Abstract
In transverse orientation, maize (Zea mays) roots are composed of a central stele that is embedded in multiple layers of cortical parenchyma. The stele functions in the transport of water, nutrients, and photosynthates, while the cortical parenchyma fulfills metabolic functions that are not very well characterized. To better understand the molecular functions of these root tissues, protein- and phytohormone-profiling experiments were conducted. Two-dimensional gel electrophoresis combined with electrospray ionization tandem mass spectrometry identified 59 proteins that were preferentially accumulated in the cortical parenchyma and 11 stele-specific proteins. Hormone profiling revealed preferential accumulation of indole acetic acid and its conjugate indole acetic acid-aspartate in the stele and predominant localization of the cytokinin cis-zeatin, its precursor cis-zeatin riboside, and its conjugate cis-zeatin O-glucoside in the cortical parenchyma. A root-specific beta-glucosidase that functions in the hydrolysis of cis-zeatin O-glucoside was preferentially accumulated in the cortical parenchyma. Similarly, four enzymes involved in ammonium assimilation that are regulated by cytokinin were preferentially accumulated in the cortical parenchyma. The antagonistic distribution of auxin and cytokinin in the stele and cortical parenchyma, together with the cortical parenchyma-specific accumulation of cytokinin-regulated proteins, suggest a molecular framework that specifies the function of these root tissues that also play a role in the formation of lateral roots from pericycle and endodermis cells.
Highlights
In transverse orientation, maize (Zea mays) roots are composed of a central stele that is embedded in multiple layers of cortical parenchyma
The goal of this study was to test three hypotheses: first, that there is significant overlap and differential protein accumulation between cortical parenchyma and stele tissues in the differentiation zone of the maize primary root; second, that these differential proteins are related to the specific functions of these tissues in root development, which might be reflected in the overall distribution of functional categories or biochemical pathways; and third, that distinct functions of these tissue types are reflected by the distribution of plant hormones and their intermediates
Soluble proteins were isolated in three independent biological replicates from cortical parenchyma and stele tissues of the maize inbred line B73
Summary
Maize (Zea mays) roots are composed of a central stele that is embedded in multiple layers of cortical parenchyma. A major drawback of the analysis of whole roots or longitudinal developmental zones of roots is the composite organization of roots, which are made up of distinct radial tissue types, each providing specific gene expression and protein accumulation profiles. The goal of this study was to test three hypotheses: first, that there is significant overlap and differential protein accumulation between cortical parenchyma and stele tissues in the differentiation zone of the maize primary root; second, that these differential proteins are related to the specific functions of these tissues in root development, which might be reflected in the overall distribution of functional categories or biochemical pathways; and third, that distinct functions of these tissue types are reflected by the distribution of plant hormones and their intermediates
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