Alarmone ppGpp modulates bacterial motility, zeamine production, and virulence of <i>Dickeya oryzae</i> through the regulation of and cooperation with the putrescine signaling mechanism

Putrescine is an important interspecies and interkingdom communication signal, modulating the bacterial motility, biofilm formation, and virulence of <i>D. oryzae</i>. The understanding of the regulation of putrescine biosynthesis and transport in <i>D. oryzae</i> is limited. In this study, we report that alarmone ppGpp hierarchically modulates putrescine biosynthesis and transport and synergistically cooperates with putrescine to regulate virulence traits and the virulence of <i>D. oryzae</i>. We found that the alarmone ppGpp synthesized by RelA regulated putrescine biosynthesis through modulating <i>speA</i> expression, and the product putrescine would thus inhibit the expression of <i>potF</i> and <i>plaP</i>. Remarkably, we unveiled the synergistic effect of alarmone ppGpp and putrescine on the modulation of swimming motility and zeamine production. Compared with the single deletion of either <i>relA</i> or <i>speA</i>, the double deletion of <i>relA</i> and <i>speA</i> could decrease the expression of RNA chaperone encoded gene <i>hfq</i> and the production of phytotoxin zeamine, which further attenuated the capability of <i>D. oryzae</i> EC1 in inhibition of rice seed germination. Collectively, the findings from this study depict alarmone ppGpp regulation on putrescine biosynthesis and transport and present the cooperation of regulation of alarmone ppGpp and putrescine in the virulence of <i>D. oryzae</i>.IMPORTANCE<i>Dickeya oryzae</i> is the causal agent of rice root rot disease. Bacterial motility and phytotoxic zeamines are characterized as two major virulent factors during <i>D. oryzea</i> infecting rice seed. Putrescine, as an interspecies and interkingdom communication signal for the infections of <i>D. oryzae</i>, has been previously demonstrated to be involved in the modulation of bacterial motility. Here we report the novel synergistic effect of putrescine signal and alarmone ppGpp on the regulation of both zeamine production and bacterial motility via modulating the expression of RNA chaperone-encoded gene <i>hfq</i>. In addition, we also showed that alarmone ppGpp hierarchically modulates putrescine biosynthesis and transport. Therefore, the findings of this study unveil the previously undetermined contribution of putrescine in the modulation of virulence determinants, and the regulatory mechanism of putrescine biosynthesis and transport in <i>D. oryzae</i>.