Challenges and Prospects of New Plant Breeding Techniques for GABA Improvement in Crops: Tomato as an Example

Abstract

Over the last seven decades, γ-aminobutyric acid (GABA) has attracted great attention from scientists for its ubiquity in plants, animals and microorganisms and for its physiological implications as a signaling molecule involved in multiple pathways and processes. Recently, the food and pharmaceutical industries have also shown significantly increased interest in GABA, because of its great potential benefits for human health and the consumer demand for health-promoting functional compounds, resulting in the release of a plethora of GABA-enriched products. Nevertheless, many crop species accumulate appreciable GABA levels in their edible parts and could help to meet the daily recommended intake of GABA for promoting positive health effects. Therefore, plant breeders are devoting much effort into breeding elite varieties with improved GABA contents. In this regard, tomato (Solanum lycopersicum), the most produced and consumed vegetable worldwide and a fruit-bearing model crop, has received much consideration for its accumulation of remarkable GABA levels. Although many different strategies have been implemented, from classical crossbreeding to induced mutagenesis, new plant breeding techniques (NPBTs) have achieved the best GABA accumulation results in red ripe tomato fruits along with shedding light on GABA metabolism and gene functions. In this review, we summarize, analyze and compare all the studies that have substantially contributed to tomato GABA breeding with further discussion and proposals regarding the most recent NPBTs that could bring this process to the next level of precision and efficiency. This document also provides guidelines with which researchers of other crops might take advantage of the progress achieved in tomato for more efficient GABA breeding programs.

Keywords: CRISPR/Cas; GABA metabolism; breeding strategies; cis-engineering; health-promoting food; new plant breeding techniques; tomato; γ-aminobutyric acid.

Figure 1

γ-Aminobutyric acid (GABA) shunt metabolism and related pathways in plant species. TCA, tricarboxylic acid cycle; GS/GOGAT, glutamine-synthetase/glutamate-synthase cycle; GAD, glutamate decarboxylase; GABA-TK, α-ketoglutarate-dependent GABA transaminase; GABA-TP, pyruvate-dependent GABA transaminase; SSADH, succinic semialdehyde dehydrogenase; SSR, succinic semialdehyde reductase; GDH, glutamate dehydrogenase; SSA, succinic semialdehyde; Suc, succinate; GHB, γ-hydroxybutyric acid; αKG, alpha-ketoglutarate; Glu, glutamate; Ala, alanine; Gly, glycine, PYR, pyruvate, GOA, glyoxylic acid.

Figure 2

Cis-engineering approaches of SlGABA-T1 and SlGAD3 promoters to enhance the GABA content in tomato. (A) Cis-engineering of the promoter region of SlGABA-T1 while SlGAD3 remained the wild type to observe the effects of SlGABA-T1 CRE editing. (B) Cis-engineering the promoter regions of SlGABA-T1 of the SlGAD3ΔC37 GABA improved line to exploit the potential synergistic effects of the cis-engineering and coding sequence targeting approaches. (C) Cis-engineering of the promoter regions of SlGABA-T1 and SlGAD3ΔC37 to maximize GABA accumulation.