Abstract
In recent years, plant genetic engineering has advanced agriculture in terms of crop improvement, stress and disease resistance, and pharmaceutical biosynthesis. Cells from land plants and algae contain three organelles that harbor DNA: the nucleus, plastid, and mitochondria. Although the most common approach for many plant species is the introduction of foreign DNA into the nucleus (nuclear transformation) via Agrobacterium- or biolistics-mediated delivery of transgenes, plastid transformation offers an alternative means for plant transformation. Since there are many copies of the chloroplast genome in each cell, higher levels of protein accumulation can often be achieved from transgenes inserted in the chloroplast genome compared to the nuclear genome. Chloroplasts are therefore becoming attractive hosts for the introduction of new agronomic traits, as well as for the biosynthesis of high-value pharmaceuticals, biomaterials and industrial enzymes. This review provides a comprehensive historical and biological perspective on plastid transformation, with a focus on current and emerging approaches such as the use of single-walled carbon nanotubes (SWNTs) as DNA delivery vehicles, overexpressing morphogenic regulators to enhance regeneration ability, applying genome editing techniques to accelerate double-stranded break formation, and reconsidering protoplasts as a viable material for plastid genome engineering, even in transformation-recalcitrant species.
Highlights
The plastids of algae and land plants are semi-autonomous organelles with their own genomes, complete with transcription and translation machinery [1] that arose from endosymbiosis between a eukaryotic ancestor and a photosynthetic cyanobacterium [2]
Agronomic characters obtained by the engineering of the chloroplast genome are listed in Table 3; most transformations relied on biolistics, these largely predate the publication of the single-walled carbon nanotubes (SWNTs) tool, which may provide a new opportunity for scientists thanks to its low cost and ease of use
High-level expression of vaccine antigens and therapeutic proteins has been achieved in plant chloroplasts or chromoplasts for antigens associated with the plague, tetanus, human immunodeficiency virus (HIV), cholera, malaria, Alzheimer’s disease and hemophilia [123,124,125,126,127,128,129,130]
Summary
The plastids of algae and land plants are semi-autonomous organelles with their own genomes, complete with transcription and translation machinery [1] that arose from endosymbiosis between a eukaryotic ancestor and a photosynthetic cyanobacterium [2]. The plastid genome is present in many copies inside the organelle and is protected from gene silencing pathways that typically lower transgene expression, thereby allowing high levels of foreign protein to accumulate, achieving 5–40% total soluble protein (TSP) [4] and up to 70% of total soluble protein in tobacco (Nicotiana tabacum) [5,6,7]. The multiple copies of the plastid genome contained within the organelle results in the high expression of foreign genes and the accumulation of the encoded proteins [10]. In addition to the crops mentioned above, recent successes in plastid transformation have been reported in the plant species bitter melon [16], and the medicinal plant sweet wormwood (Artemisia annua) [17] and licorice weed (Scoparia dulcis) [18,19] (Table 1)
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