Prima facie evidence that a phytocystatin for transgenic plant resistance to nematodes is not a toxic risk in the human diet

Genetically engineered bananas resistant to Xanthomonas wilt disease and nematodes

Banana is an important staple food crop feeding more than 100 million Africans, but is subject to severe productivity constraints due to a range of pests and diseases. Banana Xanthomonas wilt caused by Xanthomonas campestris pv. musacearum is capable of entirely destroying a plantation while nematodes can cause losses up to 50% and increase susceptibility to other pests and diseases. Development of improved varieties of banana is fundamental in order to tackle these challenges. However, the sterile nature of the crop and the lack of resistance in Musa germplasm make improvement by traditional breeding techniques either impossible or extremely slow. Recent developments using genetic engineering have begun to address these problems. Transgenic banana expressing sweet pepper Hrap and Pflp genes have demonstrated complete resistance against X. campestris pv. musacearum in the field. Transgenic plantains expressing a cysteine proteinase inhibitors and/or synthetic peptide showed enhanced resistance to a mixed species population of nematodes in the field. Here, we review the genetic engineering technologies which have potential to improve agriculture and food security in Africa.

Expression of a Cystatin Transgene in Eggplant Provides Resistance to Root-knot Nematode, Meloidogyne incognita

Root-knot nematodes (RKN) cause substantial yield decline in eggplant and sustainable management options to minimize crop damage due to nematodes are still limited. A number of genetic engineering strategies have been developed to disrupt the successful plant-nematode interactions. Among them, delivery of proteinase inhibitors from the plant to perturb nematode development and reproduction is arguably the most effective strategy. In the present study, transgenic eggplant expressing a modified rice cystatin (OC-IΔD86) gene under the control of the root-specific promoter, TUB-1, was generated to evaluate the genetically modified nematode resistance. Five putative transformants were selected through PCR and genomic Southern blot analysis. Expression of the cystatin transgene was confirmed in all the events using western blotting, ELISA and qPCR assay. Upon challenge inoculation, all the transgenic events exhibited a detrimental effect on RKN development and reproduction. The best transgenic line (a single copy event) showed 78.3% inhibition in reproductive success of RKN. Our results suggest that cystatins can play an important role for improving nematode resistance in eggplant and their deployment in gene pyramiding strategies with other proteinase inhibitors could ultimately enhance crop yield.

Review: The future of cystatin engineering

Plant cystatins are naturally occurring protease inhibitors that prevent proteolysis by papain-like cysteine proteases. Their protective action against environmental stresses has been relatively well characterised. Still, there is a need to greatly improve both potency and specificity based on the current rather poor performance of cystatins in biotechnological applications. Research in creating more potent and specific cystatins, including amino acid substitutions in either conserved cystatin motifs and/or at variable amino acid sites, is reviewed. Existing gaps for better understanding of cystatin-protease interactions are further explored. Current knowledge on multi-cystatins or hybrid protease inhibitors involving cystatins as an additional option for cystatin engineering is further outlined along with the nuances of how cystatins with rather unusual amino acid sequences might actually help in cystatin engineering. Finally, future opportunities for application of cystatins are highlighted which include applications in genetically modified transgenic plants for environmental stress protection and also as nutraceuticals, as part of more nutritious food. Further opportunities might also include the possible management of diseases and disorders, often associated with lifestyle changes, and the most immediate and promising application which is inclusion into plant-based recombinant protein production platforms.

Positive selection of digestive Cys proteases in herbivorous Coleoptera

Positive selection is thought to contribute to the functional diversification of insect-inducible protease inhibitors in plants in response to selective pressures exerted by the digestive proteases of their herbivorous enemies. Here we assessed whether a reciprocal evolutionary process takes place on the insect side, and whether ingestion of a positively selected plant inhibitor may translate into a measurable rebalancing of midgut proteases in vivo. Midgut Cys proteases of herbivorous Coleoptera, including the major pest Colorado potato beetle (Leptinotarsa decemlineata), were first compared using a codon-based evolutionary model to look for the occurrence of hypervariable, positively selected amino acid sites among the tested sequences. Hypervariable sites were found, distributed within -or close to- amino acid regions interacting with Cys-type inhibitors of the plant cystatin protein family. A close examination of L. decemlineata sequences indicated a link between their assignment to protease functional families and amino acid identity at positively selected sites. A function-diversifying role for positive selection was further suggested empirically by in vitro protease assays and a shotgun proteomic analysis of L. decemlineata Cys proteases showing a differential rebalancing of protease functional family complements in larvae fed single variants of a model cystatin mutated at positively selected amino acid sites. These data confirm overall the occurrence of hypervariable, positively selected amino acid sites in herbivorous Coleoptera digestive Cys proteases. They also support the idea of an adaptive role for positive selection, useful to generate functionally diverse proteases in insect herbivores ingesting functionally diverse, rapidly evolving dietary cystatins.

Potential use of phytocystatins in crop improvement, with a particular focus on legumes

Phytocystatins are a well-characterized class of naturally occurring protease inhibitors that function by preventing the catalysis of papain-like cysteine proteases. The action of cystatins in biotic stress resistance has been studied intensively, but relatively little is known about their functions in plant growth and defence responses to abiotic stresses, such as drought. Extreme weather events, such as drought and flooding, will have negative impacts on the yields of crop plants, particularly grain legumes. The concepts that changes in cellular protein content and composition are required for acclimation to different abiotic stresses, and that these adjustments are achieved through regulation of proteolysis, are widely accepted. However, the nature and regulation of the protein turnover machinery that underpins essential stress-induced cellular restructuring remain poorly characterized. Cysteine proteases are intrinsic to the genetic programmes that underpin plant development and senescence, but their functions in stress-induced senescence are not well defined. Transgenic plants including soybean that have been engineered to constitutively express phytocystatins show enhanced tolerance to a range of different abiotic stresses including drought, suggesting that manipulation of cysteine protease activities by altered phytocystatin expression in crop plants might be used to improve resilience and quality in the face of climate change.

Generation of transgenic plantain (Musa spp.) with resistance to plant pathogenic nematodes

Plant parasitic nematodes impose a severe constraint on plantain and banana productivity; however, the sterile nature of many cultivars precludes conventional breeding for resistance. Transgenic plantain cv. Gonja manjaya (Musa AAB) plants, expressing a maize cystatin that inhibits nematode digestive cysteine proteinases and a synthetic peptide that disrupts nematode chemoreception, were assessed for their ability to resist nematode infection. Lines were generated that expressed each gene singly or both together in a stacked defence. Nematode challenge with a single species or a mixed population identified 10 lines with significant resistance. The best level of resistance achieved against the major pest species Radopholus similis was 84% ± 8% for the cystatin, 66% ± 14% for the peptide and 70% ± 6% for the dual defence. In the mixed population, trial resistance was also demonstrated to Helicotylenchus multicinctus. A fluorescently labelled form of the chemodisruptive peptide underwent retrograde transport along certain sensory dendrites of R. similis as required to disrupt chemoreception. The peptide was degraded after 30 min in simulated intestinal fluid or boiling water and after 1 h in nonsterile soil. In silico sequence analysis suggests that the peptide is not a mammalian antigen. This work establishes the mode of action of a novel nematode defence, develops the evidence for its safe and effective deployment against multiple nematode species and identifies transgenic plantain lines with a high level of resistance for a proposed field trial.

Strategies for transgenic nematode control in developed and developing world crops

Nematodes cause an estimated $118b annual losses to world crops and they are not readily controlled by pesticides or other control options. For many crops natural resistance genes are unavailable to plant breeders or progress by this approach is slow. Transgenic plants can provide nematode resistance for such crops. Two approaches have been field trialled that control a wide range of nematodes by either limiting use of their dietary protein uptake from the crop or by preventing root invasion without a direct lethality. In addition, RNA interference increasingly in tandem with genomic studies is providing a range of potential resistance traits that involve no novel protein production. Transgenic resistance can be delivered by tissue specific promoters to just root tissues where most economic nematodes invade and feed rather than the harvested yield. High efficacy and durability can be provided by stacking nematode resistance traits including any that natural resistance provides. The constraints to uptake centre on market acceptance and not the availability of appropriate biotechnology. The need to deploy nematode resistance is intensifying with loss of pesticides, an increased need to protect crop profit margins and in many developing world countries where nematodes severely damage both commodity and staple crops.

Recombinant protease inhibitors for herbivore pest control: a multitrophic perspective

Protease inhibitors are a promising complement to Bt toxins for the development of insect-resistant transgenic crops, but their limited specificity against proteolytic enzymes and the ubiquity of protease-dependent processes in living organisms raise questions about their eventual non-target effects in agroecosystems. After a brief overview of the main factors driving the impacts of insect-resistant transgenic crops on non-target organisms, the possible effects of protease inhibitors are discussed from a multitrophic perspective, taking into account not only the target herbivore proteases but also the proteases of other organisms found along the trophic chain, including the plant itself. Major progress has been achieved in recent years towards the design of highly potent broad-spectrum inhibitors and the field deployment of protease inhibitor-expressing transgenic plants resistant to major herbivore pests. A thorough assessment of the current literature suggests that, whereas the non-specific inhibitory effects of recombinant protease inhibitors in plant food webs could often be negligible and their 'unintended' pleiotropic effects in planta of potential agronomic value, the innocuity of these proteins might always remain an issue to be assessed empirically, on a case-by-case basis.

Plant cystatins

Plant cystatins have been the object of intense research since the publication of a first paper reporting their existence more than 20 years ago. These ubiquitous inhibitors of Cys proteases play several important roles in plants, from the control of various physiological and cellular processes in planta to the inhibition of exogenous Cys proteases secreted by herbivorous arthropods and pathogens to digest or colonize plant tissues. After an overview of current knowledge about the evolution, structure and inhibitory mechanism of plant cystatins, we review the different roles attributed to these proteins in plants. The potential of recombinant plant cystatins as effective pesticidal proteins in crop protection is also considered, as well as protein engineering approaches adopted over the years to improve their inhibitory potency and specificity towards Cys proteases of biotechnological interest.

Heterologous expression of taro cystatin protects transgenic tomato against Meloidogyne incognita infection by means of interfering sex determination and suppressing gall formation

Plant-parasitic nematodes are a major pest of many plant species and cause global economic loss. A phytocystatin gene, Colocasia esculenta cysteine proteinase inhibitor (CeCPI), isolated from a local taro Kaosiang No. 1, and driven by a CaMV35S promoter was delivered into CLN2468D, a heat-tolerant cultivar of tomato (Solanum lycopersicum). When infected with Meloidogyne incognita, one of root-knot nematode (RKN) species, transgenic T1 lines overexpressing CeCPI suppressed gall formation as evidenced by a pronounced reduction in gall numbers. In comparison with wild-type plants, a much lower proportion of female nematodes without growth retardation was observed in transgenic plants. A decrease of RKN egg mass in transgenic plants indicated seriously impaired fecundity. Overexpression of CeCPI in transgenic tomato has inhibitory functions not only in the early RKN infection stage but also in the production of offspring, which may result from intervention in sex determination.

Deleterious effects of plant cystatins against the banana weevil Cosmopolites sordidus

The general potential of plant cystatins for the development of insect-resistant transgenic plants still remains to be established given the natural ability of several insects to compensate for the loss of digestive cysteine protease activities. Here we assessed the potential of cystatins for the development of banana lines resistant to the banana weevil Cosmopolites sordidus, a major pest of banana and plantain in Africa. Protease inhibitory assays were conducted with protein and methylcoumarin (MCA) peptide substrates to measure the inhibitory efficiency of different cystatins in vitro, followed by a diet assay with cystatin-infiltrated banana stem disks to monitor the impact of two plant cystatins, oryzacystatin I (OC-I, or OsCYS1) and papaya cystatin (CpCYS1), on the overall growth rate of weevil larvae. As observed earlier for other Coleoptera, banana weevils produce a variety of proteases for dietary protein digestion, including in particular Z-Phe-Arg-MCA-hydrolyzing (cathepsin L-like) and Z-Arg-Arg-MCA-hydrolyzing (cathepsin B-like) proteases active in mildly acidic conditions. Both enzyme populations were sensitive to the cysteine protease inhibitor E-64 and to different plant cystatins including OsCYS1. In line with the broad inhibitory effects of cystatins, OsCYS1 and CpCYS1 caused an important growth delay in young larvae developing for 10 days in cystatin-infiltrated banana stem disks. These promising results, which illustrate the susceptibility of C. sordidus to plant cystatins, are discussed in the light of recent hypotheses suggesting a key role for cathepsin B-like enzymes as a determinant for resistance or susceptibility to plant cystatins in Coleoptera.

Nematode resistance

Plant-parasitic nematodes are major pests of both temperate and tropical agriculture. Many of the most damaging species employ an advanced parasitic strategy in which they induce redifferentiation of root cells to form specialized feeding structures able to support nematode growth and reproduction over several weeks. Current control measures, particularly in intensive agriculture systems, rely heavily on nematicides but alternative strategies are required as effective chemicals are withdrawn from use. Here, we review the different approaches that are being developed to provide resistance to a range of nematode species. Natural, R gene-based resistance is currently exploited in traditional breeding programmes and research is ongoing to characterize the molecular basis for the observed resistant phenotypes. A number of transgenic approaches hold promise, the best described being the expression of proteinase inhibitors to disrupt nematode digestion. The application of plant-delivered RNA interference (RNAi) to silence essential nematode genes has recently emerged as a potentially valuable resistance strategy.

Prototype demonstration of transgenic resistance to the nematode Radopholus similis conferred on banana by a cystatin

Cavendish banana was transformed using Agrobacterium tumefaciens to express a protein engineered rice cystatin (OcIdeltaD86) of value for control of plant parasitic nematodes. Expression for each line was under control of a constitutive promoter from the maize ubiquitin gene (UBI-1), a constitutive, chimeric promoter from the octopine and mannopine synthase genes of A. tumefaciens or a promoter from a root-preferentially expressed tubulin gene Arabidopsis (TUB-1). Lines were selected as of potential interest after 8 weeks challenge in containment if their mean R. similis/25 g roots for three sibling plants were more than 1 standard normal variate below the grand mean for all plants in c7-15 lines challenged concurrently. A total of 16 lines were selected on this basis as putative positives. Western blots confirmed that eight of these lines expressed cystatin with a mean of 0.08 +/- 0.04% tsp. All but two of 19 negatively selected lines from bioassays did not express cystatin. The mean resistance level of the confirmed positive lines was 69 +/- 17%. ELISA established the positive lines under control of UBI provided significantly higher expression levels of OcIdeltaD86 than recorded for the other two promoters. Lines of interest were confirmed as producing a transcript for OcIdeltaD86 by RT-PCR. Eight plants of one UBI promoter line expressing only 0.1 +/- 0.004% tsp as cystatin were re-challenged with R. similis and achieved a resistance of 70 +/- 10%. Subsequent repeat western blotting confirmed that this line still produced the cystatin after the trial. This is the first report of transgenic resistance against a major pest or disease of banana.

Environmental biosafety and transgenic potato in a centre of diversity for this crop

The Nuffield Council on Bioethics suggests that introgression of genetic material into related species in centres of crop biodiversity is an insufficient justification to bar the use of genetically modified crops in the developing world. They consider that a precautionary approach to forgo the possible benefits invokes the fallacy of thinking that doing nothing is itself without risk to the poor. Here we report findings relevant to this and other aspects of environmental biosafety for genetically modified potato in its main centre of biodiversity, the central Andes. We studied genetically modified potato clones that provide resistance to nematodes, principal pests of Andean potato crops. We show that there is no harm to many non-target organisms, but gene flow occurs to wild relatives growing near potato crops. If stable introgression were to result, the fitness of these wild species could be altered. We therefore transformed the male sterile cultivar Revolucion to provide a genetically modified nematode-resistant potato to evaluate the benefits that this provides until the possibility of stable introgression to wild relatives is determined. Thus, scientific progress is possible without compromise to the precautionary principle.