Construction of a species-specific vector for improved plastid transformation efficiency in Capsicum annuum L

Recent trends and advances in chloroplast engineering and transformation methods

Chloroplast transformation technology has become a powerful platform for generating plants that express foreign proteins of pharmaceutical and agricultural importance at high levels. Chloroplasts are often chosen as attractive targets for the introduction of new agronomic traits because they have their own genome and protein synthesis machinery. Certain valuable traits have been genetically engineered into plastid genomes to improve crop yield, nutritional quality, resistance to abiotic and biotic stresses, and the production of industrial enzymes and therapeutic proteins. Synthetic biology approaches aim to play an important role in expressing multiple genes through plastid engineering, without the risk of pleiotropic effects in transplastomic plants. Despite many promising laboratory-level successes, no transplastomic crop has been commercialized to date. This technology is mostly confined to model species in academic laboratories and needs to be expanded to other agronomically important crop species to capitalize on its significant commercial potential. However, in recent years, some transplastomic lines are progressing in field trials, offering hope that they will pass regulatory approval and enter the marketplace. This review provides a comprehensive summary of new and emerging technologies employed for plastid transformation and discusses key synthetic biology elements that are necessary for the construction of modern transformation vectors. It also focuses on various novel insights and challenges to overcome in chloroplast transformation.

Developing an Optimized Protocol for Regeneration and Transformation in Pepper

Capsicum annuum L. is extensively cultivated in subtropical and temperate regions globally, respectively, when grown in a medium with 8 holding significant economic importance. Despite the availability of genome sequences and editing tools, gene editing in peppers is limited by the lack of a stable regeneration and transformation method. This study assessed regeneration and transformation protocols in seven chili pepper varieties, including CM334, Zunla-1, Zhongjiao6 (ZJ6), 0818, 0819, 297, and 348, in order to enhance genetic improvement efforts. Several explants, media compositions, and hormonal combinations were systematically evaluated to optimize the in vitro regeneration process across different chili pepper varieties. The optimal concentrations for shoot formation, shoot elongation, and rooting in regeneration experiments were determined as 5 mg/L of 6-Benzylaminopurine (BAP) with 5 mg/L of silver nitrate (AgNO3), 0.5 mg/L of Gibberellic acid (GA3), and 1 mg/L of Indole-3-butyric acid (IBA), respectively. The highest regeneration rate of 41% was observed from CM334 cotyledon explants. Transformation optimization established 300 mg/L of cefotaxime for bacterial control, with a 72-h co-cultivation period at OD600 = 0.1. This study optimizes the protocols for chili pepper regeneration and transformation, thereby contributing to genetic improvement efforts.

CRISPR/Cas9 based genome editing of Phytoene desaturase (PDS) gene in chilli pepper (Capsicum annuum L.)

An effective CRISPR/Cas9 reagent delivery system has been developed in a commercially significant crop, the chilli pepper using a construct harboring two distinct gRNAs targeting exons 14 and 15 of the Phytoene desaturase (CaPDS) gene, whose loss-of-function mutation causes a photo-bleaching phenotype and impairs the biosynthesis of carotenoids. The construct carrying two sgRNAs was observed to create visible albino phenotypes in cotyledons regenerating on a medium containing 80 mg/L kanamycin, and plants regenerated therefrom after biolistic-mediated transfer of CRISPR/Cas9 reagents into chilli pepper cells. Analysis of CRISPR/Cas9 genome-editing events, including kanamycin screening of mutants and assessing homozygosity using the T7 endonuclease assay (T7E1), revealed 62.5 % of transformed plants exhibited successful editing at the target region and displayed both albino and mosaic phenotypes. Interestingly, the sequence analysis showed that insertions and substitutions were present in all the plant lines in the targeted CaPDS region. The detected mutations were mostly 12- to 24-bp deletions that disrupted the exon-intron junction, along with base substitutions and the insertion of 1-bp at the protospacer adjacent motif (PAM) region of the target site. The reduction in essential photosynthetic pigments (chlorophyll a, chlorophyll b and carotenoid) in knockout chilli pepper lines provided further evidence that the CaPDS gene had been functionally disrupted. In this present study, we report that the biolistic delivery of CRISPR/Cas9 reagents into chilli peppers is very effective and produces multiple mutation events in a short span of time.

Plastid-expressed AdDjSKI enhances photosystem II stability, delays leaf senescence, and increases fruit yield in tomato plants under heat stress

Heat stress substantially reduces tomato (Solanum lycopersicum) growth and yield globally, thereby jeopardizing food security. DnaJ proteins, constituents of the heat shock protein system, protect cells from diverse environmental stresses as HSP-70 molecular co-chaperones. In this study, we demonstrated that AdDjSKI, a serine-rich DnaJ III protein induced by pathogens, plays an important role in stabilizing photosystem II (PSII) in response to heat stress. Our results revealed that transplastomic tomato plants expressing the AdDjSKI gene exhibited increased levels of total soluble proteins, improved growth and chlorophyll content, reduced malondialdehyde (MDA) accumulation, and diminished PSII photoinhibition under elevated temperatures when compared with wild-type (WT) plants. Intriguingly, these transplastomic plants maintained higher levels of D1 protein under elevated temperatures compared with the WT plants, suggesting that overexpression of AdDjSKI in plastids is crucial for PSII protection, likely due to its chaperone activity. Furthermore, the transplastomic plants displayed lower accumulation of superoxide radical (O2 •─) and H2O2, in comparison with the WT plants, plausibly attributed to higher superoxide dismutase (SOD) and ascorbate peroxidase (APX) activities. This also coincides with an enhanced expression of corresponding genes, including SlCuZnSOD, SlFeSOD, SlAPX2, and SltAPX, under heat stress. Taken together, our findings reveal that chloroplastic expression of AdDjSKI in tomatoes plays a critical role in fruit yield, primarily through a combination of delayed senescence and stabilizing PSII under heat stress.

Prospects of chloroplast metabolic engineering for developing nutrient-dense food crops

Addressing nutritional deficiencies in food crops through biofortification is a sustainable approach to tackling malnutrition. Biofortification is continuously being attempted through conventional breeding as well as through various plant biotechnological interventions, ranging from molecular breeding to genetic engineering and genome editing for enriching crops with various health-promoting metabolites. Genetic engineering is used for the rational incorporation of desired nutritional traits in food crops and predominantly operates through nuclear and chloroplast genome engineering. In the recent past, chloroplast engineering has been deployed as a strategic tool to develop model plants with enhanced nutritional traits due to the various advantages it offers over nuclear genome engineering. However, this approach needs to be extended for the nutritional enhancement of major food crops. Further, this platform could be combined with strategies, such as synthetic biology, chloroplast editing, nanoparticle-mediated rapid chloroplast transformation, and horizontal gene transfer through grafting for targeting endogenous metabolic pathways for overproducing native nutraceuticals, production of biopharmaceuticals, and biosynthesis of designer nutritional compounds. This review focuses on exploring various features of chloroplast genome engineering for nutritional enhancement of food crops by enhancing the levels of existing metabolites, restoring the metabolites lost during crop domestication, and introducing novel metabolites and phytonutrients needed for a healthy daily diet.

Plastid Transformation: How Does it Work? Can it Be Applied to Crops? What Can it Offer?

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.