Sucrose synthase (SuSy) is one of two enzyme families capable of catalyzing the first degradative step in sucrose utilization. Several earlier studies examining SuSy mutants in Arabidopsis failed to identify obvious phenotypic abnormalities compared with wild-type plants in normal growth environments, and as such a functional role for SuSy in the previously proposed cellulose biosynthetic process remains unclear. Our study systematically evaluated the precise subcellular localization of all six isoforms of Arabidopsis SuSy via live-cell imaging. We showed that yellow fluorescent protein (YFP)-labeled SuSy1 and SuSy4 were expressed exclusively in phloem companion cells, and the sus1/sus4 double mutant accumulated sucrose under hypoxic conditions. SuSy5 and SuSy6 were found to be parietally localized in sieve elements and restricted only to the cytoplasm. SuSy2 was present in the endosperm and embryo of developing seeds, and SuSy3 was localized to the embryo and leaf stomata. No single isoform of SuSy was detected in developing xylem tissue of elongating stem, the primary site of cellulose deposition in plants. SuSy1 and SuSy4 were also undetectable in the protoxylem tracheary elements, which were induced by the vascular-related transcription factor VND7 during secondary cell wall formation. These findings implicate SuSy in the biological events related to sucrose translocation in phloem.

中文翻译：

---

拟南芥蔗糖合酶定位表明韧皮部蔗糖易位的主要作用。

蔗糖合酶（SuSy）是能够催化蔗糖利用中第一个降解步骤的两个酶家族之一。与正常生长环境中的野生型植物相比，在拟南芥中检测SuSy突变体的几项早期研究未能发现明显的表型异常，因此，对于SuSy在先前提出的纤维素生物合成过程中的功能性作用尚不清楚。我们的研究通过活细胞成像系统地评估了拟南芥SuSy的所有六个同工型的精确亚细胞定位。我们显示黄色荧光蛋白（YFP）标记的SuSy1和SuSy4仅在韧皮部陪伴细胞中表达，并且sus1 / sus4双重突变体在缺氧条件下积累了蔗糖。发现SuSy5和SuSy6局部位于筛网元件中，仅限于细胞质。SuSy2存在于发育中的种子的胚乳和胚芽中，而SuSy3则位于胚芽和叶片气孔中。在伸长的茎的木质部组织（植物中纤维素沉积的主要部位）中未检测到SuSy的单一同工型。SuSy1和SuSy4在protoxylem气管元件中也未检测到，这是在次级细胞壁形成过程中由血管相关转录因子VND7诱导的。这些发现暗示SuSy参与韧皮部中蔗糖易位相关的生物学事件。在伸长的茎的木质部组织（植物中纤维素沉积的主要部位）中未检测到SuSy的单一同工型。SuSy1和SuSy4在protoxylem气管元件中也未检测到，这是在次级细胞壁形成过程中由血管相关转录因子VND7诱导的。这些发现暗示SuSy参与韧皮部中蔗糖易位相关的生物学事件。在伸长的茎的木质部组织（植物中纤维素沉积的主要部位）中未检测到SuSy的单一同工型。SuSy1和SuSy4在protoxylem气管元件中也未检测到，这是在次级细胞壁形成过程中由血管相关转录因子VND7诱导的。这些发现暗示SuSy参与韧皮部中蔗糖易位相关的生物学事件。