Virus-resistant potato: NewLeaf^TM Plus and NewLeaf^TM Y

Virus resistance in potato has been developed using a range of approaches and genetic constructs which include sequences of virus coat proteins, movement proteins, replicases, untranslatable sense or antisense RNAs, proteases, defective interfering RNAs, and satellites. Expression of ribozymes, a double stranded RNA-specific ribonuclease, antiviral proteins, a plant pathogen resistance gene and ‘plantibodies’ have also provided virus resistance (see Kawchuk and Prufer (1999) for a review of approaches used to deliver virus resistance).

NewLeaf^TMPlus produced by NatureMark^® is a high yielding Russet Burbank with combined CPB (cry3A expression) and potato leaf roll virus (PLRV) resistance (produced using the constitutive Figwort Mosaic Virus Promoter (FMV) within a construct designed to prevent virus replication). PLRV can cause yield losses of as much as 50% and nearly all commercial varieties are susceptible to infection with world-wide losses estimated at 10% (van der Wilk et al., 1991). PLRV also causes net necrosis (phloem cells affected), which greatly reduces the value of tubers for fresh and processing use. Freedom from such internal necroses provides a more consistent product and better financial returns per hectare by reducing processing costs in French fry and chip (crisp) industries and by helping to deliver improved seed quality. The transgenics are capable of reducing insecticide usage by up to 100%. NewLeaf^TM Plus was approved in the USA for consumption in August 1997. The FDA and EPA in the US determined that the potato was as safe to eat as any other Russet Burbank. Large-scale agronomic trials were grown in the USA in 1998.

NewLeaf^TM Y cultivars Russet Burbank and Shepody have been developed with combined CPB and potato virus Y (PVY) resistances. PVY is considered one of the most damaging potato viruses because it causes economically significant yield depression. Severe infestations can reduce yield by as much as 80% (Bemster and de Boks, 1987). The PVY coat protein gene used to generate resistance is also expressed using the FMV promoter and is more effective at PVY control than any insecticide programme allowing more sustained crop protection through reduced insecticide usage. Protection against PVY has reduced seed de-certification risk for seed growers and helped to maximise yields for commercial growers. Other benefits include improved processing quality and storage and higher tuber set in cultivars such as Shepody (more uniform tuber size distributions have been claimed through improved line selections). As far as NewLeaf^TM Y is concerned the FDA and Health Canada completed their review in May 1999 and agreed they were safe for human consumption. The EPA (August1997), USDA (February 1999) and Canadian Food Inspection Agency (April 1999) determined that NewLeaf^TM Y poses no concern for unreasonable effects on the environment or livestock.

The steps from initial clone production to the commercialisation of NewLeaf^TM Plus and NewLeaf^TM Y is shown in Fig. 9.1. The process started with ca. 3,000 original clones for each with the first selection occurring after transplanting to soil and removal of off types. True to type plants were then subjected to a range of tests including phenotypic analysis, efficacy tests against CPB, PLRV and PVY, susceptibility to insect, fungal and bacterial pests, evaluation of tuber yield, tuber appearance, quality and additional extensive field performance evaluation. Field evaluations took place over several years, in different environments and under various agronomic conditions with emphasis on potential environmental impact (Kaniewski and Thomas, 1999). From this process six clones were selected for commercialisation. Rogan et al. (2000) analysed key nutritional, quality and anti-nutritional components of NewLeaf^TM Plus and NewLeaf^TM Y lines to assess their substantial equivalence to the parent cultivar. Dry matter content, vitamin C, soluble sugar, soluble protein, glycoalkaloids, vitamin B₆, niacin, copper, magnesium, potassium, amino acids, fat, ash, calories, total protein and crude fibre were quantified. The data confirmed that tubers produced by insect and virus protected varieties were substantially equivalent to tubers produced by conventional varieties(Tables 9.4, 9.5, 9.6).









Riebe and Zalewski (submitted for publication) surveyed insecticide usage on NewLeaf^TM Plus potatoes in paired field comparisons with conventional Russet Burbank. The work showed that insecticide use could be greatly reduced or eliminated with the transgenic material, providing significant environmental and economic benefit (Tables 9.7 and 9.8). Net savingsto growers averaged $212 and $313 per hectare in 1998 and 1999, respectively. On the 140 ha monitored in 1998, NewLeaf^TM Plus allowed growers to reduce insecticides and miticides by a total of 2,870 kg of active ingredient and 7,700 kg of formulated material as compared with an adjacent field of conventional crop. Data indicate that more than 500,000 kg of active ingredients could be eliminated annually in the Columbia Basin, USA, if NewLeaf^TM Plus replaced all of the 35,000 ha of Russet Burbank grown in the region.