The Biosynthesis of Dihydrochalcone (DHC) Glycosides Necessary for the Development of Apple Seeds is Regulated by Two MYB Like Transcription Factors

Pengmin Li’s team from Northwest A&F University has made new progress in the study of the biosynthesis mechanism of dihydrochalcone (DHC) glycosides in apples. The research results published in The Plant Journal (IF=7.2, Q1).

Dihydrochalcones (DHCs) are an uncommon class of flavonoid compounds. Dihydrochalcone (DHCs), including phloridzin (phloridzin-20 '- O-glucoside) and its positional isomer trefoil glycoside (phloridzin-40' - O-glucoside), as a tool and drug for human physiological research, especially in the treatment of type 2 diabetes, has aroused widespread concern in the scientific community. So far, more than 200 types of DHCs have been identified in more than 40 plant families, including Asteraceae, Leguminosae, Rosaceae, and Rutaceae. Among these plants, the high content and rich diversity of DHCs in apples make them excellent model plants for studying the biosynthesis and regulation of DHC compounds.

A study analyzed transgenic apple plants and found that two glycosyltransferase genes (PGT1 and PGT2) are involved in the biosynthesis of dihydrochalcone (DHC) glycosides in plants. So far, the transcriptional regulation of PGT1 and PGT2 is still unclear. Transcription factors (TFs) such as MYB10 that regulate phenylpropanoid and anthocyanin biosynthesis in apples do not regulate dihydrochalcone (DHC) glycoside synthesis. The biological functions of DHCs in apples are not fully understood, and their roles in auxin transport inhibition, allelopathy, and growth regulation have been confirmed. So far, there have been no reports on apple species that do not contain DHCs, indicating that DHCs play an unknown but important role in the adaptability or survival of apples, and there is relatively little research in this area.

This study investigated the genetic basis of dihydrochalcone (DHC) glycosides in different genotypes of apples. Two transcriptional regulatory factors (TFs) that determine the synthesis pattern of DHCs were identified in apple tissue. The important role of DHCs in apple survival has also been confirmed.

This study showed through hybrid population segregation and mixed segregation analysis that the synthesis of root bark glycosides and trefoil glycosides in apple leaves is controlled by a single gene. The promoter sequences of two glycosyltransferase genes, PGT1 and PGT2, involved in the synthesis of dihydrochalcone (DHC) glycosides, have been shown to distinguish different dihydrochalcone (DHC) glycoside synthesis patterns.

Mode 1: In leaves, flowers, fruits, embryos, seed coats, and stems, the transcription regulatory factor PRR2L regulates the transcription of PGT1 and PGT2, as well as the production of phloroside (P) and phloroside (T). The PGT1 promoter lacks a MYB binding site of~3500 bp upstream of the start codon at 160, 354, 365, 536, and 627 bp, while the PGT2 promoter mutates the MYB binding site at 303 bp upstream of the start codon, resulting in hindered gene expression and product synthesis;

Mode 2: In roots and stems, the transcription of PGT1 and the production of root bark glycosides are regulated by MYB8L through a binding site located 24 bp upstream of the start codon in the promoter. MYB8L does not regulate the expression of PGT2. For genotype A * B * (AABB, Aa BB, AABb, Aa Bb), both root bark glycoside and trefoil glycoside are synthesized; For the A * bb (AAbb, Aabb) genotype, only anthocyanins are synthesized; For the aaB * (aABB, aaBb) genotype, synthesize trilobin; For the genotype of aabb, no dihydrochalcone was synthesized (dihydrochalcone may affect embryonic development through interaction with auxin).

▲Genetic regulation model of dihydrochalcone glycoside synthesis in apple plants

To confirm the differences in PGT1 and PGT2 promoter activity among different genotypes of apple plants. The author used the Biolight Biotechnology PlantView in vivo plant imaging system to capture the leaves and roots of transgenic apples fused with GFP promoter. Md PGT1pro:: GFP and Mt PGT1pro:: GFP both have strong GFP signals in the roots, while Mt PGT2.3pro:: GFP and Md PGT2.1pro:: GFP were not observed in transgenic apple roots. The expression of PGT1 and PGT2 is correlated with different dihydrochalcone (DHC) glycoside synthesis patterns in roots, with high expression of PGT1 and low expression of PGT2. The expression levels of PGT1 or PGT2 alleles are similar in roots.

▲ Activity of PGT1 and PGT2 promoters in apples of different genotypes

(a) Expression of GFP in transgenic apple plant leaves mediated by Agrobacterium tumefaciens

(b) Agrobacterium mediated expression of GFP in transgenic apple roots

Reference

https://doi.org/10.1111/tpj.16444