Rhamnogalacturonan I with β-(1,4)-Galactan Side Chains as an Ever-Present Component of Tertiary Cell Wall of Plant Fibers

Abstract

The cellulose-enriched tertiary cell walls present in many plant fibers have specific composition, architecture, machinery of formation, and function. To better understand the mechanisms underlying their mode of action and to reveal the peculiarities of fibers from different plant species, it is necessary to more deeply characterize the major components. Next to overwhelming cellulose, rhamnogalacturonan I (RG-I) is considered to be the key polymer of the tertiary cell wall; however, it has been isolated and biochemically characterized in very few plant species. Here, we add RG-I to the list from the phloem fibers of the Phaseolus vulgaris stem that was isolated and analyzed by nuclear magnetic resonance (NMR), dynamic light scattering, and immunolabeling, both within tissue and as an isolated polymer. Additionally, fibers with tertiary cell walls from nine species of dicotyledonous plants from the orders Malphigiales, Fabales, and Rosales were labeled with RG-I-related antibodies to check the presence of the polymer and compare the in situ presentation of its backbone and side chains. The obtained results confirm that RG-I is an obligatory polymer of the tertiary cell wall. However, there are differences in the structure of this polymer from various plant sources, and these peculiarities may be taxonomically related.

Gelatinous fibers develop asymmetrically for posture support of bends and coils in common bean vine

“…To further investigate these varied results, we stained sections with 2% Toluidine Blue, and found that only the outer S-layers stained blue, while the G-layer was clear ( Fig. 3B,C ), indicating the G-layer is devoid of lignin, corroborating negative results with Phloroglucinol-HCl from (Chernova et al ., 2023). Lastly, analyzing unstained sections under UV fluorescence revealed signal in the outer S-layers and the innermost G-layer (the “Gn-layer” (Roach et al ., 2011; Goudenhooft, Bourmaud and Baley, 2019), but not the main body of the G-layer ( Fig.…”

“…Many vines have highly modified stem anatomies, including wholesale reorganization of the vascular tissues (Angyalossy, Pace and Lima, 2014; Cunha Neto, 2023), but common bean stems have a typical eustele and regular secondary growth. Therefore, the only clear anatomical candidate related to climbing in this species are the G-fibers; their presence has been previously reported (Chery et al 2021; Chernova et al, 2023). In young internodes, G-fibers first appear via differentiation of existing pericyclic fibers ( Fig.…”

“…Therefore, the only clear anatomical candidate related to climbing in this species are the G-fibers; their presence has been previously reported (Chery et al 2021;Chernova et al, 2023). In young internodes, G-fibers first appear via differentiation of existing pericyclic fibers (Fig.…”

“…2A). Our anatomical interpretation differs from (Chernova et al, 2023) in one aspect -the innermost layer of the cortex has starch grains and can be interpreted as endodermis in bean, thus the cells just interior to this endodermis are the pericycle (Cattai and Menezes, 2010;Tamaio et al, 2009). We adopt the term "pericyclic fibers", instead of "primary phloem fibers" as in .…”