Abstract:
Pantoea ananatis causes disease symptoms in a wide range of economically important plants such as Eucalyptus, maize and onions. This study specifically focussed on the interactions of P. ananatis LMG20103 with onion seedlings where it causes symptoms that include water-soaked lesions, wilting and bleaching of the leaves, and maceration of the bulbs. The pathogenicity of P. ananatis is not well understood, however, motility plays an important role in many other well-known phytopathogens such as Ralstonia solanacearum and Pseudomonas syringae. It is therefore hypothesised that P. ananatis uses motility to colonise and infect its hosts.
Chapter 1 reviews the literature on how motility aids phytopathogenic bacteria in locating their host, attaching and initiating infection, as well as dissemination. The two dominant forms of motility utilised are swimming and twitching motility. Swimming motility is essentially the rotation of flagella which propels the bacterial cell forward through a fluid environment in response to chemotactic signals. The motor that drives the flagella is made up of several proteins that include the MotAB proteins and its function is dependent on a proton motive force. While twitching motility is not as fast as swimming motility, it is a rapid means of surface colonisation. Bacteria twitch by extending their type IV pili, attaching to the surface, and then retracting, bringing the whole cell closer to the point of contact. This motion is powered by the ATPase PilT.
In Chapter 2 the flagellum and type IV pilus biosynthetic gene clusters are compared between strains of P. ananatis and closely related enterobacterial strains. The four fully annotated and sequenced P. ananatis strains used in this study were isolated from various different sources and provided a greater understanding of how P. ananatis exploits its flagella and type IV pili to infect such a wide variety of hosts. While Chapter 3 focuses on the creation of four motility mutants and their respective complements in P. ananatis LMG20103, Chapter 4 consists of an array of tests and assays comparing the mutants to the wild-type strain to elucidate the role of swimming and twitching motility in the colonisation and infection of P. ananatis in onions. Chapter 5 is a published article titled, “Draft genome sequences of the onion centre rot pathogen Pantoea ananatis PA4 and maize brown stalk rot pathogen P. ananatis BD442.” Both strains are South African isolates and were sequenced using the Illumina HiSeq 2500 platform.
A greater understanding of how P. ananatis uses motility to target tissues and infect its host plant increases the currently limited body of knowledge available to develop strategies to limit the damage caused by this pathogen to agronomic crops in plantations and nurseries.
