دانلود رایگان ترجمه مقاله متابولیسم و انباشت قندهای انتقال یافته به میوه و تنظیم آنها (نشریه J-Stage 2010)

متابولیسم و تجمع قندهای انتقال یافته به میوه و تنظیم آنها

Metabolism and Accumulation of Sugars Translocated to Fruit and Their Regulation

Photoassimilates needed for fruit development are supplied from leaves, converted in fruit to substances relating to the specific quality of the fruit, then accumulate in the fruit. There are various regulation steps in the process from photoassimilate synthesis in leaves to sugar accumulation in fruit: photosynthesis, synthesis of translocation sugars, loading of translocation sugars, their translocation, their unloading, their membrane transport, their metabolic conversion, and compartmentation in vacuoles. Thus, it is important to clarify the mechanism and regulation of each step in fruit development. In this review, mainly the metabolic conversion of translocation sugars and their regulation at the genetic level in fruit are described because the metabolic conversion in fruit contributes greatly to produce the sink activity needed for fruit development.

1 Introduction

Photoassimilate in fruit depends mainly on supply from leaves, but some fruits in their early stages of development can supply it by their own photosynthesis. Fruit is a heterotrophic organ. However, specific substances in some fruits are generated in the fruit by themselves from photoassimilates etc., and can accumulate in fruit. Therefore, they depend on sugars translocated from leaves. The force attracting translocation sugars in fruit is called the sink strength. If the sink strength of the fruit is weak, the fruit can not grow sufficiently: growth is retarded or sometimes the fruit shrinks because of sugar deprivation. This review discusses the physiological conditions influencing sink strength during the various steps between photoassimilate synthesis in leaves and its accumulation in fruit (Fig. 1). 1.

Photosynthesis: Photoassimilate is synthesized by photosynthesis in leaves. 2. Synthesis of translocation sugars: Photoassimilate is converted to translocation sugars, such as sucrose and sorbitol. 3. Loading: Sucrose and sorbitol are carried in the phloem by a transporter. 4. Translocation: Sugar can flow from high to low sugar concentration according to the pressure flow theory. 5. Unloading: Sugars translocated in fruit tissue are carried out from phloem tissue. 6. Membrane transport: Sugars unloaded apoplastically are taken up in cells by a transporter on the plasma membrane. 7. Metabolic conversion: Translocation sugars unloaded in fruit by the symplasmic or apoplastic pathway are converted to various substances; first, invertase and sucrose synthase metabolize sucrose, sorbitol dehydrogenase metabolizes sorbitol, and then sucrose phosphate synthase synthesizes sucrose. 8. Compartmentation: Sugars in cells are mainly compartmented in vacuoles. High accumulation of sugars in vacuoles produces a high osmotic pressure that stimulates influx of water into vacuoles, leading to a high turgor pressure that is the driving force for cellular enlargement. Physiological steps for controlling the sink strength of fruit are mainly unloading, membrane transport, metabolic conversion, and compartmentation, if sufficient photoassimilate is supplied. Among these four major steps, the role of enzymes in metabolic conversion seems to be the most important for producing the sink strength of fruit because this step correlates closely with unloading and compartmentation of sugar. Thus, in this review, I want to describe mainly “metabolic conversion”.