Nitrogen Levels Regulate Sugar Metabolism and Transport in the Shoot Tips of Crabapple Plants.

Simin Sun,Baiyun Li,Baiquan Ma,Mingjun Li,Yonghui Liang,Lihua Zhang,Zhengyang Wang,Songya Ma

doi:10.3389/fpls.2021.626149

Abstract

To comprehensively understand the responses of carbohydrate metabolism and transport to different levels of nitrogen supply in growing shoot tips of crabapple (Malus hupehensis Rehd), enzyme activities and related genes involved in the sugar metabolism pathway were investigated. The nitrogen and chlorophyll content of plants increased with increasing nitrogen supply. High nitrogen application increased the net photosynthesis rate and the growth rate of shoot tips but decreased the synthesis capability of sucrose and sorbitol in mature leaves. However, the shoot tips of plants under high-nitrogen treatment had higher contents of sucrose and sorbitol than did those under low-nitrogen treatment, while the activity of sucrose phosphate synthase and aldose-6-phosphate was increased and the transporters MdSOT and MdSUT were up-regulated. Moreover, the activities of enzymes involved in sucrose and hexose metabolism (including sucrose synthase, fructokinase, and hexokinase) were enhanced in the shoot tips of plants under high-nitrogen conditions, and the expression levels of MdSUSY3 and MdHK1 were significantly up-regulated. These findings indicate that a high nitrogen supply increases the metabolic capacity of assimilatory substances in shoot tips, accelerates the efficiency of sugar utilization and eventually leads to a rapid increase in the growth of shoot tips. Our results highlight that high nitrogen increases the capacity of sugar unloading and metabolic utilization in growing shoot tissues.

Highlights

As the core mineral nutrient element, nitrogen (N) influences the processes of photosynthesis, respiration, and carbohydrate and signal transport in plants by synthesizing active substances such as proteins, nucleic acids, hormones, chlorophyll, vitamins, and several metabolic enzymes and participates in material and energy metabolism in plant cells (Krapp, 2015)
The results showed that the total nitrogen content of the leaves and roots was approximately 23 mg/g under normal conditions
The nitrogen content of the leaves and roots was approximately 27 and 25 mg/g, respectively; in addition, the growth rate of the shoots increased by 10.2% compared with that of the control, but the new root biomass decreased by 37.8%

Summary

Introduction

As the core mineral nutrient element, nitrogen (N) influences the processes of photosynthesis, respiration, and carbohydrate and signal transport in plants by synthesizing active substances such as proteins, nucleic acids, hormones, chlorophyll, vitamins, and several metabolic enzymes and participates in material and energy metabolism in plant cells (Krapp, 2015). In other parts of shoot tips, sucrose can be transported passively, moving along the concentration gradient between cells through plasmodesmata or via active transport involving transporters against the concentration gradient, such as membrane-localized sucrose transporters (SUTs) and SWEET proteins (Riesmeier et al, 1992; Lemoine, 2000; Milne et al, 2018) Soluble sugars such as sucrose, glucose (Glc), and fructose (Fru) can be transported into the vacuole by corresponding transporters localized on the vacuolar membrane (SOT, SUT, TST, etc.), and increasing sugar concentrations in the vacuoles activate enzymes and genes associated with the sugar metabolism system, regulating the osmotic balance in different compartments of the cell (Hedrich et al, 2015)

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Results

Conclusion