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Dickeya dadantii forms elongated cells during the infection of potato tubers: causal conditions, molecular basis, and implications to pathogenicity

Zhouqi Cui: Department of Plant Pathology & Ecology, The Connecticut Agricultural Experiment Station

<div><em>Dickeya dadantii</em> is a phytopathogenic bacterium that causes soft rot disease on potatoes. Tubers are nutrient storage organs of potato and are highly susceptible to <em>D. dadantii </em>infection. We observed that during the infection of potato tubers, some <em>D. dadantii </em>cells formed elongated cell morphology. To better understand this phenotype and its biological significance, we first investigated the conditions under which the long cells are formed. Our results demonstrated that <em>D. dadantii </em>formed more long cells under dry conditions with no free water, at the surface of the infected tubers, as oppose to under the wet conditions with free water, and at the junction of healthy and infected tuber tissues. We further characterized the molecular basis for the cell elongation. We observed a negative correlation between the cell length and the level of nucleotides guanosine tetraphosphate (ppGpp) in <em>D. dadantii</em>. Average cell length of <em>D. dadantii </em>is longer in mutant of ppGpp synthesis gene <em>relA</em>, and shorter in mutant of ppGpp degradation gene <em>spoT</em>. We also report a positive correlation between the ppGpp level and the bacterial virulence, type III secretion system, production of pectate lyase, and motility. Our results suggest that by adjusting cellular levels of ppGpp, <em>D. dadantii </em>places the elongated, non-mobile and low virulent cells at the surface of infection whereas places the shorter, motile and more virulent cells at the interface of infection.</div>