Su Jing, Zhang Chunxia, Zhu Lingcheng, Yang Nanxiang, Yang Jingjing, Ma Baiquan, Ma Fengwang, Li Mingjun
State Key Laboratory of Crop Stress Biology for Arid Areas, Shaanxi Key Laboratory of Apple, College of Horticulture, Northwest A&F University, Yangling, 712100, Shaanxi, China.
College of Forestry, Northwest A&F University, Yangling, 712100, Shaanxi, China.
Biotechnol Biofuels. 2021 Jun 15;14(1):137. doi: 10.1186/s13068-021-01989-9.
Cellulose is not only a common component in vascular plants, but also has great economic benefits for paper, wood, and industrial products. In addition, its biosynthesis is highly regulated by carbohydrate metabolism and allocation in plant. MdFRK2, which encodes a key fructokinase (FRK) in apple, showed especially high affinity to fructose and regulated carbohydrate metabolism.
It was observed that overexpression of MdFRK2 in apple decreased sucrose (Suc) and fructose (Fru) with augmented FRK activity in stems, and caused the alterations of many phenotypic traits that include increased cellulose content and an increase in thickness of the phloem region. To further investigate the involved mechanisms, we generated FRK2-OE poplar lines OE#1, OE#4 and OE#9 and discovered (1) that overexpression of MdFRK2 resulted in the huge increased cellulose level by shifting the fructose 6-phosphate or glucose 6-phsophate towards UDPG formation, (2) a direct metabolic pathway for the biosynthesis of cellulose is that increased cleavage of Suc into UDP-glucose (UDPG) for cellulose synthesis via the increased sucrose synthase (SUSY) activity and transcript levels of PtrSUSY1, (3) that the increased FRK activity increases the sink strength overall so there is more carbohydrate available to fuel increased cambial activity and that resulted in more secondary phloem. These results demonstrated that MdFRK2 overexpression would significantly changes the photosynthetic carbon flux from sucrose and hexose to UDPG for increased cellulose synthesis.
The present data indicated that MdFRK2 overexpression in apple and poplar changes the photosynthetic carbon flux from sucrose and hexose to UDPG for stem cellulose synthesis. A strategy is proposed to increase cellulose production by regulating sugar metabolism as a whole.
纤维素不仅是维管植物中的常见成分，而且对纸张、木材和工业产品具有巨大的经济效益。此外，其生物合成在植物中受到碳水化合物代谢和分配的高度调控。MdFRK2在苹果中编码一种关键的果糖激酶（FRK），对果糖表现出特别高的亲和力并调节碳水化合物代谢。
观察到苹果中MdFRK2的过表达降低了茎中的蔗糖（Suc）和果糖（Fru），同时增强了FRK活性，并导致许多表型性状发生改变，包括纤维素含量增加和韧皮部区域厚度增加。为了进一步研究其中涉及的机制，我们构建了FRK2过表达杨树株系OE#1、OE#4和OE#9，并发现：（1）MdFRK2的过表达通过将6-磷酸果糖或6-磷酸葡萄糖转向UDPG的形成，导致纤维素水平大幅增加；（2）纤维素生物合成的直接代谢途径是，通过增加蔗糖合酶（SUSY）活性和PtrSUSY1的转录水平，增加蔗糖裂解为UDP-葡萄糖（UDPG）用于纤维素合成；（3）FRK活性的增加总体上增强了库强，因此有更多的碳水化合物可用于促进形成层活性增加，从而产生更多的次生韧皮部。这些结果表明，MdFRK2的过表达将显著改变光合碳通量，从蔗糖和己糖转向UDPG以增加纤维素合成。
目前的数据表明，苹果和杨树中MdFRK2的过表达改变了光合碳通量，从蔗糖和己糖转向UDPG用于茎纤维素合成。提出了一种通过整体调节糖代谢来增加纤维素产量的策略。
