Branched Pectic Galactan in Phloem-Sieve-Element Cell Walls: Implications for Cell Mechanics

Abstract

A major question in plant biology concerns the specification and functional differentiation of cell types. This is in the context of constraints imposed by networks of cell walls that both adhere cells and contribute to the form and function of developing organs. Here, we report the identification of a glycan epitope that is specific to phloem sieve element cell walls in several systems. A monoclonal antibody, designated LM26, binds to the cell wall of phloem sieve elements in stems of Arabidopsis (Arabidopsis thaliana), Miscanthus x giganteus, and notably sugar beet (Beta vulgaris) roots where phloem identification is an important factor for the study of phloem unloading of Suc. Using microarrays of synthetic oligosaccharides, the LM26 epitope has been identified as a b-1,6-galactosyl substitution of b-1,4-galactan requiring more than three backbone residues for optimized recognition. This branched galactan structure has previously been identified in garlic (Allium sativum) bulbs in which the LM26 epitope is widespread throughout most cell walls including those of phloem cells. Garlic bulb cell wall material has been used to confirm the association of the LM26 epitope with cell wall pectic rhamnogalacturonan-I polysaccharides. In the phloem tissues of grass stems, the LM26 epitope has a complementary pattern to that of the LM5 linear b-1,4-galactan epitope, which is detected only in companion cell walls. Mechanical probing of transverse sections of M. x giganteus stems and leaves by atomic force microscopy indicates that phloem sieve element cell walls have a lower indentation modulus (indicative of higher elasticity) than companion cell walls.

Differential metabolism of pectic galactan in tomato and strawberry fruit: detection of the LM26 branched galactan epitope in ripe strawberry fruit

“…As recently described by Torode et al , the LM26 branched galactan epitope is specifically detected in phloem sieve elements in several systems. In the case of the strawberry fruit the LM26 epitope was detected in all primary cell walls of ripe fruit albeit at a relatively low level but at an increased level relative to unripe fruit cell walls (Fig.…”

“…The identification and functional significance of these specific sets of vascular cells with the branched galactan epitope is far from clear. Previous studies have demonstrated both phloem sieve element specificity in certain systems and also the widespread distribution of the LM26 epitope in garlic bulb cell walls (Torode et al ). The occurrence of branched galactan has been implicated with a higher elasticity of cell walls (Mikshina et al , Torode et al ).…”

“…Previous studies have demonstrated both phloem sieve element specificity in certain systems and also the widespread distribution of the LM26 epitope in garlic bulb cell walls (Torode et al ). The occurrence of branched galactan has been implicated with a higher elasticity of cell walls (Mikshina et al , Torode et al ). The vascular tissue of strawberry fruits is under high positive turgor pressures especially during the green‐white developmental stage (Pomper and Breen ) and increased cell wall elasticity in particular cells may be required during fruit enlargement.…”

“…mAbs used in this study included LM5 binding to pectic 1,4-galactan (Andersen et al 2016), LM26 to branched pectic galactan (Torode et al 2018), INRA-RU1 to the RG-I backbone (Ralet et al 2010), LM6-M to pectic 1,5-arabinan (Cornuault et al 2017), LM13 to linear 1,5-arabinan (Verhertbruggen et al 2009a), JIM7, LM18, LM19 and LM20 to pectic HG with differential levels of esterification (Verhertbruggen et al 2009b) and LM25 to xyloglucan (Pedersen et al 2012).…”

Nanometrology of Biomass for Bioenergy: The Role of Atomic Force Microscopy and Spectroscopy in Plant Cell Characterization

“…AM-AFM has been successful in characterizing biomass (Kirby et al, 1996;Salvadori et al, 2014;Farahi et al, 2017) and in particular the architecture of plant cell walls at the molecular level which in turn is essential in improving lignocellulosic feedstock properties (Zhang et al, 2013b(Zhang et al, , 2017Keplinger et al, 2014;Torode et al, 2018). AM-AFM mapping of cellulose topography revealed the right-handed twisted nature of microfibrils and the periodic distribution of glucose and fiber unit along the microfibrils (Hanley et al, 1997).…”