Role of the EPS levan in the life cycle of Pseudomonas syringae pv. actinidiae biovar 3, the agent of kiwifruit canker pandemic

The ability of Pseudomonas syringae (Ps) to produce different types of exopolysaccharides (EPSs) is well documented. Some EPSs allow Ps to thrive in different environments, while others, such as levan, a polyfructan polymer synthesized from sucrose, are considered to be storage molecules. In Pseudomonas, levan is synthesized by levansucrase enzymes (lsc). Among Ps pathovars, the presence of two functional lsc enzymes, lscB and lscC, has been documented in Ps pv. glycinea (Psg). We have previously shown that the pandemic lineage of Ps pv. actinidiae biovar 3 (Psa3) possesses two isoforms, lscβ and lscγ, which differ in their amino acidic sequence at the putative N-terminus as well as in their biochemical properties. In this work, by means of single and double knock-out mutants, we found that lscβ and lscγ have different roles in the physiology of Psa3 in vitro. Only the expression of lscβ is temperature-dependent, ten-fold higher at 5°C compared to 28°C, and only lscβ is responsible for inducing the characteristic levaniform colony when Psa3 is grown on NA with 5% sucrose, both at 5°C and 28°C. On the contrary in Psg, lscB and lscC are both thermoresponsive, and both lscB and lscC confer the carrier the ability to differentiate levaniform colonies. These findings suggest that the two levansucrases may serve different functions in the different species of the Ps complex. Experiments are underway to assess the role of levan on stress tolerance.