Abstract
In maize seed germination, the endosperm and the scutellum nourish the embryo axis. Here, we examined the mRNA relative amount of the SWEET protein family, which could be involved in sugar transport during germination since high [14-C]-glucose and mainly [14-C]-sucrose diffusional uptake were found in embryo tissues. We identified high levels of transcripts for SWEETs in the three phases of the germination process: ZmSWEET4c, ZmSWEET6b, ZmSWEET11, ZmSWEET13a, ZmSWEET13b, ZmSWEET14b and ZmSWEET15a, except at 0 h of imbibition where the abundance of each ZmSWEET was low. Despite the major sucrose (Suc) biosynthesis capacity of the scutellum and the high level of transcripts of the Suc symporter SUT1, Suc was not found to be accumulated; furthermore, in the embryo axis, Suc did not decrease but hexoses increased, suggesting an efficient Suc efflux from the scutellum to nourish the embryo axis. The influx of Glc into the scutellum could be mediated by SWEET4c to take up the large amount of transported sugars due to the late hydrolysis of starch. In addition, sugars regulated the mRNA amount of SWEETs at the embryo axis. These results suggest an important role for SWEETs in transporting Suc and hexoses between the scutellum and the embryo axis, and differences in SWEET transcripts between both tissues might occur because of the different sugar requirements and metabolism.
Highlights
Sucrose (Suc) is the main carbohydrate synthesized during photosynthesis, and is mainly exported through the vascular system and distributed to non-photosynthetic sink tissues to support its growth and development
Maize SWEETs are distributed among the four clades, but most fall into clade III, which groups most of the SWEET transporters from A. thaliana and rice
We studied some of the events that take place in the three phases of germination; in order to do so, the scutellum and the embryo axis were isolated at 0, 18 and 30 h to cover phases I and II of germination, and for phase III
Summary
Sucrose (Suc) is the main carbohydrate synthesized during photosynthesis, and is mainly exported through the vascular system and distributed to non-photosynthetic sink tissues to support its growth and development. Apoplastic transport depends on several families of transporters: SUTs Transporter) and STPs (Sugar Transporter Protein) are families of Suc and hexose transporters, respectively, that drive the accumulation of sugars into the cells or vacuoles at the expense of the proton gradient [1,2]. The SWEET family is composed of transporters that move Suc or hexoses in the direction of the concentration gradient at the plasma membrane or tonoplast [3]. Suc efflux from the parenchyma phloem cells is mediated by SWEET11 and 12 in Arabidopsis thaliana [3,4]. In Zea mays, SWEET13a, b and c are essential for allocating the sugars from the photosynthetic cells to the phloem [5]. Sugar secretion from the nectar parenchyma in eudicots is produced by AtSWEET9 [6]
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