The colonization processes of Myrtus communis by strains of Pseudomonas savastanoi with a differential ability to produce phytohormones

Abstract

The ability to produce indole-3-acetic acid (IAA) through the indole-3-acetamide (IAM) pathway as well as cytokinins is a common trait of Pseudomonas savastanoi populations causing disease on oleander and olive. These phytohormones are required for the induction and development of an outgrowth of plant cell tissue termed a knot. However, in myrtle orchards of Sardinia (Italy), strains of P. savastanoi unable to produce cytokinins were found coexisting with cytokininproducing strains. Data presented here show that the ability to produce IAA through the IAM pathway is also a variable trait within this population, raising questions on the exact role of these plant growth substances in the disease process on myrtle. Three P. savastanoi strains were selected based on their differential ability to produce phytohormones in vitro, and their interaction with the host was investigated over a period of 8 months using histological methods. All strains successfully invaded the infected twigs, moving systemically (unhalted by host defences) upward and downward from the inoculation point, both by completely degrading the cell walls and by taking advantage of the xylem vessels and intercellular spaces. Moreover, all strains induced the development of cankers, which slowly evolved into typical knots only on the twigs inoculated with the phytohormone-producing strains. This study further demonstrates that cytokinins and IAA are essential for knot development; moreover, it ascertains that bacterial production of cytokinins is not necessary for host colonization and for the expression of pathogenicity (i.e. the ability to cause disease) of P. savastanoi on myrtle.

A novel function of the key nitrogen-fixation activator NifA in beta-rhizobia: Repression of bacterial auxin synthesis during symbiosis

“…savastanoi. While A. tumefaciens uses the IAM pathway as a virulence factor to induce crown gall tumors in plants ( Zupan et al, 2000 ), in P. syringae IAA induces virulence ( Djami-Tchatchou et al, 2020 , 2022 ) and in P. savastanoi the iaaMH genes are involved in the colonization process of Mytus communis ( Schiff et al, 2019 ). Orthologs of the P. phymatum STM815 iaaMH genes were also present in Burkholderia species belonging to the Bcc group, which includes human opportunistic pathogens ( Supplementary Table 3 ).…”

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

“…nerii strains cured of plasmids encoding ptz are able to multiply at a rate similar to the wild-type strain in oleander leaves ( Iacobellis et al., 1994 ). Furthermore, P. savastanoi strains isolated from Myrtus lacking gene ptz and producing undetectable amounts of CKs successfully invaded the host tissues and moved systemically, apparently unaffected by host defenses, such as the synthesis of tannins and the lignin deposits around the inoculation sites ( Schiff et al., 2019 ). Thus, the role of CKs in the ability of P. savastanoi to multiply and survive in plant tissues might be strain- and/or host-dependent and could be influenced by other virulence factors varying among strains ( Moreno-Pérez et al., 2020 ).…”

“…Nevertheless, plasmid curing could completely abolish or diminish CK production, indicating that more than one gene could participate in CK biosynthesis. Diverse studies have also found a positive association between the amount of CKs naturally produced by certain P. syringae strains and the size of knots induced in the plant host ( Surico et al., 1985b ; Cinelli et al., 2014 ; Schiff et al., 2019 ), although this is not always true, suggesting the existence of additional strain-specific genes contributing to virulence. Similarly, a strain of P. syringae pv.…”

Knot formation and spread along the shoot stem in 13 olive cultivars inoculated with an indigenous pathobiome of 7 species of Pseudomonas including Pseudomonas savastanoi

“…savastanoi (herein referred to as Pss) has been recognized as the primary agent of olive knot disease [4], in the recent years evidence has emerged to suggest that a consortium of bacteria (pathobiome) may favor the development of the disease [5][6][7][8][9]. In addition, Pss can affect several other species [10][11][12][13] including oleander (Nerium oleander L.), pomegranate (Punica granatum L.) and myrtle (Myrtus communis L.).…”