Recently, scientists at Stanford University, Iowa State University, and the University of California found that during the domestication of corn and rice, SWEET (a type of sugar transport protein) mediated hexose transport regulates the seed filling process.
The ability of soluble sugars to transport to developing seeds determines the size and number of cells and ultimately affects the size of the seeds. Seeds must efficiently absorb nutrients from mature tissues to promote their own development. Previously, it was widely believed that the long-term domestication of crops affected the regulation mechanism of hormones on plants. However, there is little evidence to show how the domestication process selects the sugar transport mechanism during seed filling. For example, the seeds of corn become larger in the selection process, but the functional sites related to carbohydrate metabolism and transport are less studied.
By comparing and screening published databases of different maize gene expressions, the researchers found that ZmSWEET4c has large differences in the early origin of maize and maize, and is highly abundant in seeds. The gene encodes a class of sugar transporters, which are responsible for transporting the phloem transported sugars to surrounding organs. It may be an important carbohydrate metabolism or transport function site during corn domestication and participate in the regulation of seed filling.
The researchers first compared the sequence differences between ZmSWEET4c in maize and alfalfa, and found that the gene has a large difference in the promoter sequence, and ZmSWEET4c has a higher expression level in maize during seed development. At the same time, the T-DNA insertion mutant of the gene showed an "empty peel" phenotype, that is, the endosperm and embryo became smaller than the wild type, but the embryo could still develop into a fertile individual, indicating that ZmSWEET4c specifically Affecting the filling process of seeds.
The researchers confirmed that the gene was localized on the cell membrane of the basal endosperm transfer layer (BETL) by means of in situ hybridization, RNA-seq, subcellular localization, etc., and through the substrate-specific absorption experiment and the yeast glucose absorption-deficient strain complementation experiment. This indicates that the gene mainly transports six carbon sugars such as glucose and fructose from the phloem to the endosperm via the basal endosperm transfer layer, thereby affecting the seed filling.
The study found that the mutant of this gene decreased the ability of hexose to transport to the basal endosperm metastasis layer, resulting in incomplete development of BELT. At the same time, the expression of OsSWEET4 was induced by glucose, and a positive feedback mechanism model for regulating grouting was proposed, namely ZmSWEET4c. After the delivery of the hexoses to BETL, an increase in the area of ​​the BETL membrane and an increase in the transport activity of ZmSWEET4c were induced, which ultimately enhanced the ability of glucose and fructose to be delivered to the endosperm.
Finally, the researchers extended the study to other species such as rice and found that the only homologous gene in rice, OsSWEET4, is also highly expressed in seeds. Its expression is induced by glucose and has six-carbon sugars such as glucose and fructose. The ability of the mutant to exhibit a phenotype of grouting defects, ie, the endosperm becomes smaller and the seed setting rate decreases. Sequence alignment between different rice varieties indicates that this gene is also an important carbohydrate metabolism or transport function site during rice domestication.
Therefore, the researchers concluded that the plant SWEET transporter regulates the seed filling process and can provide new ideas for exploring high-yield maize and rice new varieties.