Studies on crops genetically modified to include lectins with the potential to enhance the plants’ resistance to insect and nematode pests.

Introduction

This short report provides some background to studies at the Rowett Research Institute undertaken to help establish whether proteins (lectins) with the potential to confer pest-resistance to crops might be suitable and safe for use in transgenic plants intended for human consumption. The early improper public comment on unpublished data from these studies unfortunately led to media reports of possible damage to the immune system of rats fed such proteins and, by implication, to that of humans should such proteins ever be used in commercial crops. As a result of this speculation, the Institute launched an independent audit of the experimental data, the outcome of which is attached.

The research in question was funded by the Scottish Office Agriculture, Environment and Fisheries Department from 1995 to December 1998 as part of a wider programme to examine efficacy and safety issues arising from the possible use of introduced lectins to increase pest resistance in crop plants. The programmme as a whole involved three collaborating scientists and their respective research groups:

(a) Dr A Pusztai, Rowett Research Institute, Aberdeen

(b) Mr W M Robertson, Scottish Crop Research Institute, Dundee

(c) Professor J A Gatehouse, University of Durham

The work at the Scottish Crop Research Institute and Durham University was concerned with the production of transgenic plants, testing their resistance to pests and ensuring the absence of effects on harmless insects. Neither organisation was involved in the feeding studies made with rats at the Rowett other than to supply transgenic material.

Lectins

Lectin is the collective name for a group of proteins which have the common property of binding to specific carbohydrate structures found in body tissues. Virtually all known plants contain lectins, usually as one of a number of proteins in seeds, but often also distributed amongst the other parts of plants including leaves, roots and storage organs. Lectins are present in most foods such as peas, beans, cereal grains, etc. and thus form part of the normal human diet.. A few lectins, such as the lectin found in red kidney beans (the PHA lectin), are, however, known to damage the structure of the small intestine, or affect the pancreas and production of digestive enzymes when eaten by humans or animals. This is why kidney beans are always soaked and boiled before use, a process which effectively destroys the lectin, making beans fit for human consumption. Most of the other lectins that have been studied appear to have no discernible effect other than their ready attachment to the surface of intestinal cells.

Work over the last 25 years has shown that lectins can play a role in resistance to insect and nematode pests in the plant, and that they are potentially important in crop protection. A simple screening procedure would therefore be valuable in allowing plant biotechnologists to assess whether genes coding for a potentially effective insecticidal lectin could be introduced into food plants with the confidence that it would be not be harmful if the plant were eaten by humans. The work at the Rowett was funded to address this issue.

Three lectins with very different effects on the digestive system of rats, judged on the basis of previously published studies originating from the Rowett and elsewhere were selected for study:

(a) PHA - the kidney bean lectin with a well documented detrimental impact on intestinal structure

(b) ConA - the lectin obtained from the jack bean and toxic to soil-borne nematodes.

(c) GNA - the snowdrop lectin, known to have insecticidal properties but considered less likely than the other two selected lectins to produce harmful effects in humans on the basis of previous feeding, histological and microbiological studies in rodents.

Transgenic plants expressing the selected lectins

A number of gene constructs were made in order to produce transgenic crop plants expressing one of the above three lectin genes. However it should be stressed that none of these constructs were intended for commercial exploitation in the short term. They were produced for experimental purposes to establish whether selected lectins introduced into crops could confer pest resistance and, if so, whether their presence introduced any hazards for humans, animals and the wider environment. In practice, much of the experimental work was done with potato. Potato was selected because it is easy to transform (introduce novel genes) and the resulting plants provide roots with which to investigate resistance to soil-borne potato cyst nematodes and leaves which can be used to investigate transgenic resistance to those insect pests which attack the aerial parts of the plant. In addition, the tubers can form the basis of diets suitable for feeding to experimental animals.

Animal Feeding experiments

In the event, only transgenic potatoes expressing GNA were grown in sufficient quantity for inclusion in rodent diets. The feeding studies were made with rats and included one of 110 days duration to investigate any effects of prolonged exposure to the transgenic tubers as well as a number of shorter term (10 day) trials.

In addition to feeding studies made with transgenic potato themselves, several experiments included the addition of isolated purified lectins (ConA and GNA) to raw and cooked potato both in amounts similar to that found in the transgenic potato and in amounts greatly in excess of those in the transgenic plant. The former was intended to detect any changes to lectin structure and properties which might have been introduced when the lectin was expressed in a new host and the latter to allow a thorough testing for an effect which might be less discernible at the far lower concentrations found in transgenic plants.

Growth of the animals fed the various potato-based diets was monitored throughout and at the end of the growth period each animal was subject to a standard toxicological examination. Also included was a simple test of immune function based on stimulating lymphocytes separated from the blood of rats fed GNA transgenic potato tubers, tubers free from the selected lectin or tubers to which isolated lectins had been added. This was done because some lectins, including PHA and ConA are known mitogens (agents which will stimulate lymphocyte cell division and elicit an immune response).

Evaluation of the results by the audit committee

Attached is the formal report of the audit group. Readers interested in seeing the detailed growth and other studies may obtain these from the Rowett Research Institute. As is made clear in the Audit Report, one long-term feeding study and several short-term studies were made with potato tubers expressing GNA. These showed, as expected, no discernable effect on growth. No comparable experiments were made with transgenic plants expressing the Con A gene. One long-term feeding study, however, was made with potato-based diets to which ConA had been added in a concentration some 5000-times that found in potato tubers expressing the ConA gene. Addition of isolated ConA at this concentration did cause a small depression in growth rate.

Studies have been made on the effects of lectins on mammalian cells in laboratory experiments, which have led to the observation that some lectins can stimulate cell division (i.e are mitogenic). However, lectins from different sources have different properties, and thus one lectin may be mitogenic, while another lectin is not. With the exception of several well-studied lectins, little is known about their precise effects when fed as part of the diet on the cells of the gut and immune systems of mammals. Given the interest in the possible future introduction of lectin genes to provide pest resistance in crops, it is important that suitable tools are developed to evaluate the potential effects of lectins on animals, including any possible effect on the immune system.

Unfortunately, in the course of developing appropriate tools, difficulties were encountered with the lymphocyte proliferation assay selected as a preliminary test of immune function These early results were far too variable to allow any conclusions to be drawn or even to allow any effects to be suggested. This led the Audit committee to their final conclusion that the existing data do not support any suggestion that the consumption by rats of transgenic potato tubers expressing GNA lectin has any effect on growth, organ development or immune function.