Characterization of arbuscular mycorrhizal fungi and endophytic bacteria in mycorrhizal and non-mycorrhizal mutant maize by a metagenomics approach

Abstract

Arbuscular mycorrhizal fungi (AMF) establish mutualistic symbiosis with almost 80% of all plants. AMF are widely used in agriculture because they offer a substitute to chemical biofertilizers as they can boost plant growth under non-ideal conditions. Maize is the most important commercial and cultural crop in Mexico. The common symbiosis pathway (CSP) is a well conserved molecular signaling pathway in all plants that can achieve mycorrhizal and rhizobia symbiosis, which starts with specific receptors that recognize each microorganism. Here, maize-associated AMF species and endophytic bacteria composition from two variants of maize, a mycorrhizal wild type and a non-mycorrhizal mutant type, were assessed by using high-throughput MiSeq-Illumina sequencing. Mutant maize did not possess the HUN gen, a CASTOR homologous gene, that is involved in the CSP by starting upstream Ca2+ oscillations and activating molecular signaling cascade. Composite root samples of wild type (n=9) and mutant maize (n=10) were used for microbial characterization. Mycorrhization was observed in all samples. A total of 22 AMF species were found with abundance >1%, and only 7 species were shared by both types of maize. Differences among AMF composition were observed, where mutant plants showed absence of the AMF generalist species Rhizophagus. Shannon and Chao1 diversity indexes were higher in mutant plants than wild type maize. Regarding endophytic bacteria, 137 and 135 amplicon sequence variants (AVS) were found in wild type and mutant maize, where 105 were shared by both treatments. Streptomyces and Bulkholderia were the most predominant among all samples. Streptomyces abundance suggest that this genus may play a main role in bacterial and AMF composition, as it is known for improving AMF germination and symbiosis due to its chitinase production. As mutation of HUN gene was not sufficient to block complete symbiosis between AMF and the plant, further characterization of downstream genes of the CSP in maize and field experimentation are required.