Genes ptz and idi, Coding for Cytokinin Biosynthesis Enzymes, Are Essential for Tumorigenesis and In Planta Growth by P. syringae pv. savastanoi NCPPB 3335.

Maite Añorga,Nuria De Diego,Ondřej Novák,Cayo Ramos,Jesús Murillo,Lydia Ugena,Adrián Pintado

doi:10.3389/fpls.2020.01294

Abstract

The phytopathogenic bacterium Pseudomonas syringae pv. savastanoi elicits aerial tumors on olive plants and is also able to synthesize large amounts of auxins and cytokinins. The auxin indoleacetic acid was shown to be required for tumorigenesis, but there is only correlational evidence suggesting a role for cytokinins. The model strain NCPPB 3335 contains two plasmid-borne genes coding for cytokinin biosynthesis enzymes: ptz, for an isopentenyl transferase and idi, for an isopentenyl-diphosphate delta-isomerase. Phylogenetic analyses showed that carriage of ptz and idi is not strictly associated with tumorigenic bacteria, that both genes were linked when first acquired by P. syringae, and that a different allele of ptz has been independently acquired by P. syringae pv. savastanoi and closely related bacteria. We generated mutant derivatives of NCPPB 3335 cured of virulence plasmids or with site-specific deletions of genes ptz and/or idi and evaluated their virulence in lignified and micropropagated olive plants. Strains lacking ptz, idi, or both produced tumors with average volumes up to 29 times smaller and reached populations up to two orders of magnitude lower than those induced by strain NCPPB 3335; these phenotypes reverted by complementation with the cloned genes. Trans-zeatin was the most abundant cytokinin in culture filtrates of NCPPB 3335. Deletion of gene ptz abolished biosynthesis of trans-zeatin and dihydrozeatin, whereas a reduced but significant amount of isopentenyladenine was still detected in the medium, suggesting the existence of other genes contributing to cytokinin biosynthesis in P. syringae. Conversely, extracts from strains lacking gene idi contained significantly higher amounts of trans-zeatin than extracts from the wild-type strain but similar amounts of the other cytokinins. This suggests that Idi might promote tumorigenesis by ensuring the biosynthesis of the most active cytokinin forms, their correct balance in planta, or by regulating the expression of other virulence genes. Therefore, gene ptz, but not gene idi, is essential for the biosynthesis of high amounts of cytokinins in culture; however, both ptz and idi are individually essential for the adequate development of tumors on olive plants by Psv NCPPB 3335.

Highlights

Plant defense against offenders is expensive, requiring an investment that could otherwise be used for reproduction
P. syringae commonly induces spots and blights of leaves, stems and fruits, with bacterial cankers and blights as the most common diseases of woody plants (Agrios, 2005; Lamichhane et al, 2014). Another frequent type of symptoms induced in woody hosts are galls or tumors (Lamichhane et al, 2014; Caballo-Ponce et al, 2017a), which are exclusively caused by P. syringae pathovars cerasicola, daphniphylli, dendropanacis, myricae, nerii, retacarpa, rhaphiolepidis, and savastanoi plus P. meliae and P. tremae, all belonging to PG3
BlastP comparisons identified homologs of the deduced products of genes ptz and idi in a variety of bacteria (Supplementary Figure S2). Phylogenetic analyses of these sequences immediately evidenced the separate evolution of these two genes before invading P. syringae because the closest relatives of Ptz and Idi are carried by phylogenetically different bacteria

Summary

Introduction

Plant defense against offenders is expensive, requiring an investment that could otherwise be used for reproduction. P. syringae commonly induces spots and blights of leaves, stems and fruits, with bacterial cankers and blights as the most common diseases of woody plants (Agrios, 2005; Lamichhane et al, 2014) Another frequent type of symptoms induced in woody hosts are galls or tumors (Lamichhane et al, 2014; Caballo-Ponce et al, 2017a), which are exclusively caused by P. syringae pathovars cerasicola, daphniphylli, dendropanacis, myricae, nerii, retacarpa, rhaphiolepidis, and savastanoi plus P. meliae and P. tremae, all belonging to PG3 P. amygdali) (Gardan et al, 1999; Gomila et al, 2017)

Methods

Results

Conclusion