Enhancing Tolerance to Potato Zebra Chip Disease Using a Transgenic Approach

Zebra Chip (ZC) disease, associated with the phloem-limited bacterium Candidatus Liberibacter solanacearum (Lso) and vectored by the potato psyllid, Bactericera cockerelli, is a serious threat to the potato industry that leads to significant economic losses. To date, there are only a few registered insecticides that are effective against the potato psyllid and not much research has been done on controlling the causal bacterium. Recent focus has shifted from chemical control of Lso and the psyllid vector, to establishing genetic tolerance to Lso in potato through breeding and genetic engineering. Our current approach to study Lso-potato interactions involves an optimized transgenic delivery system of a plant-derived antimicrobial gene family into the commercially important chipping variety, ‘Atlantic’. To efficiently target Lso, the use of co-delivery of antimicrobial genes and targeted transgene vascular expression were optimized. The first component consists of using one delivery vector to deliver two different gene expression cassettes and identifying the most effective combination of these genes. The second component is to target gene expression in phloem vascular tissues, where Lso is confined, by using phloem-specific promoters. The transformants were then screened for reduction of Lso titer and the loss of disease symptoms. Effectiveness of the introduced antimicrobial genes was evaluated by exposing transgenic lines to Lso positive potato psyllids in controlled cages. Data from plant health observations and semi-quantitative PCR analysis after 14 days and 24 days of exposure to the psyllids showed that several of the transgenic ­potato lines remained Lso free after 24 days of exposure to Lso positive psyllids. We are now investigating the use of Lso-potato system as a model for identification of potential transgenic for delivering resistance in citrus to the greening disease (Huanglongbing) caused by a closely related bacterium, Candidatus Liberibacter asiaticus. The Lso-potato model is ideal to study Candidatus Liberibacter-plant interactions, as it presents the ability to screen many different potentially tolerant genes in a short period of time due to the rapid growth, propagation, and ease of transformation of the potato. We hope that our Lso-potato model system can greatly reduce the use of chemical control and provide useful insights on using transgenic approaches to control other related bacteria in economically important crops, such as citrus, tomato, and bell pepper.