Disruption of Secondary Wall Cellulose Biosynthesis Alters Cadmium Translocation and Tolerance in Rice Plants

Tricheary elements （TEs）, wrapped by secondary cell wall, play essential roles in water, mineral, and nutrient transduction. Cadmium （Cd） is a toxic heavy metal that is absorbed by roots and transported to shoot, leaves, and grains through vascular systems in plants. As rice is a major source of Cd intake, many efforts have been made to establish ＇low- Cd rice＇. However, no links have been found between cellulose biosynthesis and cadmium accumulation. We report here a rice brittle culm13 mutant, resulting from a novel missense mutation （G101K） in the N-terminus of cellulose synthase subunit 9 （CESA9）. Except for the abnormal mechanical strength, the mutant plants are morphologically indistinguishable from the wild-type plants. Transmission electron microscopy （TEM） and chemical analyses showed a slight reduction in secondary wall thickness and 22% decrease in cellulose content in bc13 plants. Moreover, this mutation unexpectedly confers the mutant plants Cd tolerance due to less Cd accumulation in leaves. Expression analysis of the genes required for Cd uptake and transport revealed complicated alterations after applying Cd to wild-type and bc13. The mutants were further found to have altered vascular structure. More importantly, Cd concentration in the xylem saps from the bc13 plants was significantly lower than that from the wild-type. Combining the analyses of CESA9 gene expression and Cd content retention in the cell-wall residues, we conclude that CESA9^G101K mutation alters cell-wall properties in the conducting tissues, which consequently affects Cd translocation efficiency that largely contributes to the low Cd accumulation in the mutant plants.

Identification of Quantitative Trait Loci Affecting Hemicellulose Characteristics Based on Cell Wall Composition in a Wild and Cultivated Rice Species

Cell wall hemicellulosic polysaccharides are structurally complex and diverse. Knowledge about the synthesis of cell wall hemicelluloses and their biological roles is limited. Quantitative trait loci （QTL） mapping is a helpful tool for the dissection of complex phenotypes for gene identification. In this study, we exploited the natural variation in cell wall monosaccharide levels between a common wild rice, Yuanj, and an elite indica cultivar, Teqing, and performed QTL map- ping with their introgression lines （ILs）. Chemical analyses conducted on the culms of Yuanj and TeqJng showed that the major alterations are found in glucose and xylose levels, which are correlated with specific hemicellulosic polymers. Gly- cosidic linkage examination revealed that, in Yuanj, an increase in glucose content results from a higher level of mixed linkage I^-glucan （MLG）, whereas a reduction in xylose content reflects a low level of xylan backbone and a varied arabi- noxylan （AX） structure. Seventeen QTLs for monosaccharides have been identified through composition analysis of the culm residues of 95 core ILs. Four major QTLs affecting xylose and glucose levels are responsible for 19 and 21% of the phenotypic variance, respectively. This study provides a unique resource for the genetic dissection of rice cell wall forma- tion and remodeling in the vegetative organs.

Galactosylation of rhamnogalacturonan-Ⅱ for cell wall pectin biosynthesis is critical for root apoplastic iron reallocation in Arabidopsis

Apoplastic iron(Fe)in roots represents an essential Fe storage pool.Reallocation of apoplastic Fe is of great importance to plants experiencing Fe deprivation,but how this reallocation process is regulated remains elusive,likely because of the highly complex cell wall structure and the limited knowledge about cell wall biosynthesis and modulation.Here,we present genetic and biochemical evidence to demonstrate that the Cdi-mediated galactosylation of rhamnogalacturonan-II(RG-II)is required for apoplastic Fe reallocation.Cdi is expressed in roots and up-regulated in response to Fe deficiency.It encodes a putative glycosyltransferase localized to the Golgi apparatus.Biochemical and mass spectrometry assays showed that Cdi catalyzes the transfer of GDP-L-galactose to the terminus of side chain A on RG-II.Disruption of Cdi essentially decreased RG-II dimerization and hence disrupted cell wall formation,as well as the reallocation of apoplastic Fe from roots to shoots.Further transcriptomic,Fourier transform infrared spectroscopy,and Fe desorption kinetic analyses coincidently suggested that Cdi mediates apoplastic Fe reallocation through extensive modulation of cell wall components and consequently the Fe adsorption capacity of the cell wall.Our study provides direct evidence demonstrating a link between cell wall biosynthesis and apoplastic Fe reallocation,thus indicating that the structure of the cell wall is important for efficient usage of the cell wall Fe pool.

Research on the underlying mechanism behind abrasive flow machining on micro-slit structures and simulation of viscoelastic media

In this study,the machining mechanism of abrasive flow machining(AFM)microstructures was analyzed in depth according to the transmission morphology and rheological behaviors of the abrasive media.The transmission morphology demonstrated the excellent combination of the polymer melt with abrasive grains at the interface,indicating that the polymer melt,combined with the uniform distribution of the polymer chains,could exert a harmonious axial force on the abrasive grains.Based on the rheological behavior analysis of the abrasive media,for example,the stress relaxation and moduli of storage and loss,a machining mechanism model was established incorporating the effect of microplastic deformation and continuous viscous flow,which was further verified by the grooves along the flow direction.In addition,the PhanThien-Tanner(PTT)model combined with a wall slipping model was employed to simulate the machining process for the first time here.The value of the simulated pressure(1.3 MPa)was similar to the measured pressure(1.45 MPa),as well as the simulated volumetric rate(0.0114 mL/s)to the measured volumetric rate(0.067 mL/s),which further proved the validity of the simulation results.The flow duration(21 s)derived from a velocity of 1.2 mm/s further confirmed the residual stretched state of the polymer chains,which favored the elasticity of the abrasive media on the grains.Meanwhile,the roughly uniform distribution of the shear rate at the main machining region exhibited the advantages of evenly spread storage and loss moduli,contributing to the even extension of indentation caused by the grains on the target surface,which agreed with the mechanism model and machined surface morphology.