Advance in starch metabolism research of kiwifruit

Abstract: Kiwifruit (Actinidia chinensis Planch.) is well known as“the king of fruit”and deeply loved by consumers at home and abroad because of its unique flavor and being rich in a variety of vitamins, dietary fiber, mineral elements and other nutrients. As the rapid development of kiwifruit industry in China, kiwifruit has become“Golden fruit”of the targeted poverty alleviation and rural revitalization. Starch, as the main carbohydrate derived from carbon with plant photosynthesis, plays an important role in plant whole growth and development. The kwifruit belongs to the starch-accumulating fruit, and the photosynthetic products are accumulated and converted into starch during the fruit growth and development close to the commercial harvest. The starch in kiwifruit is present in the form of particles, which increase from 3-4 μm to 10-12 μm during the fruit growth and then decrease to 6-8 μm with maturity and then disappeared finally when fruit ripens. The starch accumulation is strongly similar among different cultivars or germplasm, but the starch content differs at same stages of fruit growth and development. Initially, there is little starch accumulation in the early stages of fruit development, and starch starts to accumulate only after the increase of the cell volume and weight and reaches to the peak close to the commercial harvest, accounting for 40% of the dry matter of the fruit. At this time, 80% of the starch in the pericarp is mainly amylopectin. During storage period after harvesting, the starch is degraded into sugar with fruit softening and ripening, leading to the increase of sweetness with about 10% of sugar content and the formation of fruit flavour. In higher plants, the starch metabolism involves starchbiosynthesis and starch degradation pathways. There are two ways to synthesis starch, including transient starch synthesis in the chloroplasts of photosynthetic tissue and storage starch synthesis in the amyloplast of non-photosynthetic tissue. The starch degradation begins with the hydrolysis of intact starch granules, and then the α-1, 6-glucoside bond is transferred to form linear dextran and finally degraded into glucose under the action of a series of enzymes. The starch metabolic pathway has been thoroughly studied in Arabidopsis thaliana and cereals, and the genes encoding enzymes involved in the starch metabolic pathway such as AGP pyrophosphorylase (AGPase) starch synthase (SSS), starch branching enzyme (SBE), starch debranching enzyme (DBE), starch phosphorylase (SP), α-amylase (AMY) and β- amylase (BAM) has also been identified. Compared with the starch degradation pathway in kiwifruit, the molecular mechanism of starch biosynthesis and accumulation before harvest is still unclear. The studies of starch content during kiwifruit growth and development have been largely reported, as well as the enzymes involved in starch biosynthesis. The AGPase enzyme is proposed to be the key enzyme for starch synthesis but without strong evidences, and the genes encoding AGPase and other biosynthetic enzymes and the molecular regulatory mechanism for starch synthesis in kiwifruit is still unknown. The kiwifruit is an atypical climacteric fruit type with softening and ripening ability after harvest, and easy to soften and decay after harvest, and does not store well for a long time at ambient temperature. How to prolong storage and shelf life periods without sacrificing fruit quality is always the hot spot of kiwifruit research. Starch, as the cell filling contents plays an essential role in maintaining cell turgor and supporting fruit firmness. Therefore, starch degradation is strongly associated with fruit softening and thus more attention has been paid, compared with the starch biosynthesis. The starch degradation in kiwifruit is regulated by not only ethylene and also low temperature. Although the kiwifruit itself produces very low amount of ethylene, but is very sensitive to exogenous ethylene. Even extremely low concentration of ethylene (0.1 μL·L^{- 1} ) still can promote starch degradation and fruit softening at low temperature. The ethylene-induced fruit ripening has been completely and deeply studied. Meanwhile, several recent reports indicated that low temperature at appropriately 10 degree could also induce starch degradation and fruit softening under no detectable ethylene present, suggesting fruit ripening induced by low temperature could be another regulation way, independent on ethylene regulation pathway. Utilizing the low temperature to induce fruit softening and ripening becomes an alternative way to provide ready-to-eat fruit for packhouse and consumers, and now this method applied in postharvest commercial management has appeared, but the scale is relatively small and the operation protocol is not well developed. The concern is also taken into account that the ripened fruit due to low temperature usually lacks volatile aroma of ethylene-induced ripe fruit. With the completion of the genome sequencing of the kiwifruit, the research of the starch metabolism in kiwifruit has gradually shifted from the traditional study of starch accumulation pattern and the change of metabolic enzyme activity to the study of important gene mining and molecular regulation mechanism, and some new progresses have been made in the molecular regulatory mechanism of the starch degradation. However, substantial breakthroughs have not been made in the molecular regulation of the starch synthesis and accumulation up to now, and the summary of the starch metabolism studies in kiwifruit is still limited. Therefore, this review focused on the physio-chemical properties of the starch in kiwifruit, the starch metabolic pathway of plant and the molecular mechanism of the starch metabolism in kiwifruit. Combined with the relationship between starch metabolism and flavor quality, ripening and softening of kiwifruit, the current status and progresses of the starch researches in kiwifruit were reviewed. In future, the molecular regulatory mechanism of the starch degradation and fruit flavor formation should be further studied, and the study ofstarch synthesis pathway and molecular regulation mechanism should be deeply strengthened, which is of great significance for creating new varieties or new germplasm with high content of starch and high quality, and controlling fruit softening and ripening to provide ready-to-eat kiwifruit.