Species Diversity, Nitrogen Fixation, and Nutrient Solubilization Activities of Endophytic Bacteria in Pea Embryos

Abstract

Endophytic bacteria, especially those that participate in nitrogen fixation, play critical roles in supplying essential nutrients for legume plant growth. Despite that there have been numerous investigations targeting bacterial microbiomes in legume roots and nodules, little is known about embryonic bacteria that facilitate plant nutrient utilization after seed germination. Here, we collected and investigated endophytic bacterial microbiome in edible pea (Pisum sativum) embryos using five representative cultivars and a pea sprout (shoot of pea [SHP]) control. Twenty-six nitrogen-fixing bacteria (NFB) were isolated from pea embryos, with three strains found in fresh grain pea (FGP) and snow pea (SP) exhibiting the strongest nitrogenase activity of above 85 nmol C2H4/mL/h. Some NFB isolates are also potassium-solubilizing bacteria (KSB) or phosphorus-solubilizing bacteria (PSB) utilizing inorganic and/or organic phosphorus. All 26 NFB showed variable levels (0.41 to 7.10 μg/mL) of indole-3-acetic acid (IAA) secretion. The nutrient-solubilizing NFB identified in our research are potential targets for biofertilizer development. They could be useful in converting nitrogen, potassium, and/or phosphorus into usable forms for the plants. At the microbiome level, high-throughput 16S ribosomal RNA (rRNA) sequencing of 40 bacterial collections from pea embryos generated 4234 operational taxonomic units (OTUs) using 97% identity as the threshold for clustering high-quality effective reads (valid tags). Analysis of OTU annotation results revealed similar species community structures, abundance, and diversity in most samples. Our embryo-derived endophytic bacterial pool provides a microbiome platform for seed dormancy and germination research of edible peas, as well as for digging new biofertilizer resources in general.