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

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.种子内生细菌和真菌（Seed endophytic bacteria and fungi）可与种子共同传播，是发育中幼苗初始内生微生物组的潜在奠基类群。种子微生物组对幼苗存活与生长的影响尤为受关注，因其可通过抵御病原菌侵染、增强植物对环境胁迫的抗性发挥关键作用。真菌病原菌Austropuccinia psidii（G. Winter）Beenken是香桃木锈病的致病菌，该病原菌可侵染全球范围内480余种桃金娘科植物，对新西兰特有物种如红花铁心木（Metrosideros excelsa Sol. ex Gaertn.）构成严重威胁。本研究推测，红花铁心木（M. excelsa）的幼苗可能携带种子来源的微生物群落，这些群落可能会导致其对A. psidii感病性的差异。本研究采用分离培养法，从30株母树采集的1580粒红花铁心木种子中，分离得到412株真菌菌株（隶属于22个真菌物种）以及156株细菌菌株（隶属于16个细菌类群）。研究结果显示，不同母树的种子携带微生物的分离频率及物种组成存在显著差异：4株母树的种子未分离得到可培养微生物，其余26株母树的种子中可培养类群最多可达8种（涵盖细菌与真菌），其中包含部分已知的有益内生菌或植物病原菌。分离频率最高的真菌类群隶属于短梗霉属（Aureobasidium）、枝孢霉属（Cladosporium）、青霉属（Penicillium）以及拟盘多毛孢属（Pestalotiopsis）。上述真菌常与芽孢杆菌属（Bacillus）、欧文氏菌属（Erwinia）以及甲基杆菌属（Methylobacterium）的细菌共同从种子中分离得到。与此同时，4株母树的种子中分离得到了已知植物病原菌：Colletotrichum aff. fioriniae（Marcelino & Gouli）Pennycook、新壳梭孢属未定种Neofusicoccum sp.以及Robbsia aff. andropogonis（Smith）Lopes-Santos et al.，这些病原菌可能会影响种子萌发。本研究首次报道了新西兰特有树木种子中存在的细菌与真菌群落，以及种子携带微生物组对种子萌发的潜在影响。