Liu Chunming's team elucidated the molecular mechanism of albumin accumulation in rice endosperm

The starch accumulated in the endosperm of grasses and stored protein are the most important food sources
for humans.
According to the different solubility in different solvents, rice endosperm storage proteins can be divided into gluten, gliadin, albumin and globulin
.
Among them, albumin is the most abundant water-soluble protein in rice endosperm and the most important allergenic protein, and the mechanism of its accumulation regulation is not clear
.
Previous results have shown that mutations in the transcription factors NAC20 and NAC26 specifically expressed in rice endosperm lead to reduced endosperm starch and storage protein accumulation, resulting in a powdery endosperm phenotype
.
Recently, Liu Chunming's team from the Institute of Botany, Chinese Academy of Sciences/School of Modern Agriculture, Peking University, found that the accumulation of gliadin in the endosperm of NAC20/26-3 of the NAC20/26 double-knockout mutant nac20/26-3 of flower 11 in rice varieties was slightly reduced, while the accumulation of 16-kD albumin almost disappeared
.
The results of mass spectrometry showed that the abundance of the four albumins encoded by Alb1, Alb2, Alb3 and Alb4 decreased sharply in the NAC20/26-3 mutants
.
qRT-PCR analysis showed that the expression of Alb1-5, all five genes encoding innocent synthesis in the mutant endosperm, was greatly reduced
.
Further experiments showed that the NAC20/26 transcription factor activated albumin gene expression in endosperm by binding to the ACG conserved motif in the Alb1-5 promoter region, thereby regulating the accumulation of rice endosperm, and the activation of NAC26 was more effective than NAC20, and this activity difference was caused
by the sequence difference of its intermediate segment.
Through yeast two-hybridization test, the researchers identified the rice gliadin-binding factor RPBF that interacted with NAC20/26, and obtained two independent mutants, rpbf-1 and rpbf-2
, using gene editing technology.
The grains of these two mutants shrank, and the content of various stored proteins in the endosperm, including albumin, was partially reduced
.
The expression of Alb in the endosperm of rpbf mutant was significantly reduced, but RPBF could not directly bind to the promoter encoding albumin gene, but it could interact with NAC20/26 to enhance the activation activity of NAC20/26 on the expression of 16-kD albumin gene, and indirectly control the accumulation
of albumin in rice endosperm.
This study elucidates the molecular regulation mechanism of albumin accumulation in rice endosperm, lays a foundation for regulating albumin content in the breeding process, and may also provide a solution
for rice consumers to cope with allergens.

Figure 1.
Molecular mechanism of RPBF and NAC20/26 interacting to regulate the accumulation of endosperm protein in rice

A.
SDS-PAGE and antibody immunoassays showed that most stored protein accumulation was reduced in rpbf mutants, while mainly 16-kD albumin disappeared
in nac20/26-3 double mutants.
B.
The most dominant water-soluble protein in rice endosperm is albumin, which almost disappears
in the NAC20/26-3 double mutant.
C.
In vitro transcriptional activation analysis showed that NAC20/26 activates Alb1/2 promoter expression, which is stronger
in the presence of RPBF.
D.
ACG motifs are core elements of NAC20 and NAC26 binding, and although RPBF does not interact directly with albumin gene promoters, it can promote its transcriptional activation activity
by binding to NAC20/26.

The research results were completed by Liu Chunming's team and the team of Academician Wan Jianmin of the Institute of Crop Science, Chinese Academy of Agricultural Sciences, and were published online in the international academic journal Plant Biotechnology Journal
on December 29, 2022.
Wu Mingwei, a postdoctoral fellow at the Institute of Botany, Chinese Academy of Sciences, is the first author of this paper, and researcher Liu Chunming is the corresponding author
.
The research was supported
by the National Key Research and Development Program of China, the China Science Innovation Project and the National Natural Science Foundation of China.

Reference:

Ming-Wei Wu, Jinxin Liu, Xue Bai, Wen-Qiang Chen, Yulong Ren, Jin-Lei Liu, Meng-Meng Chen, Heng Zhao, Xuefeng Yao, Jin-Dan Zhang, Jianmin Wan and Chun-Ming Liu, Transcription factors NAC20 and NAC26 interact with RPBF to activate albumin accumulations in rice endosperm.
Plant Biotechnology Journal, 29 Dec.
2022.
doi:10.
1111/pbi.
13994