11B Solid-state NMR Investigation of the Rhamnogalacturonan II-borate Complex in Plant Cell Walls

Abstract

IntroductionBoron (B) is an essential trace element for vascular plants. 1,2 We have been studying the chemical form and function of B in plants. [3][4][5][6][7][8][9] Our in vivo solution state 11 B NMR study revealed that B in plant tissues exists in two forms, that is, water-soluble and water-insoluble, which is in vivo NMR observable and unobservable, respectively, and that the water-soluble B is a borate ester with low-molecular-weight diol compounds (1:1 and 1:2) as well as boric acid (Fig. 1). 3,4 Matoh's group reported that water-insoluble B is located in the cell walls 10 and that a dimeric rhamnogalacturonan II-borate (dRG-II-B) complex, in which two monomeric RG-IIs are cross-linked by borate-diol (1:2) esterification, was isolated and identified from enzymatic digests of radish cell walls. 11 Recently, we showed that RG-II is covalently linked to homogalacturonan, a major component of pectin in plant cell walls, 7 and that dRG-II-B formation in cell walls contributes to wall stabilization.8 From these studies, we have concluded that the primary function of B in plants is to covalently cross-link wall pectin by dRG-II-B formation, and thereby to contribute to the strength and integrity of cell walls. 2,8 Solution NMR studies have proven that enzymatically solubilized dRG-II-B from cell walls has the borate-diol (1:2) structure; 5,6,9,11,12 however, there is no direct evidence that water-insoluble and solid state B in the cell walls is present as a borate-diol ester (1:2) that cross-links pectin molecules. Thus, a nondestructive analysis of the chemical form of B in plant cell walls is needed to confirm our conclusions regarding the function of B in these walls.Solid-state NMR is an emerging analytical methodology for molecular structure, which is not limited by either insolubility or difficulty in crystallization, and makes it possible to address a variety of structural and dynamic questions at atomic-level resolution. 13,14 The 11 B nucleus has an electric quadrupole moment (I = 3/2), which frequently gives rise to a characteristic asymmetric NMR line shape and difficulty in observing the isotropic chemical shift. 15 However, in a tetrahedral B environment the local electric field gradient (EFG) is small. Moreover, the quadrupole interaction is inversely proportional to the applied magnetic field. Therefore, we would be able to observe the isotropic chemical shift of 11 B in a tetrahedral configuration by using high-magnetic field NMR with magic angle spinning (MAS). Consequently, to clarify the chemical form of B in plant cell walls, we performed 192-and 96-MHz 11 B MAS NMR measurements for solid state dRG-II-B and plant cell wall (sugar beet fiber). The boron in plant cell walls, which is water-insoluble and in the solid state, is solubilized by pectinase digestion to give a dimeric rhamnogalacturonan II-borate (dRG-II-B) complex. To clarify the nondestructive structure of boron present in plant cell walls (as represented by sugar beet fiber), we performed 192-and 96-MHz 11 B solid state NMR measur...

Natural and Synthetic Small Boron-Containing Molecules as Potential Inhibitors of Bacterial and Fungal Quorum Sensing

“…Boron alcohol complexes were discovered more than 30 years ago by Alan Darvill and co-workers, who described one of the most complex carbohydrates found in nature . Rhamnogalacturan II, or RG-II, is found in plant cell walls . The carbohydrate is found in all higher plants and requires a host of different proteins to manufacture …”

“…In the mutant, a little bit of the structure of the RGII has been changed, and because of the change in shape, it cannot hold the boron quite as well. Fertilizing mutants with high levels of boron also reversed dwarfing because the high amount of available boron effectively forced RG-II to cross-link. , …”

Borate Cross-Linked/Total Rhamnogalacturonan II Ratio in Cell Walls for the Biochemical Diagnosis of Boron Deficiency in Hydroponically Grown Pumpkin

“…8,11 Waterinsoluble B has been revealed to exist in cell walls as a dimeric rhamnogalacturonan II-borate (dRG-II-B) complex, in which a borate-apiose (1:2) ester cross-links two monomeric RG-IIs. [12][13][14][15][16] The pectic polysaccharide RG-II is covalently bound to homogalacturonan, which is a major component of pectin. 17 The borate cross-linked RG-II or dRG-II-B/total RG-II ratio in cell walls of B-sufficient and deficient plants is reported to be 0.8 -0.9 and 0 -0.2, respectively.…”