diabetes and obesity. In contrast, multidisciplinary experimental research shows that digestion-resistant starch could increase the healthful microbial communities of the gastrointestinal tract, reducing the occurrence of constipation, and lowering the risk of colon cancer. Altering potato starch composition is a viable way to increase ‘dietary fiber’ content and to enhance colonic health. This can be achieved by either physical, chemical, or enzymatic modifications of purified starch, e.g., etherification, esterification, or by fine-tuning the activity of starch biosynthetic enzymes. Reduction or knockout of SBEs in a range of species have reliably led to an increase in the resistant starch content in various species including horticultural crops e.g., potato, sweet potato, and cassava. Interestingly, SBE2 is not the dominant isoform expressed in storage tubers and roots, but it exerts a major function in amylopectin synthesis. Very high levels of RS can be achieved by the combined suppression of SBE1 and SBE2, but with a yield penalty. The transcriptional profiles and functions of SBE3 are unclear in the developing tubers . In addition, potato tubers suffer from a post harvest disorder: cold-induced sweetening . Potato tubers are stored at low temperatures to extend shelf life and to meet year-round demand. However, sugars accumulate from starch breakdown, a process referred to as CIS. Although a problem for the potato industry, CIS could be a mechanism to allow tubers to cope with chilling stress. CIS negatively affects the quality of fried or baked potato products: reducing sugars react with free amino acids at high temperature cooking through the Maillard reaction, to form carcinogenic acrylamide.
Changes in the enzymes involved in starch biosynthesis and degradation are involved in CIS. SBEs are actively expressed in CIS susceptible tubers, and in StVInvsilenced, CIS-resistant tubers, SBEs transcriptional level were suppressed. Naturally occurring high RS potato varieties, also, have less susceptibility to CIS. Therefore, evidence points to a positive association of SBE activity with CIS severity in some potato genotypes.Starch is a major component of the dry mass of fruits at commercial harvesting time. Starch is transiently synthesized and stored in unripe fruits with a peak just before ripening. Starch appears to be a critical feature of climacteric fruit metabolism, known for their bursts of respiratory activity and ethylene production upon ripening. Climacteric fruits contain more starch, and, more active starch biosynthesis than non-climacteric fruit after anthesis. In tomato, the functional genomics model for freshy climacteric fruit, starch fulfilled 40% of the carbon needed for respiratory processes based on a constraint-based flux model. Experimental evidence from post harvest metabolism also supports the model: tomato fruits stored post harvest under low or chilling temperatures undergo bursts of stress-related carbon dioxide and ethylene production when allowed to recover at room temperature, with an accompanying and corresponding decrease in starch reserves. A similar inverse relationship between starch content and respiratory activity was observed in ripening banana, ginger rhizomes sunberry, apple and durian. Te relationship between tissue starch content and respiration may not be perfectly linear in all species, e.g., in stored ginger, starch showed a biphasic accumulation pattern as respiration progressed, a trend not seen in other tissues examined . Furthermore, the relationship between these variables may also differ among genotypes within a species. Apart from climacteric characteristics, after the onset of ripening, starch content plummets sharply accompanied by starch decomposition into soluble sugars, and the total soluble sugar content continues to rise proportionally . This dynamic metabolic process had been reported for both climacteric and non-climacteric species including tomato, apple, banana, plantain, mango, kiwifruit, pear, and strawberry. Adequate storage of the starch-derived soluble sugars, is essential to produce an acceptably favored horticultural produce of appropriate sweetness. Accompanying the starch-sugar dynamics,plastic growing bag amylopectin-to-amylose ratio , also changes interactively . The difference in the AP/AM ratio in fruit development is expected to influence the structure of starch and its degradability. In the ripening tomato, the rate of decrease of amylose was greater than that for amylopectin . Thus, the AP/AM ratio increased dramatically during ripening, in concert with the increase in soluble sugar content and fruit color change from green to red. This phenomenon where the proportion of amylopectin increases relative to amylose, was also evident in ripening apple and banana. It is possible to speculate that of the available starch left during fruit ripening, the amylose, or long chained amylopectin was converted into amylopectin whose branch-like structure has a much higher susceptibility to enzyme attack, allowing the rapid process of starch degradation into soluble sugars and supply for respiration. However, this mechanism may not be universal for all fruit. For example, the changes in AP/ AM ratio in kiwifruit are similar to those in developing potato tubers, where the ratio of AP/AM almost remains constant during tuber development. In ripening tomato fruit with sharp increases in AP/AM, up-regulation of SBEs transcriptional expression is expected. Among SBEs, the class 2 SBE has the major effect on altering starch compositions .
Elevated expression of SBE2 transcripts does parallel the changes in the AP/AM in ripening tomato, apples, and banana. We propose that ultimately, this change in glucan structure indirectly contributes to favor, quality, and commodity value.Starch, in general, plays an essential role in balancing the plant’s carbon budget as a reserve of glucose that is tightly related to sucrose metabolism and sugar signaling pathways. Starch is considered as an integrative mediator throughout the plant life cycle, regulating plant vegetative growth, reproductive growth, maturation and senescence, and response to abiotic stresses. This comprehensive regulation is achieved by changes in the synthesis and degradation of starch to balance glucose levels, after developmental and environmental triggers in different organs. Transitory starch and its biosynthesis have been well studied in the model plant Arabidopsis, but little research has been conducted on post harvest leafy greens. Quality metrics such as shelf-life, favor, color, firmness, and texture are of consumers’ choice, and they are related to the limited pools of storage compounds in detached leaves, which cells rely on to maintain basic cellular activities. A hypothesized function for the starch in packaged leaves could be presented as such: starch may act as a bufer against sugar starvation, and protect against cellular autophagy, by serving as an alternative energy source. If the biosynthesis and degradation of starch could be adjusted in a controlled way, then the modulated release of sugars may influence the post harvest shelf-life in detached leafy greens . A continuous, paced supply of sugars may preserve vacuolar nutrients and water content, leaf cellular structure and integrity, and, thus extend the ‘best by’ post harvest date of the produce. Although the eco-physiological role of amylose is poorly understood in Arabidopsis, the AP/AM ratio may set a threshold for the optimum usage of starch. SBE action in leafy crops may differ from those in Arabidopsis given the dissimilar numbers of their isoforms and domain features . Modifying the quantity and quality of the starch in leafy greens such as spinach, lettuce, and watercress, by targeting starch bio-synthetic enzymes, may provide evidence to its post harvest function in terms of produce longevity. Resistant starch is a popular nutritional additive to produce food with enhanced quality attributes, i.e., higher fiber content, and starchy horticultural commodities are similarly attractive. Te yield penalty of high amylose crops may be alleviated by picking an ideal AP/AM ratio through a coordinate change in the relative balance of starch bio-synthetic enzymes. In the case of potato, it is plausible that down regulation of SBEs not only produces healthy fiber-starch, but also lessens the CIS severity and acrylamide problem . However, the sugars derived from starch during CIS may be an adaptive mechanism to enhance plant chilling tolerance. Rapid sugar accumulation upon cold stress have been reported in fruit . The sugars freed from starch may promote metabolic activity and serve as an osmoprotectant, thus alleviating chilling injury. Te major functional SBEs were found to be upregulated in cold-stressed banana fruit, potato tuber, and Arabidopsis leaf, which may facilitate the ‘sugaring’ process. Modulating SBE activities may alter the rate of sugar released from the highly digestible starch polymers, thus changing the fruit/tuber cold responses. In fruiting species, the importance of ‘transitory-storage starch’ may be underestimated due to the lack of enough direct knowledge of its function, gained from experimental data. Tomato serves as a functional genomics model for feshy fruit, as it is easily transformed and genetically manipulated. The putative function of ‘transitory-storage starch’ in fruit ripening, respiration, and sweetness enhancement may be revealed by engineering AP/AM ratio through over expression or suppression of SBEs. We hypothesize that high amylose, resistant starch tomato fruit may have reduced available starch, sugars, and changes in fruit ripening and other processes that are dependent on starch as a carbon supply and source of energy post harvest. Tomato SBEs may not reffect the functionality of all fruit SBEs, but it would produce fundamental knowledge and expand our understanding of species-, organ- and developmental-specifc regulations of the core starch bio*synthetic enzymes.