Bacterial endophytes in the leaves of Pavetta spp. with a specific focus on those causing leaf nodules

Abstract

Bacteria filled leaf nodules can be found on some plant species within the genus Pavetta. The identity of the bacterial endophytes within leaf nodules has been described for certain leaf nodulated plant species such as Psychotria kirkii and Pavetta schumanniana. These bacteria were found to belong to the genus Burkholderia. The bacterial endophyte population within the leaves of three Pavetta spp. indigenous to South Africa, viz. P. lanceolata, P. edentula and P. schumanniana, was investigated in this study with a focus on the species of bacterial endophytes that form the nodules on the leaves of these plants. To achieve this, a combination of culturing and Denaturing Gradient Gel Electrophoresis (DGGE) was used. The bacterial endophyte population within the leaves was found to be different between the three plant species but harboured bacterial genera that have been found in other plants. The nodule-forming bacterial endophyte in P. edentula and P. schumanniana was found to be related to those Burkholderia spp. previously described. The nodule forming bacterial endophyte in P. lanceolata specimens growing in pots was found to be a different species, i.e. from the family Bradyrhizobiaceae. This was compared to P. lanceolata specimens that were growing in open soil where a bacterium belonging to the genus Burkholderia was identified within the nodules. Further sampling and experimentation is necessary to determine if this finding is a rare incident and why the bacterial species within the leaf nodules of P. lanceolata differed between the two sites. Leaf nodule development and morphology has been well documented within the Psychotria, another plant genus that includes leaf nodulated species. The leaf nodule development in Pavetta spp. has not been documented. With the aid of electron microscopy, leaf nodule development was studied in the three indigenous Pavetta spp. Differences such as the number of bacteria at various leaf nodule ages and the shape of the bacteria were noted between the three plant species. The most notable difference was between the leaf nodules of P. edentula and P. schumanniana which were shown to be colonized by Burkholderia spp. in comparison to the P. lanceolata leaf nodules that were filled with a bacterium from the Bradyrhizobiaceae. Once again further analysis is required to determine if this difference in nodule morphology is due to the nodule-forming bacterial species or the species of Pavetta. The presence of the nodule-forming bacteria has been observed within the leaf nodules and on rare occasions within the flowers and seeds of some Pavetta spp. but their presence within stem tissue and deciduous hosts during winter has not been reported. With the use of Burkholderia specific primers the presence of the nodule-forming bacterial endophyte was investigated in different tissue types of P. schumanniana in winter and summer and within P. edentula. A Burkholderia sp. was detected within the older leaf buds of both winter and summer samples of P. schumanniana as well as in the flowers. No Burkholderia spp. were detected in the stem tissue of either plant species or the leaf buds of P. edentula.