Osmotic stress enhances suberization of apoplastic barriers in barley seminal roots: analysis of chemical, transcriptomic and physiological responses

Abstract

Barley (Hordeum vulgare) is more drought tolerant than other cereals, thus making it an excellent model for the study of the chemical, transcriptomic and physiological effects of water deficit. Roots are the first organ to sense soil water deficit. Therefore, we studied the response of barley seminal roots to different water potentials induced by polyethylene glycol (PEG) 8000. We investigated changes in anatomical parameters by histochemistry and microscopy, quantitative and qualitative changes in suberin composition by analytical chemistry, transcript changes by RNA-sequencing (RNA-Seq), and the radial water and solute movement of roots using a root pressure probe. In response to osmotic stress, genes in the suberin biosynthesis pathway were upregulated that correlated with increased suberin amounts in the endodermis and an overall reduction in hydraulic conductivity (Lp ). In parallel, transcriptomic data indicated no or only weak effects of osmotic stress on aquaporin expression. These results indicate that osmotic stress enhances cell wall suberization and markedly reduces Lp of the apoplastic pathway, whereas Lp of the cell-to-cell pathway is not altered. Thus, the sealed apoplast markedly reduces the uncontrolled backflow of water from the root to the medium, whilst keeping constant water flow through the highly regulated cell-to-cell path.

Seminal roots of wild and cultivated barley differentially respond to osmotic stress in gene expression, suberization, and hydraulic conductivity

“…The qualitative suberin composition in terms of substance classes and single detected suberin monomers was identical between cultivated and wild barley, and it also fits to published data of other barley cultivars such as Golf (Ranathunge et al, ) and Scarlett (Kreszies et al, ). This suggests that the root suberin monomer composition is genetically well conserved in barley even under stress conditions.…”

“…Comparing the RNA‐Seq data of the wild barley Pakistan with published RNA‐Seq data from the modern cultivar Scarlett (Kreszies et al, ) grown under exactly the same experimental conditions, at FDR ≤ 5% and|Log 2 FC|≥ 1, resulted in 5,749 and 5,730 unique upregulated genes and 5,092 and 6,889 unique downregulated genes in control and osmotic stress, respectively (Figure b,c, Table S4). The functional categorization and singular enrichment analysis of GO terms of the comparison between wild barley Pakistan and modern cultivar Scarlett showed 717 enriched GO terms.…”

“…So far it has been described that very different from other crops, for example, wheat, maize, and rice (Ranathunge, Lin, et al, ; Ranathunge, Schreiber, Bi, & Rothstein, ; Schreiber, Franke, Hartmann, Ranathunge, & Steudle, ), barley does not form an exodermis characterized by Casparian bands and suberin lamellae (Coffey et al, ; Gitto & Fricke, ; Jackson, ; Knipfer & Fricke, ; Kreszies et al, ; Ranathunge et al, ). Thus, it was an exciting observation that in response to osmotic stress, the wild barley accession Jordan formed an exodermis in the basal parts of about 20% of the investigated seminal roots (Figure ).…”

“…Lateral roots, which started to develop in the zone C, were removed with a razor blade. For gas chromatography, root segments were enzymatically digested, soluble lipids extracted and the remaining samples were transesterified using BF 3 ‐methanol as described earlier (Kolattukudy & Agrawal, ; Kreszies et al, ; Zeier & Schreiber, , ). Three independent biological replicates were used for each experiment.…”

“…The Bioconductor package limma (Smyth, ) in R (R Version 3.4.0, limma_3.32.2) was used to test the data quality via a multidimensional scaling plot. To assess differences in gene expression between osmotic stress treatment and control in each root tissue of the Pakistan wild barley and for the comparison between the Pakistan wild barley and the cultivar Scarlett (Raw Data from Kreszies et al, , SRA accession: SRP136092), linear models were fitted, including a fixed effect for tissue and treatment or tissue and genotype and an interaction effect for both terms. After shrinking the variances over all genes in the fitted model towards a common value by using an empirical Bayes approach (Smyth, ), pair‐wise comparisons between stress treatment and control treatment for each tissue and between the different genotypes were computed.…”