An alternative to the use of pesticides: chitosan as an elicitor of systemic acquired resistance (SAR) in plants

Kiwifruit is a commercial crop threatened by attack of its most dangerous pathogen, Pseudomonas syringae pv. actinidiae (PSA). Currently, the European Economic Community has engaged in a policy of replacing chemical pesticides with natural substances. Following the European trend, this study is dealing with the prevention of disease outbreaks by growing plants able to better resist pathogen infection through the use of chitosan, a biodegradable, well-known elicitor of Systemic Acquired Resistance (SAR). The aim of this work was to improve the quality of Actinidia plants by a sustainable approach and reduce costs for the containment of phytopathological disorders. It was a 3-years project. In the first year, in vitro cultures of Actinidia deliciosa (A. Chev) C.F. Liang & A.R. Ferguson were chosen as model system to investigate the mechanisms of interaction between chitosan (elicitor) and the host plant. In order to understand the response to elicitation in planta, some effects of chitosan on the detoxification of reactive oxygen species (by guaiacol peroxidase [G-POD] and ascorbate peroxidase [APX]) and on the metabolism of phenolic compounds (by phenylalanine ammonia lyase [PAL] and polyphenol oxidase [PPO]) were monitored. Effectively, chitosan has been able to increase activity of the selected SAR biochemical markers in multiplication and rooting stages. In the second year, to study chitosan-host plant interaction in the presence of a pathogen, 2 year old plants belonging to two different species (Actinidia chinensis Planch. and A. deliciosa) were artificially inoculated with PSA in greenhouse experiments. The symptoms and SAR’s onset were investigated using morphological and biochemical analyses. Chitosan had the ability to enhance several activities of enzymes involved in detoxification processes (G-POD and APX) and in increasing plant defence barriers (PAL and PPO). Furthermore, plant’s defence responses were studied at molecular level. After a screening of possible SAR markers, Pathogenesis-Related proteins (PR1 and PR5) were chosen. Chitosan’s elicitation effect was determined by qRT-PCR and compared to the action of the most common SAR elicitors, such as salicylic acid, methyl jasmonate and the ethylene precursor 1-aminocyclopropane-1-carboxylic acid (ACC). As shown by the changes in PR transcription profiles, chitosan elicited a systemic response, with intensity comparable to other well-known signalling molecules. During the third year the validation of previously obtained results was undertaken. The changes in PR transcription profiles were corroborated in 2 year old plants, confirming that chitosan also elicits a systemic response in kiwifruit plants. In addition, a field trial with a natural presence of the pathogen was carried out and the kiwi plant response was determined by molecular techniques. If field trials will confirm the efficacy of chitosan in improving the behaviour of Actinidia plants under pathogen attack, further evidence will be added to support consideration of chitosan as sustainable and eco-friendly alternative for the reduction of pesticide use. In practice, a controlled induction of SAR with chitosan may be useful to alternate and integrate chemical treatments in a modern management of crop protection based on integrated control programs, including the use of environmentally safe products.