Department of Plant and Environmental Sciences, New Mexico State University, Las Cruces, NM, 88003, USA.
Department of Pharmaceutical Sciences, University of New Mexico, Albuquerque, NM, USA.
Planta. 2019 Nov;250(5):1743-1755. doi: 10.1007/s00425-019-03261-9. Epub 2019 Aug 17.
In alfalfa, the B form of Sucrose phosphate synthase synthesizes sucrose in the leaves while the A form participates in regulatory cycles of synthesis/breakdown of sucrose/starch in the root nodules. Sucrose (Suc) is the major stable product of photosynthesis that is transported to all heterotrophic organs as a source of energy and carbon. The enzyme sucrose phosphate synthase (SPS) catalyzes the synthesis of Suc. Besides the leaves, SPS is also found in heterotrophic organs. There are two isoforms of SPS in alfalfa (Medicago sativa): SPSA and SPSB. While SPSA is expressed in the vasculature of all the organs and in the N-fixing zone in the nodules, SPSB is exclusively expressed in the photosynthetic cells. Two classes of alfalfa transformants were produced, one with a gene construct consisting of the alfalfa SPSA promoter and the other with the SPSB promoter-both driving the maize SPS coding region-referred to as SPSA-ZmSPS and SPSB-ZmSPS, respectively. Both classes of transformants showed increased growth compared to control plants. The SPSB-ZmSPS transformants showed increased SPS protein levels and activity along with a significant increase in the Suc levels in the leaves. The SPSA-ZmSPS transformants showed an increase in the SPS protein level and enzyme activity both in the leaves and the nodules with no increase in Suc content in the leaves but a substantial increase in the nodules. Both SPSA and SPSB have unique roles in the nodules (sink) and leaves (source). SPSB is responsible for the synthesis of Suc in the photosynthetic cells and SPSA participates in a regulatory cycle in which Suc is simultaneously degraded and re-synthesized; both these functions contribute to plant growth in rhizobia nodulated alfalfa plants.
在紫花苜蓿中，B 型蔗糖磷酸合酶在叶片中合成蔗糖，而 A 型参与根瘤中蔗糖/淀粉合成/分解的调节循环。蔗糖（Suc）是光合作用的主要稳定产物，被运输到所有异养器官作为能量和碳的来源。蔗糖磷酸合酶（SPS）催化蔗糖的合成。除了叶片，SPS 也存在于异养器官中。紫花苜蓿中有两种 SPS 同工型：SPSA 和 SPSB。虽然 SPSA 在所有器官的脉管系统和根瘤的固氮区表达，但 SPSB 仅在光合细胞中表达。产生了两类苜蓿转化体，一种带有由紫花苜蓿 SPSA 启动子和另一种带有 SPSB 启动子组成的基因构建体-分别驱动玉米 SPS 编码区-分别称为 SPSA-ZmSPS 和 SPSB-ZmSPS。与对照植物相比，这两类转化体的生长都有所增加。SPSB-ZmSPS 转化体表现出 SPS 蛋白水平和活性的增加，以及叶片中 Suc 水平的显著增加。SPSA-ZmSPS 转化体在叶片和根瘤中均表现出 SPS 蛋白水平和酶活性的增加，而叶片中 Suc 含量没有增加，但根瘤中增加了很多。SPSA 和 SPSB 在根瘤（汇）和叶片（源）中都有独特的作用。SPSB 负责光合细胞中蔗糖的合成，而 SPSA 参与蔗糖同时降解和重新合成的调节循环；这两种功能都有助于根瘤菌结瘤苜蓿植物的生长。
