Pseudomonas syringae pv. actinidiae chemotaxis and the kiwifruit phyllosphere

Pseudomonas syringae pv. actinidiae (Psa) is causative agent of bacterial canker of kiwifruit. Outbreaks of bacterial canker disease cause significant ecological impact and economic losses both in New Zealand and globally. The first stage of infection is host colonisation. Psa must navigate from the plant surface (i.e., the phyllosphere) to the interior (i.e., apoplast). The molecular basis of how Psa navigates through these environments is unclear.

Emerging evidence suggest that chemicals may act as ‘signposts’ for entry into the apoplast. Firstly, plants exude an array of chemicals onto the leaf surface through organelles such as stomata. Secondly, motile bacteria such as Psa can move toward or away from chemical signals in their environment. This behaviour – called chemotaxis – allows them to migrate towards favourable conditions for growth and survival. Chemotaxis is mediated by chemoreceptors that sense chemicals through their sensor domain, feeding the signal into an intracellular signalling cascade to bias motility. Chemoreceptors vary widely in sensor domain structure allowing for chemotactic responses to a wide range of chemicals.

This thesis aimed to explore the significance of chemotaxis in Psa. The Psa genome contains 43 putative chemoreceptors, of which only four have been characterised. The characterized Psa chemoreceptors respond to numerous organic acids including malic, glycolic, formic, and propionic acids. The first aim was to determine the presence of these four organic acids on the kiwifruit leaf surface. Matrix-assisted laser-desorption/ionisation time-of-flight imaging mass spectrometry revealed that all four organic acids were detected and were heterogeneously distributed on the leaf surface. The second aim was to characterise the sensory repertoire of three putative Psa chemoreceptors in vitro to explore the chemical signals (i.e., ligands) they detect. Among the three chemoreceptors tested, Psa_006420 binds to the heavy metals zinc and copper. The third aim was to validate the role of Psa_006420 in chemotaxis using an in vivo gene deletion approach. Surprisingly, Psa_006420 does not appear to be a chemotaxis protein. Gene expression analyses suggests that Psa_006420 is putatively involved in two separate two-component systems associated with calcium and heavy metal signalling. Together, these findings provide the first proof of concept for chemotaxis in the phyllosphere and contribute to the fundamental understanding of bacterial chemotaxis systems.