SAfETY Assessment of the PLRvrep Protein Present in NEwLEAf Plus Potatoes

Potato is one of the most important sources of human food in the world, ranking fourth behind wheat, maize, and rice.68 Effective control of pests in potato is one of the key factors impacting production of high-quality tubers. Potato leafroll virus (PLRV) is a common potato virus that can be transmitted from one crop to the next through the use of potato tubers as seed,69 and severe infections with PLRV can cause yield losses of as much as 50%.70 Introduction of a viral sequence encoding potato leafroll virus replicase (PLRVrep)71-73 into the potato genome induced resistance to PLRV. Potato clones resistant to PLRV as a result of the insertion of the viral gene were referred to as NewLeaf® Plus potatoes.

The introduction of viral sequences into the genome of a host plant has often given rise to pathogen-derived resistance (PDR). The basis for such a control method lies in the observation that insertion of a portion of a viral genome into a host plant's DNA can lead to induction of resistance in the host plant to the virus from which the genetic material was derived.71 The expression of viral protein derived from the inserted sequences does not always correlate with resistance to the virus. This observation has resulted in much speculation concerning the mechanism of action for PDR. The absence of detectable protein has led to the hypothesis that resistance to virus is achieved via a nucleic acid-mediated mechanism of action.74 Consistent with this hypothesis, PLRVrep protein was not detected in leaf or tubers of NewLeaf®

* NewLeaf® is a registered trademark of Monsanto Technology LLC. Copyright 2008 by Taylor and Francis Group, LLC

Plus potato plants, even though messenger RNA (mRNA) was produced.73 However, evidence indicated that expression of a protein from the PLRVrep gene is required for effective resistance, although probably at a very low level. During the development of NewLeaf® Plus potatoes, potato plants (Russet Burbank) were transformed with several experimental constructs and field-tested for control of PLRV. Constructs, which produced mRNA that would translate a PLRVrep protein, were found to be the most effective at reducing infection of potato by PLRV. Constructs that produced mRNA but did not translate a protein were not effective.75

Given an extremely low exposure to the PLRVrep protein in NewLeaf® Plus potatoes, acute oral gavage with the PLRVrep protein and other associated protein safety assessments were not warranted. Rather, the safety assessment focused on the history of safe use. PLRV is a common potato virus and has been a component of the food supply for many years. By 1900, it was recognized that potato diseases, such as leaf curling and rolling and leaf mottling (Potato Virus Y) were transmitted from one crop to the next through the use of potato tubers as seed.76 In the early part of the twentieth century, there were no insecticides available to protect potatoes against insect damage during cultivation (which could make potatoes more susceptible to viral infection). Despite the application of techniques such as heat treatment, meristem culture, and potato seed certification which enabled potato growers to reduce the spread of viruses in potato, it was still common to have nearly 100% of tubers infected with PLRV by the end of the growing season.77 Indeed, in a broader historical context, potatoes consumed in the eighteenth and nineteenth centuries were undoubtedly widely infected with a plethora of viruses commonly harbored by potatoes. Bawden78 recounts that in 1775 potatoes in different parts of Europe were so severely infected with viruses that their cultivation had to be abandoned. Therefore, the PLRVrep protein has a history of safe human and animal consumption from the widespread consumption of PLRV-infected potatoes.

Although it is well known that exposure to potato viruses via consumption of infected tubers is a common occurrence, no quantitative data were available to support the amount of exposure to PLRV. Therefore, in order to obtain an estimate of exposure to the PLRV virus, a study was performed by Noteborn79 using tubers obtained from commercial outlets in five different European countries — the United Kingdom, the Netherlands, Italy, Germany, and Denmark. Tuber samples from popular European varieties were obtained at randomly selected dates over a four-month period. The amount of PLRV in the tuber (peel and flesh) was determined by a quantitative enzyme-linked immunosorbent assay (ELISA) method.79 The results from this study confirmed that PLRV is commonly found in fresh market potato tubers. The amount of PLRV detected and the variety tested are presented in Table 10.2. Although the level of PLRVrep protein was not directly measured, this protein is obligatory for the virus to multiply, assemble virions, and move throughout the plant. Therefore, the presence of PLRV in infected plants indicates that the replicase protein from PLRV is present. The maximum amount of PLRV detected was 5.28 ^g/100 g tuber fresh weight. Given that the average European consumption of potatoes is 240 g/day,80 the dietary exposure to the PLRV virion can be as high as 12.7 ^g, which is significantly higher than expression of the PLRVrep protein in NewLeaf® Potato.

In summary, PLRV is a common potato virus with established significant human exposure to the virus and its associated obligatory proteins in multiple potato varieties.

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