Screening for leaf-associated endophytes in the genus Psychotria (Rubiaceae)

Abstract

Burkholderia endophytes were identified within the leaves of non-nodulated members of the genus Psychotria. In contrast to leaf-nodulated Psychotria species, which are known to accommodate their endosymbionts into specialized endosymbiont-housing structures, non-nodulated species lack bacterial leaf nodules and harbor endosymbionts intercellularly between mesophyll cells. Based on molecular data (rps16, trnG, and trnLF), the phylogenetic reconstruction of the host plants revealed a separate origin of leaf-nodulated and non-nodulated Psychotria species. Despite a distinct phylogenetic position of the two host clades, the endophytes of the non-nodulated plants were not placed into a single monophyletic group but were found to be closely related to the leaf-nodulated endosymbionts. The observation of genetically similar endophytes in both nodulated and non-nodulated Psychotria lineages suggests that the host plant is playing a crucial role in the induction of leaf nodule formation. Moreover, the concentration of endosymbionts into specialized leaf nodules may be considered as a more derived evolutionary adaptation of the host plant, serving as an interface structure to facilitate metabolic exchange between plant and endosymbiont.

Symbiotic ß-Proteobacteria beyond Legumes: Burkholderia in Rubiaceae

“…This finding is peculiar because all Vanguerieae endophytes so far are member of the PBE group (sensu Suárez-Moreno et al) and the Globulostylis species do not have visible bacterial galls in their leaf blades. These results are, however, corroborated by the observation of non-nodulating endophytes in Psychotria species, the genus that is especially known for its leaf galls [23]. The closest relative of all these nodulating endophytes is B. glathei , a free-living soil bacterium that is not considered to be part of the PBE group according to Suárez-Moreno et al; instead it is placed in the pathogenic Burkholderia clade.…”

Evidence of horizontal gene transfer between obligate leaf nodule symbionts

“…The simplest explanation is that the symbiont of Pa. schumanniana was recently replaced with the leaf nodule symbiont of P. kirkii. The possibility of a reinfection by bacteria living in a soil reservoir as previously hypothesized (Lemaire et al, 2012) seems unlikely. The high degree of genome reduction, together with the very high sequence homology observed between the genomes of the symbionts of Pa. schumanniana and P. kirkii, rather suggest that both species have been restricted to a host for millions of years.…”

“…This association has been described in three genera of Rubiaceae: Psychotria, Pavetta and Sericanthe (Lemaire et al, 2012). Nodulated species are endemic to tropical and sub-tropical Africa, with most nodulated Psychotria found in savannah habitats (Lachenaud, 2013).…”

Phylogeny and generic limits in the sister tribes Psychotrieae and Palicoureeae (Rubiaceae): Evolution of schizocarps in Psychotria and origins of bacterial leaf nodules of the Malagasy species

“…The generic status of all these genera has been questioned by some authors (e.g., Piesschaert, 2001 ;Schatz, 2001 ;Davis et al, 2007 ), as they appear to fall within Psychotria 's limits based on their general morphology. Before this study less than 1% of Psychotrieae and Palicoureeae species from Madagascar had been included in previous molecular systematic studies (e.g., Andersson, 2002b ;Lemaire et al, 2011Lemaire et al, , 2012a, and no species from the neighboring islands had been sequenced.…”

“…Bacterial symbiosis in the form of leaf nodules has been recorded in about 500 species of Rubiaceae and in the family Primulaceae ( Miller, 1990 ). The symbionts are known to play an important role for the survival of their plant hosts (e.g., Lemaire et al, 2011Lemaire et al, , 2012a. Within Rubiaceae, all leaf-nodulated species belong to Psychotria and Apomuria (Psychotrieae) and the genera genus Carapichea Aubl.…”