ptxD/Phi as alternative selectable marker system for genetic transformation for bio-safety concerns: a review

Abstract

Antibiotic and herbicide resistance genes are the most common marker genes for plant transformation to improve crop yield and food quality. However, there is public concern about the use of resistance marker genes in food crops due to the risk of potential gene flow from transgenic plants to compatible weedy relatives, leading to the possible development of “superweeds” and antibiotic resistance. Several selectable marker genes such as aph, nptII, aaC3, aadA, pat, bar, epsp and gat, which have been synthesized to generate transgenic plants by genetic transformation, have shown some limitations. These marker genes, which confer antibiotic or herbicide resistance and are introduced into crops along with economically valuable genes, have three main problems: selective agents have negative effects on plant cell proliferation and differentiation, uncertainty about the environmental effects of many selectable marker genes, and difficulty in performing recurrent transformations with the same selectable marker to pyramid desired genes. Recently, a simple, novel, and affordable method was presented for plant cells to convert non-metabolizable phosphite (Phi) to an important phosphate (Pi) for developing cells by gene expression encoding a phosphite oxidoreductase (PTXD) enzyme. The ptxD gene, in combination with a selection medium containing Phi as the sole phosphorus (P) source, can serve as an effective and efficient system for selecting transformed cells. The selection system adds nutrients to transgenic plants without potential risks to the environment. The ptxD/Phi system has been shown to be a promising transgenic selection system with several advantages in cost and safety compared to other antibiotic-based selection systems. In this review, we have summarized the development of selection markers for genetic transformation and the potential use of the ptxD/Phi scheme as an alternative selection marker system to minimize the future use of antibiotic and herbicide marker genes.

A novel dominant selection system for plant transgenics based on phosphite metabolism catalyzed by bacterial alkaline phosphatase

“…However, the biosafety associated with the residual marker genes in traditional plant genetic transformation remains as an onerous challenge [4][5][6]. In recent years, an exquisite solution has been hallmarked by the PtxD/Phi system using PtxD as the dominant selective marker and Phi as the selection agent [11,24]. Additionally, PtxD PLOS ONE transgenic plants can convert toxic Phi into bio-utilizable Pi, while non-transgenic plants and wild weeds do not own the ability to metabolize Phi, thus allowing Phi can play an extra dual role as a new P-fertilizer and herbicide [26].…”

“…Additionally, PtxD PLOS ONE transgenic plants can convert toxic Phi into bio-utilizable Pi, while non-transgenic plants and wild weeds do not own the ability to metabolize Phi, thus allowing Phi can play an extra dual role as a new P-fertilizer and herbicide [26]. Therefore, the PtxD/Phi system is indeed advantaged with an aggregate of safe selective marker, plant P utilization and weed management, and has emerged as a powerful Agro-biotechnology with splendid application prospects [11,12,24,[26][27][28]. As BAP has a Phi-oxidizing activity similar to that of PtxD [39], herein we investigated whether it could replace PtxD to establish an analogous BAP/Phi system, particularly keeping an eye on its use as a selective marker.…”

“…The above properties and advantages of Phi had evoked the establishment of a PtxD/Phi-coupled selection system for plant transgenics [ 11 , 24 ]. In this system, PtxD is a phosphite dehydrogenase (PTDH) from the Phi-autotrophic bacteria Pseudomonas stutzeri , which can efficiently catalyze the oxidative conversion of Phi to Pi and the reduction of cofactor NAD + to NADH [ 25 ].…”

“…This pleiotropic PtxD/Phi selection system has been shortly exploited to develop transgenic variants for both monocot and dicot plant species including Arabidopsis [ 24 ], tobacco [ 24 , 30 ], and other more important crops such as cotton [ 27 ], maize [ 31 ] and sorghum [ 32 ], since its invention several years ago, with fascinating application prospects in sustainable agriculture [ 11 ]. Moreover, it has also been extensively used to genetically engineer important microorganisms (e.g.…”

“…With the increasing concerns on their inherent defects, i.e. the potential threats to ecological environment and food safety [4][5][6], development of a biosafety-secured dominant selection marker is of great significance and necessity, and always ongoing [7][8][9][10][11].…”