The ability of polysaccharides from two species of domesticated Ulva seaweed to reduce disease incidence on kiwifruit caused by Pseudomonas syringae pv. actinidiae (Psa) and on tomato caused by P. syringae pv tomato (Pto) has been demonstrated in glasshouse experiments on potted kiwifruit plants and tomato seedlings. We also showed that those two ulvans induce a defence reaction in kiwifruit plants.
Psa, the causal agent of bacterial canker on kiwifruit, is the economically most important pathogen of kiwifruit. The 2010 New Zealand outbreak threatened the survival of the New Zealand kiwifruit industry and cost the economy over NZ$ 930 million in the first four years of the outbreak (Vanneste, 2017). Today the industry has recovered thanks mainly to the wide planting of a Psa tolerant cultivar. However, the number of tools available for the control of this disease is still very limited and Psa has shown that it can evolve rapidly. Therefore, this pathogen is still a threat to the New Zealand kiwifruit industry. The goal of this project is to develop an elicitor that can be used for control of Psa by kiwifruit growers including organic kiwifruit growers.
We showed that spraying ulvans from two species of domesticated Ulva seaweed onto the leaves of potted kiwifruit plants before inoculation with Psa, significantly reduced disease incidence. Gene expression analysis in kiwifruit plants treated with the two different ulvans confirmed that these ulvans act as elicitors. Seven days post treatment several genes that we had previously demonstrated to be involved in SAR in kiwifruit against Psa, were significantly overexpressed in kiwifruit plants treated with ulvan. Particularly effective were AcDMR6, which is a possible modulator of the salicylic acid (SA) response; Benzyl Alcohol Dehydrogenase (BAD), a dehyrogenase of SA derivatives; NIM-interacting protein 2 (NIMIN2), a SA-induced NPR1-independent gene involved in regulation of PR1; and a Glucan endo-1,3-β-glucosidase (PR2 protein), which is a common marker of the SA resistance pathway. Those genes were also shown to be overexpressed after treatment with acibenzolar-s-methyl, a well-characterised commercial elicitor sold as Actigard® or Bion®.
While economically important outbreaks of P. syringae pv. tomato (Pto) are relatively rare, this pathogen has been widely used to study the molecular mechanisms of host responses to infection. In this study we demonstrated that treatment with ulvans significantly reduced disease incidence tomato seedlings when the plants were treated 7 days before inoculation, and also when the plants were treated just before inoculation, suggesting that ulvans could, in addition to being elicitors, directly affect the pathogen.
Previous studies have shown that the efficacy of ulvan-elicitors depends on their structure including their molecular weight. Since the two ulvans we have used in this study are structurally distinct from each other and well characterised, we are in a position to further investigate the structure/activity relationships of ulvans as elicitors.