MATE-Type Proteins Are Responsible for Isoflavone Transportation and Accumulation in Soybean Seeds

Abstract

Soybeans are nutritionally important as human food and animal feed. Apart from the macronutrients such as proteins and oils, soybeans are also high in health-beneficial secondary metabolites and are uniquely enriched in isoflavones among food crops. Isoflavone biosynthesis has been relatively well characterized, but the mechanism of their transportation in soybean cells is largely unknown. Using the yeast model, we showed that GmMATE1 and GmMATE2 promoted the accumulation of isoflavones, mainly in the aglycone forms. Using the tobacco BrightYellow-2 (BY-2) cell model, GmMATE1 and GmMATE2 were found to be localized in the vacuolar membrane. Such subcellular localization supports the notion that GmMATE1 and GmMATE2 function by compartmentalizing isoflavones in the vacuole. Expression analyses showed that GmMATE1 was mainly expressed in the developing soybean pod. Soybean mutants defective in GmMATE1 had significantly reduced total seed isoflavone contents, whereas the overexpression of GmMATE1 in transgenic soybean promoted the accumulation of seed isoflavones. Our results showed that GmMATE1, and possibly also GmMATE2, are bona fide isoflavone transporters that promote the accumulation of isoflavones in soybean seeds.

The Poly-Glutamate Motif of GmMATE4 Regulates Its Isoflavone Transport Activity

“…The poly-glutamate motif of GmMATE4 was converted from EEEEEEE to AAAEEEE (GmMATE4∆3ala) or from EEEEEEE to AAAAAAA (GmMATE4∆7ala) with the use of specially designed primers. Then, the DNA fragments were cloned into the vector pGBKT7∆BD [7]. The primers and restriction enzymes used are listed in Table S1.…”

“…The yeast isoflavone uptake assays were performed, as described previously [7,25] with slight modifications. Untransformed Y2HGold was compared to GmMATE4-transformed yeast to rule out that the uptakes of isoflavones in GmMATE4 expressing yeast cells were solely due to the endogenous yeast proteins.…”

“…Structurally, MATE proteins are generally characterized by the typical 12 TMDs [5]. However, the different amino acid residues of different MATE proteins enable different substrate specificities [1,6,7]. In plants, MATE transporters have been reported to be responsible for the movement of alkaloids, phytohormones, ion chelators, and flavonoids including anthocyanins and isoflavones across membranes [1,[6][7][8][9].…”

“…However, the different amino acid residues of different MATE proteins enable different substrate specificities [1,6,7]. In plants, MATE transporters have been reported to be responsible for the movement of alkaloids, phytohormones, ion chelators, and flavonoids including anthocyanins and isoflavones across membranes [1,[6][7][8][9]. The transport direction is largely associated with the nature of the substrate to be transported.…”

“…The transport direction is largely associated with the nature of the substrate to be transported. For example, it has been reported that GmMATE1 and GmMATE2 mediate the transport of isoflavones into the vacuole for storage [7]. In plant genomes, MATE genes usually compose a big family.…”