A putative novel type of tight adherence (tad) like gene cluster of Pseudomonas chlororaphis PCL1606 exhibits a crucial role in avocado roots colonization, fostering its biological control activity

Abstract

Aims Pseudomonas chlororaphis PCL1606 (PcPCL1606), displays strong antagonistic and biological control abilities against several soil-borne fungal pathogens mainly due to the production of the antifungal molecule 2-hexyl, 5-propyl resorcinol (HPR). HPR governs other beneficial phenotypes, suggesting its additional regulatory activity. Published transcriptomic data identifying HPR-regulated genes involved in the interaction of PcPCL1606 with the avocado rhizosphere were used as a target database to identify putative genes involved in avocado roots colonization. Methods The induction of several consecutive genes that showed homology with genes encoding a putative type IV Flp/Tad (tight adherence) pilus but with a few differences from the Tad type A and B was observed. To study the role of this tad- like gene cluster in the biology of PcPCL1606, a chromosomal deletion mutant was constructed. The molecular characterization of the tad -like gene cluster and different in vitro and in vivo phenotypes related to colonization were addressed in the mutant strain respect to PcPCL1606. Results The tad- like gene cluster was composed of five independent transcriptional units. Furthermore, the tad -like deletion mutant was impaired in early attachment, early biofilm formation, bacterial autoggregation and in root competitiveness in avocado plants and biocontrol activity against R. necatrix . Conclusions These results expand our understanding about the role of HPR as a putative signalling molecule. This study revealed the importance of a putative novel type of a Tad system of PcPCL1606 in the avocado roots colonization, confirming that initial attachment to roots is a fundamental mechanism for the PcPCL1606 rhizospheric performance.

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