Transitions in wheat endosperm metabolism upon transcriptional induction of oil accumulation by oat endosperm WRINKLED1

Abstract

Background: Cereal grains, including wheat (Triticum aestivum L.), are major sources of food and feed, with wheat being dominant in temperate zones. These end uses exploit the storage reserves in the starchy endosperm of the grain, with starch being the major storage component in most cereal species. However, oats (Avena sativa L.) differs in that the starchy endosperm stores significant amounts of oil. Understanding the control of carbon allocation between groups of storage compounds, such as starch and oil, is therefore important for understanding the composition and hence end use quality of cereals. WRINKLED1 is a transcription factor known to induce triacylglycerol (TAG; oil) accumulation in several plant storage tissues. Results: An oat endosperm homolog of WRI1 (AsWRI1) expressed from the endosperm-specific HMW1Dx5 promoter resulted in drastic changes in carbon allocation in wheat grains, with reduced seed weight and a wrinkled seed phenotype. The starch content of mature grain endosperms of AsWRI1-wheat was reduced compared to controls (from 62 to 22% by dry weight (dw)), TAG was increased by up to nine-fold (from 0.7 to 6.4% oil by dw) and sucrose from 1.5 to 10% by dw. Expression of AsWRI1 in wheat grains also resulted in multiple layers of elongated peripheral aleurone cells. RNA-sequencing, lipid analyses, and pulse-chase experiments using 14 C-sucrose indicated that futile cycling of fatty acids could be a limitation for oil accumulation.Conclusions: Our data show that expression of oat endosperm WRI1 in the wheat endosperm results in changes in metabolism which could underpin the application of biotechnology to manipulate grain composition. In particular, the striking effect on starch synthesis in the wheat endosperm indicates that an important indirect role of WRI1 is to divert carbon allocation away from starch biosynthesis in plant storage tissues that accumulate oil.

Accumulation and deposition of triacylglycerols in the starchy endosperm of wheat grain

“…The presence of Ole-2 transcripts encoding the 16 kDa oleosin isoform suggests that these bodies are stabilised by oleosins as in most other oil-storing seed tissues. Although we have not directly demonstrated a role of oleosin in stabilizing lipid bodies in this paper, recent studies carried out in transgenic wheat support this suggestion: overexpression of the AsWRI1 transcription factor from oat in the starchy endosperm of wheat resulted in increases in accumulation of TAGs by up to nine-fold and in up-regulation of oleosin-encoding genes ( Grimberg et al, 2020 ). The fact that oleosins have not been reported in published proteomic studies of wheat starchy endosperm or white flour probably reflects technical problems, as oleosins are highly hydrophobic and not readily extracted in buffers used for proteomic studies.…”

“…However, the acyl-CoA independent pathway clearly also operates and the precise contributions of de novo DAG (Kennedy pathway) and PC-derived DAG remain to be determined. The importance of the Kennedy pathway is supported by the transcript analysis of wheat grains reported by Grimberg et al (2020) which also showed low levels of PDAT activity early in grain development and higher levels of DGAT/TAG1 activity.…”

Molecular and Transcriptional Regulation of Seed Development in Cereals: Present Status and Future Prospects

“…In addition, synergy between RPBF and RISBZ1 has been implicated in mediating the regulatory networks essential for seed development by binding to the GCN4 motif to trans-activate the expression of seed storage proteins in rice [29,129]. Recently, Grimberg et al [130] identified an oat endosperm homolog of WRINKLED1 transcription factor (AsWRI1), which when expressed under the control of endosperm-specific HMW1Dx5 promoter, causes substantial alterations in carbon allocation in wheat grains, including lower seed weight and a wrinkled seed phenotype.…”

An integrated omics analysis reveals the gene expression profiles of maize, castor bean, and rapeseed for seed oil biosynthesis

“…Many transcription factors (TFs) also participate in seed development and nutrient accumulation. TFs such as WRINKLED1 (WRI1), LEAFY COTYLEDON1/2 (LEC1/2), ABSCISIC ACID INSENSITIVE3 (ABI3), FUSCA3 (FUS3), PICKLE (PKL), and VP1/ABI3-LIKE (VAL) have been proved to regulate SOC in multiple species such as Arabidopsis, rapeseed, soybean, castor bean, and others [ 14 , 21 – 24 ].…”