Variation in seed-borne microbial communities of Metrosideros excelsa Sol. ex Gaertn. with consequences for germination success

Abstract

ABSTRACT Seed endophytic bacteria and fungi are co-dispersed with seeds and are likely founders of the initial endophytic microbiome in developing seedlings. The effects of the seed microbiome on seedling survival and growth are of particular interest for their roles in protection against pathogens and plant resistance to environmental stress. The fungal pathogen Austropuccinia psidii (G. Winter) Beenken is the causal agent of myrtle rust, which infects more than 480 species of Myrtaceae globally and poses a severe threat to New Zealand endemic species such as Metrosideros excelsa Sol. ex Gaertn. We hypothesised that seedlings of M. excelsa may contain seed-derived microbial communities that may contribute to variation in susceptibility to A. psidii. Culture based methods were used to recover 22 species of fungi (n = 412 isolates) and 16 bacterial taxa (n = 156 isolates) from 1580 M. excelsa seeds collected from 30 trees. Our results indicate broad variation in the isolation frequency and species composition of seed-borne microorganisms among individual trees: while seeds sampled from four trees were free of culturable microorganisms, the other 26 trees accumulated up to eight culturable taxa (both bacteria and fungi) in their seeds, including some species known to be beneficial endophytes or phytopathogens. The most frequently isolated fungal species belong to genera Aureobasidium, Cladosporium, Penicillium, and Pestalotiopsis. These fungi were often isolated from seeds also containing bacteria of the genera Bacillus, Erwinia, and Methylobacterium. At the same time, seeds collected from four trees contained known plant pathogens Colletotrichum aff. fioriniae (Marcelino & Gouli) Pennycook, Neofusicoccum sp., and Robbsia aff. andropogonis (Smith) Lopes-Santos et al., which could affect seed germination. This is the first report of bacterial and fungal communities residing in seeds of endemic New Zealand trees and the potential effects of seed-borne microbiomes on germination.