Abstract
Oryza sativa, a model plant for Arbuscular Mycorrhizal (AM) symbiosis, has both host and non-host roots. Large lateral (LLR) and fine lateral (FLR) roots display opposite responses: LLR support AM colonization, but FLR do not. Our research aimed to study the molecular, morphological and physiological aspects related to the non-host behavior of FLR. RNA-seq analysis revealed that LLR and FLR displayed divergent expression profiles, including changes in many metabolic pathways. Compared with LLR, FLR showed down-regulation of genes instrumental for AM establishment and gibberellin signaling, and a higher expression of nutrient transporters. Consistent with the transcriptomic data, FLR had higher phosphorus content. Light and electron microscopy demonstrated that, surprisingly, in the Selenio cultivar, FLR have a two-layered cortex, which is theoretically compatible with AM colonization. According to RNA-seq, a gibberellin inhibitor treatment increased anticlinal divisions leading to a higher number of cortex cells in FLR. We propose that some of the differentially regulated genes that lead to the anatomical and physiological properties of the two root types also function as genetic factors regulating fungal colonization. The rice root apparatus offers a unique tool to study AM symbiosis, allowing direct comparisons of host and non-host roots in the same individual plant.
Highlights
One of the most important biological novelties that evolved in plant colonization of land was the root apparatus, an organ specialized to anchor the plant body, and to absorb and store water and nutrients
Through mRNA-seq, we compared Large lateral (LLR) vs. fine lateral (FLR) in control and mycorrhizal conditions and we focused our attention on candidate transcripts that may: (i) define the differences in anatomy and different roles of LLR and FLR, and (ii) make FLR not susceptible to fungal colonization
RNA was isolated from LLR and FLR of rice plants, grown in the absence (LLRc and FLRc) or the presence (LLRmyc and FLRmyc) of the mycorrhizal fungus R. irregularis
Summary
One of the most important biological novelties that evolved in plant colonization of land was the root apparatus, an organ specialized to anchor the plant body, and to absorb and store water and nutrients. Current knowledge indicates that the ancient alliance between non-rooted plants and symbiotic fungi, such as Glomeromycota and Mucoromycotina, promoted this morphological innovation and has played a key role in the origin of land flora (Brundrett, 2002; Bonfante and Genre, 2008; Gutjahr and Paszkowski, 2013 and citations therein). This ancient alliance continues with most modern plants, as approximately 80% of vascular plant species establish arbuscular mycorrhizal (AM) symbiosis with fungi of the Glomeromycota (Redecker et al, 2013). AM fungi preferentially colonize lateral roots and rarely colonize taproots (eudicotyledons) or crown roots (monocotyledons) (Hooker et al, 1992; Gutjahr et al, 2009)
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