Abstract
Key messageLignification and suberization of Casparian strips occurred simultaneously during the development of Cunninghamia lanceolata primary roots.The Casparian strips in the cell walls of vascular plant root endodermis and exodermis cells are vital for the exclusion of salts and pathogens, selective nutrient uptake, and other processes related to the transport and assimilates of water. Despite the importance of Casparian strips, their chemical composition and its relevance to their diffusion barrier functions remain a matter of debate, especially in woody plants. Here, we describe the cytological features revealed by fluorescent staining of developing Casparian strips in the primary root endodermis of Chinese fir (Cunninghamia lanceolata), one of the most important timber species native to China. Using apoplastic permeability tests, we revealed that the Casparian strips provide their barrier function from the secondary stage of endodermis development, after lignin and suberin are deposited onto the surface of the protoplast next to tangential cell walls. Furthermore, we enzymatically isolated Casparian strips from the primary root endodermis and analyzed their chemical composition using Fourier transform infrared spectroscopy, revealing that they consist largely of lignin and suberin. Using confocal Raman microscopy for label-free, in situ, structural and biochemical analysis, we showed that lignin and suberin accumulated simultaneously in the developing Casparian strips. Taken together, these results elucidated Casparian strip development and characterized the lignification and suberization processes in situ in the primary roots of Chinese fir. Since this species shows broad adaptability to varying environmental conditions, our findings may facilitate work to understand the barrier function of Casparian strips in relationship to environmental stresses.
Highlights
Plant roots grow in direct contact with the soil to facilitate their uptake of water and necessary solutes, but must simultaneously avoid the uptake of unwanted or toxic solutes
The Casparian strips in the walls of root endodermis and exodermis cells are comprised of lignin and suberin in most species; the development of Casparian strips can be strikingly different between species (Barnabas and Peterson 1992; Enstone and Peterson 1997; Zeier et al 1999a)
We used berberine-aniline blue staining and fluorescence microscopy to dissect the development of the endodermal Casparian strips in the Chinese fir primary root, revealing that they initially formed in the cell walls 0.5 cm from the root tip but became thicker and larger in the cells approximately 1–4 cm from the root tip
Summary
Plant roots grow in direct contact with the soil to facilitate their uptake of water and necessary solutes, but must simultaneously avoid the uptake of unwanted or toxic solutes. Casparian strips were first described by Robert Caspary (1865) as a single layer that very closely approximated cells, and were first isolated by Priestley and North (1922). They were more comprehensively illustrated by Schreiber et al (1994), and have since been described in a wide range of plants. The function of Casparian strips as apoplastic transport barriers in roots has been established in many different plant species
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