Using RNAi to improve plant
nutritional value: from mechanism to
application
Guiliang Tang
1
and Gad Galili
2
1
Department of Biochemistry and Molecular Pharmacology, University of Massachusetts Medical School, Worcester, MA 01605,
USA
2
Department of Plant Science, The Weizmann Institute of Science, Rehovot 76100, Israel
RNA interference (RNAi) is an ancient mechanism of
gene suppression, whose machinery and biological
functions are only partially understood. Intensive
studies have focused on developing RNAi technologies
for treating human diseases and for improving plant
traits. Yet application of RNAi to improving the nutri-
tional value of plants for human and animal nutrition,
and development of the related RNAi technologies are
still in their infancy. Here we discuss current knowledge
of plant RNAi function, as well as concepts and
strategies for the improvement of plant nutritional
value through the development of plant RNAi
technologies.
Although total yield is still the first priority for both
traditional plant breeding and contemporary plant genetic
engineering in developing countries, the goal of improving
the nutritional value of plants is receiving increasing
attention [1,2]. Some major diseases, such as heart disease
and cancer, can be prevented by dietary supplements of
specific nutrients [3]. In particular, essential amino acids,
minerals, fatty acids and vitamins are key factors for
robust human health and growth [2], and a diet of plant
foods rich in essential nutrients can significantly improve
human health and life expectancy [4].
Traditional breeding has been tremendously successful
in improving the nutritional value of food and feed [5];
however, this process is time-consuming and the limited
genetic resources of most crops have left little room for
continued improvement by these means. Over the past few
decades, the possibilities for improvement have been
broadened by extensive gene mapping and identification,
whole-genome sequencing of model plants and crops, and
the development of gene transfer technologies. Directed
efforts are now underway to use genetic engineering of
metabolic pathways to alter plant nutrients [2]. These
efforts depend on a detailed understanding of plant
metabolic pathways and their constituent enzymes.
Currently, the principal strategy for transgenic
enhancement of plant nutrients involves increasing the
expression of anabolic biosynthetic genes. Unfortunately,
the efficacy of this approach for crop engineering has been
restricted by two main obstructions. First, the introduc-
tion of extra gene copies can have the non-intuitive effect
of decreasing expression from both the introduced and
homologous endogenous loci – a phenomenon of gene
silencing known as
CO
-
SUPPRESSION
(See Glossary) [6].
Second, feedback metabolic loops tend to maintain
homeostatic levels of nutrients. For example, plants
might respond to an increase in nutrient production
induced by overexpression of biosynthetic genes by
activating a degradation pathway that either negates
nutrient accumulation or converts nutrients into undesir-
able metabolites [7]. It is therefore necessary to consider
schemes for metabolic engineering that decrease the levels
of catabolic enzymes; effective and expedient methods to
achieve this, however, have been wanting in the past.
Ironically, the very phenomenon of co-suppression that
plagues some overexpression efforts might be useful for
realizing such a reduction in catabolic enzymes. Co-
suppression has been recently recognized as a manifes-
tation of RNA
INTERFERENCE
(RNAi) – an endogenous
pathway of negative posttranscriptional regulation.
RNAi has revolutionized the possibilities for creating
customized ‘knock-down’ of gene activity. RNAi operates
in both plants and animals and uses double-stranded RNA
(dsRNA) as a trigger that targets homologous mRNAs for
degradation. Methods that introduce dsRNA into plant
and animal cells have been enormously successful in
decreasing cognate gene expression in vivo [8–11].
In this review, we first examine current understanding
of the endogenous RNAi pathway in plants. We then
discuss strategies and applications of RNAi for improving
plant nutritional value via the coordinated overexpression
and suppression of genes in plants. Finally, we discuss the
development of plant RNAi technologies.
The RNAi pathway in plants
The phenomenon of plant co-suppression was accidentally
discovered during attempts to alter the pigmentation of
commercial petunia flowers [6]. It was supposed that
deeper flower colors might result from the overexpression
of a chalcone synthase gene driven by the constitutive 35S
promoter. Instead, both endogenous and transgenic
chalcone synthase genes were silenced in these plants,
Corresponding author: Guiliang Tang (guiliang.tang@umassmed.edu).
Available online 29 July 2004
www.sciencedirect.com 0167-7799/$ - see front matter Q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.tibtech.2004.07.009
Review
TRENDS in Biotechnology Vol.22 No.9 September 2004