Influence of the production of antibacterial and antifungal proteins by transgenic plants on the saprophytic soil microflora

Root colonization and rhizospheric community structure of Arbuscular Mycorrhizal Fungi in BADH transgenic maize BZ-136 and its recipient under salt stress and neutral soil

Betaine aldehyde dehydrogenase (BADH) transgenic maize has a capability to grow under drought and salt stress; the risk of planting BADH transgenic maize on symbiotic microorganisms remains problematic, however. A pot experiment was carried out to assess the impact of BADH transgenic maize BZ-136 on arbuscular mycorrhizal fungi (AMF) colonization in root and community structure in rhizosphere soil compared with that of parental maize Zheng58 in neutral and saline-alkaline soil. Microscope observation found that BZ-136 only had a significantly reduced effect on AMF colonization at the elongation stage (9-14%). High-throughput sequencing analysis revealed that the AMF taxonomic composition kept consistency at the genus level between transgenic BZ-136 and non-transgenic parental Zheng58. NMDS analysis verified the slight difference in community structure between BZ-136 and Zheng58 presented an agrotype-dependent pattern. AMF community indices showed that BZ-136 had a higher richness at the flowering stage in saline-alkaline soil and had a higher diversity at the mature stage in neutral soil. Heatmap analysis also illuminated AMF community structure of transgenic maize at species level was similar to that of non-transgenic maize. In summary, cropping transgenic BADH maize has minor or transient effects on AMF colonization and rhizospheric soil AMF community structure, while agrotype has a stronger effect on AMF community structure.

Citrus Genetic Engineering for Disease Resistance: Past, Present and Future

Worldwide, citrus is one of the most important fruit crops and is grown in more than 130 countries, predominantly in tropical and subtropical areas. The healthy progress of the citrus industry has been seriously affected by biotic and abiotic stresses. Several diseases, such as canker and huanglongbing, etc., rigorously affect citrus plant growth, fruit quality, and yield. Genetic engineering technologies, such as genetic transformation and genome editing, represent successful and attractive approaches for developing disease-resistant crops. These genetic engineering technologies have been widely used to develop citrus disease-resistant varieties against canker, huanglongbing, and many other fungal and viral diseases. Recently, clustered regularly interspaced short palindromic repeats (CRISPR)-based systems have made genome editing an indispensable genetic manipulation tool that has been applied to many crops, including citrus. The improved CRISPR systems, such as CRISPR/CRISPR-associated protein (Cas)9 and CRISPR/Cpf1 systems, can provide a promising new corridor for generating citrus varieties that are resistant to different pathogens. The advances in biotechnological tools and the complete genome sequence of several citrus species will undoubtedly improve the breeding for citrus disease resistance with a much greater degree of precision. Here, we attempt to summarize the recent successful progress that has been achieved in the effective application of genetic engineering and genome editing technologies to obtain citrus disease-resistant (bacterial, fungal, and virus) crops. Furthermore, we also discuss the opportunities and challenges of genetic engineering and genome editing technologies for citrus disease resistance.

Antimicrobial peptide expression in a wild tobacco plant reveals the limits of host-microbe-manipulations in the field

Plant-microbe associations are thought to be beneficial for plant growth and resistance against biotic or abiotic stresses, but for natural ecosystems, the ecological analysis of microbiome function remains in its infancy. We used transformed wild tobacco plants (Nicotiana attenuata) which constitutively express an antimicrobial peptide (Mc-AMP1) of the common ice plant, to establish an ecological tool for plant-microbe studies in the field. Transgenic plants showed in planta activity against plant-beneficial bacteria and were phenotyped within the plants´ natural habitat regarding growth, fitness and the resistance against herbivores. Multiple field experiments, conducted over 3 years, indicated no differences compared to isogenic controls. Pyrosequencing analysis of the root-associated microbial communities showed no major alterations but marginal effects at the genus level. Experimental infiltrations revealed a high heterogeneity in peptide tolerance among native isolates and suggests that the diversity of natural microbial communities can be a major obstacle for microbiome manipulations in nature.

Can functional hologenomics aid tackling current challenges in plant breeding?

Molecular plant breeding usually overlooks the genetic variability that arises from the association of plants with endophytic microorganisms, when looking at agronomic interesting target traits. This source of variability can have crucial effects on the functionality of the organism considered as a whole (the holobiont), and therefore can be selectable in breeding programs. However, seeing the holobiont as a unit for selection and improvement in breeding programs requires novel approaches for genotyping and phenotyping. These should not focus just at the plant level, but also include the associated endophytes and their functional effects on the plant, to make effective desirable trait screenings. The present review intends to draw attention to a new research field on functional hologenomics that if associated with adequate phenotyping tools could greatly increase the efficiency of breeding programs.

Chitinases: in agriculture and human healthcare

Biological control of phytopathogenic fungi and insects continues to inspire the research and development of environmentally friendly bioactive alternatives. Potentially lytic enzymes, chitinases can act as a biocontrol agent against agriculturally important fungi and insects. The cell wall in fungi and protective covers, i.e. cuticle in insects shares a key structural polymer, chitin, a β-1,4-linked N-acetylglucosamine polymer. Therefore, it is advantageous to develop a common biocontrol agent against both of these groups. As chitin is absent in plants and mammals, targeting its metabolism will signify an eco-friendly strategy for the control of agriculturally important fungi and insects but is innocuous to mammals, plants, beneficial insects and other organisms. In addition, development of chitinase transgenic plant varieties probably holds the most promising method for augmenting agricultural crop protection and productivity, when properly integrated into traditional systems. Recently, human proteins with chitinase activity and chitinase-like proteins were identified and established as biomarkers for human diseases. This review covers the recent advances of chitinases as a biocontrol agent and its various applications including preparation of medically important chitooligosaccharides, bioconversion of chitin as well as in implementing chitinases as diagnostic and prognostic markers for numerous diseases and the prospect of their future utilization.

Interactions between engineered tomato plants expressing antifungal enzymes and nontarget fungi in the rhizosphere and phyllosphere

The introduction of genetically modified (GM) plants in agroecosystems raises concern about possible effects on nontarget species. The impact of a tomato line transformed for constitutive expression of tobacco beta-1,3-glucanase and chitinase on indigenous nonpathogenic fungi was investigated. In greenhouse experiments, no significant differences were found in the colonization by arbuscular mycorrhizal fungi. Diversity indices computed from over 20 500 colonies of culturable rhizosphere and phyllosphere saprotrophic microfungi, assigned to 165 species (plus > 80 sterile morphotypes), showed no significant differences between GM and wild-type plants. Differences were found by discriminant analysis in both the rhizosphere and the phyllosphere, but such effects were minor compared with those linked to different plant growth stages.

Will transgenic plants adversely affect the environment?

Transgenic insecticidal plants based on Bacillus thuringiensis (Bt) endotoxins, on proteinase inhibitors and on lectins, and transgenic herbicide tolerant plants are widely used in modern agriculture. The results of the studies on likelihood and non-likelihood of adverse effects of transgenic plants on the environment including: (i) effects on nontarget species; (ii) invasiveness; (iii) potential for transgenes to 'escape' into the environment by horizontal gene transfer; and (iv) adverse effects on soil biota are reviewed. In general, it seems that large-scale implementation of transgenic insecticidal and herbicide tolerant plants do not display considerable negative effects on the environments and, moreover, at least some transgenic plants can improve the corresponding environments and human health because their production considerably reduces the load of chemical insecticides and herbicides.

Assessing effects of transgenic crops on soil microbial communities

Deleterious effects of transgenic plants on soils represent an often expressed concern, which has catalyzed numerous studies in the recent past. In this literature review, studies addressing this question have been compiled. A total of 60 studies has been found, and their findings as well as their analytical approaches are summarized. These studies analyzed the effects of seven different types of genetically engineered traits, i.e., herbicide tolerance, insect resistance, virus resistance, proteinase inhibitors, antimicrobial activity, environmental application, and biomolecule production. Sixteen genetically engineered plant species were investigated in these studies including corn, canola, soybean, cotton, potato, tobacco, alfalfa, wheat, rice, tomato, papaya, aubergine, and silver birch. Many of these plants and traits have not been commercialized and represent experimental model systems. Effects on soil microbial characteristics have been described in various studies, indicating the sensitivity and feasibility of the analytical approaches applied. However, classification of the observed effects into acceptable and unacceptable ones has not been possible so far. Establishment of validated indicators for adverse effects represents a scientific challenge for the near future, and will assist risk assessment and regulation of transgenic plants commercially released to the field.

Cry1Ab protein from Bt transgenic rice does not residue in rhizosphere soil

Expression of Cry1Ab protein in Bt transgenic rice (KMD) and its residue in the rhizosphere soil during the whole growth in field, as well as degradation of the protein from KMD straw in five soils under laboratory incubation were studied. The residue of Cry1Ab protein in KMD rhizosphere soil was undetectable (below the limit of 0.5 ng/g air-dried soil). The Cry1Ab protein contents in the shoot and root of KMD were 3.23-8.22 and 0.68-0.89 microg/g (fresh weight), respectively. The half-lives of the Cry1Ab protein in the soils amended with KMD straw (4%, w/w) ranged from 11.5 to 34.3d. The residence time of the protein varied significantly in a Fluvio-marine yellow loamy soil amended with KMD straw at the rate of 3, 4 and 7%, with half-lives of 9.9, 13.8 and 18d, respectively. In addition, an extraction method for Cry1Ab protein in soil was developed, with extraction efficiencies of 46.4-82.3%.

Genetically modified crops: environmental and human health concerns

About 10,000 years ago subsistence farmers started to domesticate plants and it was only much later, after the discovery of the fundaments of genetics, those organisms were submitted to rational genetic improvement mainly by selecting of traits of interest. Breeders used appropriate gene combinations to produce new animal races, plant varieties and hybrids, as well as improved microorganisms such as yeasts. After the introduction of recombinant DNA techniques, the transfer of DNA between species belonging to different genera, families or kingdoms became possible. The release of transgenic plants has aroused debates about several aspects of the environmental and human risks that could result from the introduction of genetically modified crops. Less effort has been dedicated to evaluate the impact of transgenic plants on their associated microorganisms, some of which (e.g. nitrogen-fixing bacteria, mycorrhizal fungi and endophytic microbiota) are extremely important for the survival of the plant. Investigations have been made regarding the horizontal transfer of genetic material between transgenic plants and microorganisms and on the disturbance of useful symbiotic associations between plants and endophytic, epiphytic and rhizosphere communities. In most cases the results do no show any adverse effect of transgenic plants on autochthonous plant-associated microorganisms. Results from our laboratory show small changes caused by genetically modified endophytic bacteria on the indigenous endophytic population of the sweet orange Citrus sinensis. In tests using appropriated fungal strains preliminary results using extracts from transgenic plants indicate that these plants do not affect haploidization, mitotic crossing-over, mutation rate or chromosomal alterations.

The release of genetically modified crops into the environment. Part II. Overview of ecological risk assessment

Despite numerous future promises, there is a multitude of concerns about the impact of GM crops on the environment. Key issues in the environmental assessment of GM crops are putative invasiveness, vertical or horizontal gene flow, other ecological impacts, effects on biodiversity and the impact of presence of GM material in other products. These are all highly interdisciplinary and complex issues. A crucial component for a proper assessment is defining the appropriate baseline for comparison and decision. For GM crops, the best and most appropriately defined reference point is the impact of plants developed by traditional breeding. The latter is an integral and accepted part of agriculture. In many instances, the putative impacts identified for GM crops are very similar to the impacts of new cultivars derived from traditional breeding. When assessing GM crops relative to existing cultivars, the increased knowledge base underpinning the development of GM crops will provide greater confidence in the assurances plant science can give on the risks of releasing such crops.

Strategies to improve plant resistance to bacterial diseases through genetic engineering

Many different genetic strategies have been proposed to engineer plant resistance to bacterial diseases, including producing antibacterial proteins of non-plant origin, inhibiting bacterial pathogenicity or virulence factors, enhancing natural plant defenses and artificially inducing programmed cell death at the site of infection. These are based on our knowledge of the mechanisms of action of antibacterial compounds and of the successive steps in plant-bacterial interactions. This article presents the different approaches and demonstrates that, even though several of these ideas have already been applied, no commercial applications have yet been achieved.