Cysteine and methionine biosynthetic enzymes have distinct effects on seed nutritional quality and on molecular phenotypes associated with accumulation of a methionine-rich seed storage protein in rice.

A - Papers appearing in refereed journals

Whitcomb, S. J., Rakpenthai, A., Bruckner, F., Fischer, A., Parmar, S., Erban, A., Kopka, J., Hawkesford, M. J. and Hoefgen, R. 2020. Cysteine and methionine biosynthetic enzymes have distinct effects on seed nutritional quality and on molecular phenotypes associated with accumulation of a methionine-rich seed storage protein in rice. Frontiers in Plant Science. 11, p. 118. https://doi.org/10.3389/fpls.2020.01118

AuthorsWhitcomb, S. J., Rakpenthai, A., Bruckner, F., Fischer, A., Parmar, S., Erban, A., Kopka, J., Hawkesford, M. J. and Hoefgen, R.
Abstract

Staple crops in human and livestock diets suffer from deficiencies in certain “essential” amino acids including methionine. With the goal of increasing methionine in rice seed, we generated a pair of “PushxPull” double transgenic lines, each containing a methionine-dense seed storage protein (2S albumin from sunflower, HaSSA) and an exogenous enzyme for either methionine (feedback desensitized cystathionine gamma synthase from Arabidopsis, AtD-CGS) or cysteine (serine acetyltransferase from E. coli, EcSAT) biosynthesis. In both double transgenic lines, the total seed methionine content was approximately 50% higher than in their untransformed parental line, Oryza sativa ssp. japonica cv. Taipei 309. HaSSA-containing rice seeds were reported to display an altered seed protein profile, speculatively due to insufficient sulfur amino acid content. However, here we present data suggesting that this may result from an overloaded protein folding machinery in the endoplasmic reticulum rather than primarily from redistribution of limited methionine from endogenous seed proteins to HaSSA. We hypothesize that HaSSA-associated endoplasmic reticulum stress results in redox perturbations that negatively impact sulfate reduction to cysteine, and we speculate that this is mitigated by EcSAT-associated increased sulfur import into the seed, which facilitates additional synthesis of cysteine and glutathione. The data presented here reveal challenges associated with increasing the methionine content in rice seed, including what may be relatively low protein folding capacity in the endoplasmic reticulum and an insufficient pool of sulfate available for additional cysteine and methionine synthesis. We propose that future approaches to further improve the methionine content in rice should focus on increasing seed sulfur loading and avoiding the accumulation of unfolded proteins in the endoplasmic reticulum.

KeywordsMethionine (Met); Cysteine (Cys); Nutritional quality; Seed Storage Protein (SSPs); Serine acetyltransferase; Cystathionine gamma-synthase; Endoplasmic reticulum (ER stress); Oryza sativa (rice)
Year of Publication2020
JournalFrontiers in Plant Science
Journal citation11, p. 118
Digital Object Identifier (DOI)https://doi.org/10.3389/fpls.2020.01118
Open accessPublished as bronze (free) open access
FunderBiotechnology and Biological Sciences Research Council
Funder project or codeDFW - Designing Future Wheat - Work package 1 (WP1) - Increased efficiency and sustainability
Publisher's version
Accepted author manuscript
Output statusPublished
Publication dates
Online22 Jul 2020
Publication process dates
Accepted07 Jul 2020
PublisherFrontiers Media SA
ISSN1664-462X

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