Soybean is a major oil crop and the main resource of vegetable oil. Seed oil content is a momentous agronomical trait of soybean targeted by domestication, and it is steadily increased during long-term breeding practice. Although multiple oil biosynthesis genes have been uncovered, the knowledge about the regulatory mechanism of seed oil biosynthesis is currently very limited especially in soybean. Based on development of soybean genomics, combined with improvement of soybean transformation technology, we can utilize genetic engineering to manipulate seed oil accumulation.
Researchers at ZHANG Jinsong’s group and CHEN Shouyi’s group from Institute of Genetics and Developmental Biology, Chinese Academy of Sciences, identified and characterized.genes regulating lipogenesis and demonstrate that seed-preferred gene GmZF351 is selected during domestication for oil accumulation.
Given that transcription factors may play important regulatory role in seed oil accumulation, we previously investigated differentially expressed genes between developing soybean seeds and vegetative organs including leaves, stems or roots based on RNA-seq analysis, and finally identified 69 seed-preferred potential transcription factor genes. In the present study, 30 of these candidate genes were overexpressed in Arabidopsis, and the lipid contents in seeds of transgenic plants were evaluated. Among these, GmZF351 appeared to participate in lipid biosynthesis, and were further investigated in details.
GmZF351 was abundantly expressed in soybean seeds, and accumulated steadily from stage H1 to stage H6 during seed development. The gene encoded tandem CCCH zinc finger proteins which located at nucleus of cells and possessed transcriptional activation activity. Overexpression of GmZF351 increased seed oil content of transgenic Arabidopsis plants, through direct activation of lipid synthesis and storage genes BCCP2, KASIII, TAG1 and OLEO2. Furthermore, GmZF351 positively regulated a transcription factor gene WRI1, which has been demonstrated to enhance seed oil accumulation in Arabidopsis and several other plants. In addition, GmZF351 also promoted expression of pyruvate kinase genes Pkpα and Pkpβ1, and increased pyruvate kinase activity. This activity would elevate pyruvate synthesis in the glycolysis pathway for further acyl-CoA synthesis, and finally facilitate fatty acid biosynthesis and seed oil accumulation.
By comparing transcript levels of GmZF351 in cultivated soybeans and wild soybeans, together with the phylogenetic analysis of promoter region, GmZF351 was found to be subjected to selection during soybean domestication. ZF351 expression positively correlated with seed oil contents. Moreover, the ZF351 haplotype from G. max group and G. soja group III was well correlated with higher gene expression, higher lipid contents and higher promoter activity compared to other haplotypes. These results suggest that GmZF351 haplotype was fixed during domestication, thereby leading to improvement of seed oil content in cultivated soybean.
Domestication-selected GmZF351 was further overexpressed in soybean and this gene facilitates lipid accumulation in transgenic soybean seeds. GmZF351 could induce expression of WRI homologues Glyma15g34770 and Glyma08g24420, BCCP2 homologue Glyma19g03530, KASIII homologue Glyma15g00550, TAG1 homologues Glyma13g16560 and Glyma17g06120 as well as OLEO2 homologues Glyma19g13060 and Glyma16g07800.
Taken together, a master transcription factor GmZF351 is identified to activate the downstream genes including WRI1, BCCP2, KASIII, TAG1, OLEO2, Pkpα and Pkpβ1, which are involved in glycolysis, the fatty acid biosynthesis, the TAG biosynthesis and oil storage process. GmZF351 may be subjected to selection during soybean domestication.
This study reveals a novel regulatory mechanism of seed oil accumulation, and manipulation of the genes may have great potential for seed oil improvement in soybean and many other oil crops.
This work entitled “Selection for a Zinc-finger Protein Contributes to Seed Oil Increase During Soybean Domestication” has been published online in Plant Physiology on Feb 9, 2017 (DOI:10.1104/pp.16.01610
), and is supported by the CAS leading project and other projects.
A model for GmZF351 regulates seed oil content during soybean domestication (Image by IGDB)
Dr. ZHANG Jinsong and Dr. CHEN Shouyi