Impact of Rootstock Propagation Method on Citrus Rhizosphere Bacterial Community Composition

Citrus rootstocks, like other tree crops, have historically been generated from seeds. However, the significant tree loss due to huanglongbing (HLB, aka citrus greening), has dramatically increased demand for some of the newly developed HLB-tolerant rootstocks for which inadequate amounts of seed are available. As a result, rootstock cuttings and tissue culture methods are now being widely used for citrus rootstock propagation. Rhizosphere recruitment has been examined in Arabidopsis, rice, and other annual plant systems, and appears to be influenced by the microbial community of the bulk soil, soil physical conditions, plant phylogeny, and the seed-associated microbiome. However, less is known about the rhizosphere microbiome of tree crops, particularly citrus, which has a wide range of commercially used rootstock genotypes. To examine the influence of propagation method and rootstock genotype on the rhizosphere community composition, three citrus rootstocks were propagated using each method. When trees were 6 months-old, rhizosphere samples were collected to determine the bacterial community composition using high-throughput sequencing. Preliminary results found differences in the bacterial community composition due to the rootstock genotype, indicating a potentially strong influence of genotype on rhizosphere bacterial recruitment. Bacterial diversity in the rhizosphere of seed-propagated trees was greater compared to tissue culture propagation, and within each genotype, the propagation method exhibited a different bacterial community composition. Seed-propagated communities were enriched compared to tissue culture communities in Bacillus spp., which contain taxa with known plant-growth promoting properties, as well Betaproteobacteria and Bacteroidetes. Additional analyses are being performed to further examine the specific taxonomic differences in associated bacterial communities between these propagation methods within and between the three rootstock genotypes. Preliminary physiological studies indicate differences in root growth between propagation methods, and analyses are being performed to determine correlations between rhizosphere composition and plant performance.