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Review
. 2017 Jan 6:7:1977.
doi: 10.3389/fpls.2016.01977. eCollection 2016.

Intron-Mediated Enhancement: A Tool for Heterologous Gene Expression in Plants?

Miriam Laxa  1
Affiliations
Review

Intron-Mediated Enhancement: A Tool for Heterologous Gene Expression in Plants?

Miriam Laxa. Front Plant Sci. .

Abstract

Many plant promoters were characterized and used for transgene expression in plants. Even though these promoters drive high levels of transgene expression in plants, the expression patterns are rarely constitutive but restricted to some tissues and developmental stages. In terms of crop improvement not only the enhancement of expression per se but, in particular, tissue-specific and spatial expression of genes plays an important role. Introns were used to boost expression in transgenic plants in the field of crop improvement for a long time. However, the mechanism behind this so called intron-mediated enhancement (IME) is still largely unknown. This review highlights the complexity of IME on the levels of its regulation and modes of action and gives an overview on IME methodology, examples in fundamental research and models of proposed mechanisms. In addition, the application of IME in heterologous gene expression is discussed.

Keywords: crop improvement; gene expression; intron-mediated enhancement; plants; tissue specificity; transcription.

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Figures

FIGURE 1
FIGURE 1
Complexity of intron-mediated enhancement (IME) on both the level of regulation and the modes of action. IME depends on positional requirements, sequences and elements within the intron and, in the case of monocots, on splicing. IME affects all levels of gene expression. IME can be measured indicate (intrinsic promote activity and enhancement) and visualized in different tissues (tissue specificity/spatial expression). Bent arrows in the gear-wheel indicate that all parameters influencing IME mainly have an additive affect rather than counter acting each other. Thickness of the arrows indicates the importance of the parameter for IME. Blue straight arrows indicate which level of gene expression is affected at a certain frequency (thickness correlates with the number of publications). Black straight arrows indicate modes of action of IME in dependence of both the level gene expression targeted and internal levels of relationship.
FIGURE 2
FIGURE 2
Introns mediate the enhancement of gene expression in both monocots and dicots. Arrows indicate the modes of action that either was reported for a specific intron or can be proposed as a mechanistic level for a group of introns. The list summarizes information on the given publications in the following way: (i) intron in combination with specific promoters (E – endogenous promoter studied, F – foreign promoter, 35S – CaMV 35S promoter, NOS – nopaline synthase promoter), (ii) plant species in which IME was tested in and the type of transformation (not specifically signed – stable transformation, P – protoplast, C – callus, SC – suspension cells, T – transient, for example in leaves, St – stable transformation, specifically indicated when multiple types of transformation were used in the study), (iii) reference, and (iv) methods and reporter genes used to test IME of the intron-promoter combination given. The different reporter genes are highlighted with a color code (blue – gusA encoding β-glucuronidase, red – cat encoding chloramphenicol acetyltransferase, green – luc encoding firefly luciferase, grey – bar encoding phosphinothricin acetyltransferase, brown – neo encoding neomycin phosphotransferase II, and purple – Adh1-S encoding ADH1-S). In case a foreign promoter was tested the gene name is given.
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