Community Structure of Endophytic Actinobacteria in a New Zealand Native Medicinal Plant Pseudowintera colorata (Horopito) and Their Influence on Plant Growth

The role of plant endophytic Actinobacteria remains poorly understood with no reports of these communities in New Zealand native plants. This first investigation of endophytic Actinobacteria in New Zealand targeted the culturally significant medicinal shrub Pseudowintera colorata (horopito) as a model plant. Community analysis in plant tissues collected from ten geographically distinct sites showed that tissue type had the strongest influence on diversity and richness of endophytic Actinobacteria. More denaturing gradient gel electrophoresis (DGGE) bands were obtained from stems (n = 18) compared to roots (n = 13). Sequencing analysis of the major bands (n = 20) identified them as uncultured bacteria, Streptomyces sp. and Angustibacter peucedani. Using two Actinobacteria-specific media, nine isolates were recovered from surface-sterilised P colorata tissues. This was approximately 12% of the total taxa and correlated well with culturable numbers in international studies. In vitro analysis of the functionality of these strains showed that Streptomyces sp. PRY2RB2 inhibited all the tested phytopathogenic fungi (n = 4), Streptomyces sp. UKCW/B and Nocardia sp. TP1BA1B solubilised phosphate and produced siderophores. The functionality of the phosphate solubilising strains (n = 2) in vivo was investigated by inoculation of P. colorata seedlings. After 4 months, the mean shoot height of seedlings treated with Nocardia sp. TP1BA1B was 1.65x longer, had higher shoot dry weight (1.6x) and number of internodes (1.67x) compared to control. This study identified for the first time a key group of endophytic Actinobacteria that arc likely to be important in the ecology of New Zealand flora.