Increased activity of MdFRK2, a high-affinity fructokinase, leads to upregulation of sorbitol metabolism and downregulation of sucrose metabolism in apple leaves

To investigate the functions of fructokinase(FRK)in apple(Malus domestica)carbohydrate metabolism,we cloned the coding sequences of MdFRK1 and MdFRK2 from the‘Royal Gala’apple.The results showed that MdFRK2 expression was extremely high in shoot tips and young fruit.Analyses of heterologously expressed proteins revealed that MdFRK2 had a higher affinity for fructose than did MdFRK1,with Km values of 0.1 and 0.62 mM for MdFRK2 and MdFRK1,respectively.The two proteins,however,exhibited similar Vmax values when their activities were significantly inhibited by high concentrations of fructose.MdFRK2 ectopic expression was associated with a general decrease in fructose concentration in transgenic lines.In leaves,increased FRK activity similarly resulted in reduced concentrations of glucose and sucrose but no alterations in sorbitol concentration.When compared with those in the untransformed control,genes involved in sorbitol synthesis(A6PR)and the degradation pathway(SDH1/2)were significantly upregulated in transgenic lines,whereas those involved in sucrose synthesis(SPS1)and other degradation processes(SUSY4,NINV1/2,and HxK2)were downregulated.The activity of enzymes participating in carbohydrate metabolism was proportional to the level of gene expression.However,the growth performance and photosynthetic efficiency did not differ between the transgenic and wild-type plants.These results provide new genetic evidence to support the view that FRK plays roles in regulating sugar and sorbitol metabolism in Rosaceae plants.

Sugar Input, Metabolism, and Signaling Mediated by Invertase： Roles in Development, Yield Potential, and Response to Drought and Heat

Invertase （INV） hydrolyzes sucrose into glucose and fructose, thereby playing key roles in primary metabolism and plant development. Based on their pH optima and sub-cellular locations, INVs are categorized into cell wall, cytoplasmic, and vacuolar subgroups, abbreviated as CWlN, CIN, and VlN, respectively. The broad importance and implications of INVs in plant development and crop productivity have attracted enormous interest to examine INV function and regulation from multiple perspectives. Here, we review some exciting advances in this area over the last two decades, focusing on （1） new or emerging roles of INV in plant development and regulation at the post-translational level through interaction with inhibitors, （2） cross-talk between INV-mediated sugar signaling and hormonal control of development, and （3） sugar- and INV-mediated responses to drought and heat stresses and their impact on seed and fruit set. Finally, we discuss major questions arising from this new progress and outline future directions for unraveling mechanisms underlying INV-mediated plant development and their potential applications in plant biotechnology and agriculture.

Overexpression of a Potato Sucrose Synthase Gene in Cotton Accelerates Leaf Expansion, Reduces Seed Abortion, and Enhances Fiber Production

Sucrose synthase （Sus） is a key enzyme in the breakdown of sucrose and is considered a biochemical marker for sink strength, especially in crop species, based on mutational and gene suppression studies. It remains elusive, however, whether, or to what extent, increase in Sus activity may enhance sink development. We aimed to address this question by expressing a potato Sus gene in cotton where Sus expression has been previously shown to be critical for normal seed and fiber development. Segregation analyses at T1 generation followed by studies in homozygous progeny lines revealed that increased Sus activity in cotton （1） enhanced leaf expansion with the effect evident from young leaves emerging from shoot apex; （2） improved early seed development, which reduced seed abortion, hence enhanced seed set, and （3） promoted fiber elongation. In young leaves of Sus overexpressing lines, fructose concentrations were significantly increased whereas, in elongating fibers, both fructose and glucose levels were increased. Since hexoses contribute little to osmolality in leaves, in contrast to developing fibers, it is concluded that high Sus activity promotes leaf development independently of osmotic regulation, probably through sugar signaling. The analyses also showed that doubling the Sus activity in 0-d cotton seeds increased their fresh weight by about 30%. However, further increase in Sus activity did not lead to any further increase in seed weight, indicating an upper limit for the Sus overexpression effect. Finally, based on the observed additive effect on fiber yield from increased fiber length and seed number, a new strategy is proposed to increase cotton fiber yield by improving seed development as a whole, rather than solely focusing on manipulating fiber growth.

Molecular regulation of sucrose catabolism and sugar transport for development~ defence and phloem function

Sucrose （Suc） is the major end product of photosynthesis in mesophyll cells of most vascular plants. It is loaded into phloem of mature leaves for long-distance translocation to non-photosynthetic organs where it is unloaded for diverse uses. Clearly, Suc transport and metabolism is central to plant growth and development and the functionality of the entire vascular system. Despite vast information in the literature about the physiological roles of individual sugar metabolic enzymes and transporters, there is a lack of systematic evaluation about their molecular regulation from transcriptional to post-translational levels. Knowledge on this topic is essential for understanding and improving plant development, optimizing resource distri- bution and increasing crop productivity. We therefore focused our analyses on molecular control of key players in Suc metabolism and transport, including： （i） the identifica- tion of promoter elements responsive to sugars and hormones or targeted by transcription factors and micro- RNAs degrading transcripts of target genes; and （ii） modulation of enzyme and transporter activities through protein-protein interactions and other post-translational modifications. We have highlighted major remaining questions and discussed opportunities to exploit current understanding to gain new insights into molecular control of carbon partitioning for improving plant performance.

Boosting Seed Development as a New Strategy to Increase Cotton Fiber Yield and Quality

Cotton （Gossypium spp.） is the most important textile crop worldwide due to its cellulosic mature fibers, which are single-celled hairs initiated from the cotton ovule epidermis at anthesis. Research to improve cotton fiber yield and quality in recent years has been largely focused on identifying genes regulating fiber cell initiation, elonga- tion and cellulose synthesis. However, manipulating some of those candidate genes has yielded no effect or only a marginally positive effect on fiber yield or quality. On the other hand, evolutionary comparison and transgenic studies have clearly shown that cotton fiber growth is intimately controlled by seed development. Therefore,I propose that enhancing seed development could be a more effective and achievable strategy to increase fiber yield and quality.

Evidence for the Role Evolutionary Increase Yield in Cotton of Transfer Cells in the in Seed and Fiber Biomass

Transfer cells （TCs） are specialized cells exhibiting invaginated wall ingrowths （Wls）, thereby amplifying their plasma membrane surface area （PMSA） and hence the capacity to transport nutrients. However, it remains unknown as to whether TCs play a role in biomass yield increase during evolution or domestication. Here, we examine this issue from a comparative evolutionary perspective. The cultivated tetraploid AD genome species of cotton and its A and D genome diploid progenitors displayed high, medium, and low seed and fiber biomass yield, respectively. In all three species, cells of the innermost layer of the seed coat juxtaposed to the filial tissues trans-differentiated to a TC morphology. Electron microscopic analyses revealed that these TCs are characterized by sequential formation of flange and reticulate Wls during the phase of rapid increase in seed biomass. Significantly, TCs from the tetraploid species developed substantially more flange and reticulate Wls and exhibited a higher degree of reticulate WI formation than their progenitors. Consequently, the estimated PMSA of TCs of the tetraploid species was about 4 and 70 times higher than that of TCs of the A and D genome progenitors, respectively, which correlates positively with seed and fiber biomass yield. Further, TCs with extensive Wls in the tetraploid species had much stronger expression of sucrose synthase, a key enzyme involved in TC Wl formation and function, than those from the A and D progenitors. The analyses provide a set of novel evidence that the development of TC Wls may play an important role in the increase of seed and fiber biomass yield through polyploidization during evolution.

Signaling Role of Sucrose Metabolism in Development

In most higher plants, sucrose is the primary organic carbon that is translocated through phloem from photosynthetic leaves （source） into non-photosynthetic tissues （sink） such as seed, fruit, and root. After phloem unloading in sinks, sucrose needs to be degraded into hexoses for diverse use by either invertase （Inv） that hydrolyses sucrose into glucose and fructose or sucrose synthase （Sus） that degrades sucrose into UDPglucose and fructose. By generating hexoses and their derivates, Inv- or Sus-mediated sucrose metabolism and re- lated transport process provide （1） energy source to power cel- lular processes; （2） starting molecules convertible to numerous metabolites and building blocks for synthesizing essential pol- ymers including starch, cellulose, callose, and proteins; and （3） a mechanism to reduce sucrose concentration at the unloading sites to facilitate its source-to-sink translocation, thereby pre- venting feedback inhibition on photosynthesis and sustaining carbon flow at the whole-plant level.

Live Long and Prosper：. Roles of Sugar and Sugar Polymers in Seed Vigor

Seed vigor refers to the overall capacity of the seed to （1） germi- nate and emerge after sowing （i.e., germination vigor） and （2） retain this potential during postharvest storage, known as seed longevity. Seed vigor plays vital roles in plant growth and realiza- tion of crop productivity. Long shelf life of seeds and their rapid and uniform germination and seedling emergence are highly desirable commercial traits. Despite its central importance, how seeds acquire their vigor is poorly understood.

Translate Plant Metabolism into Modern Agriculture： A Starting Point

Plants, the primary producers on the Earth, use solar energy to convert CO2 and H2O into various organic compounds. Over the past 10 000 years, humans have exploited the plant synthesizing capability and developed agricultural practice to meet the basic needs for food, fiber, energy, chemicals, and medicines. Actually, all of the products are an assortment of compounds synthesized in plants though a variety of metabolic processes. It has been estimated that any given plant species can synthesize 5000-25 000 different compounds （Trethewey, 2004）. More than 100 000 metabolites, which may account for approximately 10% of the total natural products, have been identified in the plant kingdom.