Results
This study examined the interactions of root longitudinal segments
(apical and basal zones), rhizocompartment (rhizosphere and endosphere),
chickpea genotype, and soil type on the assemblage of the prokaryotic
microbiome by sequencing 16S rRNA genes. We used three chickpea
genotypes, including one early flowering (EF) and two late flowering
(LF1 and LF2), based on previous long-term and multi-environmental field
evaluations. Thermal time to flowering of EF, LF1, and LF2 were 962 ±
21.5 ºCd, 1041 ± 27.4 ºCd, and 1124 ± 29.2 ºCd, respectively (Sadras et
al., 2016). Genomic analysis confirmed that genotype EF carried a
mutated form of the gene Efl1 with 11-bp deletion compared with
genotype LF1 and LF2 (Fig. S2). The 11-bp deletion of Efl1 was
associated with early flowering in the global chickpea germplasm (Ridge
et al., 2017).
Five soils were collected from rainfed cropping lands with contrasting
physical, chemical and biological properties (Table S1). Using the
rhizotron system with a removable transparent side, root longitudinal
segments of apical zone and basal zone in the same chickpea plant were
sampled, based on the rapid establishment of root microbiome reported
previously (DeAngelis et al., 2009; Edwards et al., 2015). Each of the
sampled longitudinal zone had the same root-soil contact period (1 day)
from different soils and genotypes. We measured the length of sampled
apical roots or basal roots, and the rooting depth at harvest under
different soil types and genotypes, and found the interactions between
soil type and genotype were significant, but genotypic variation of
sampled root length or rooting depth was not associated with flowering
time (Fig. S3). As root elongation rate differs among genotypes and soil
types, our rhizotron system can ensure that the sampled apical / basal
zone was at the same soil residence period across different treatments.