Arabidopsis XTH4 and XTH9 Contribute to Wood Cell Expansion and Secondary Wall Formation

Abstract

Xyloglucan is the major hemicellulose of dicotyledon primary cell walls, affecting the load-bearing framework with the participation of xyloglucan endo-transglycosylase/hydrolases (XTHs). We used loss-and gain-of function approaches to study functions of XTH4 and XTH9 abundantly expressed in cambial regions during secondary growth of Arabidopsis (Arabidopsis thaliana). In secondarily thickened hypocotyls, these enzymes had positive effects on vessel element expansion and fiber intrusive growth. They also stimulated secondary wall thickening but reduced secondary xylem production. Cell wall analyses of inflorescence stems revealed changes in lignin, cellulose, and matrix sugar composition indicating an overall increase in secondary versus primary walls in mutants, indicative of higher xylem production compared with the wild type (since secondary walls were thinner). Intriguingly, the number of secondary cell wall layers compared with the wild type was increased in xth9 and reduced in xth4, whereas the double mutant xth4x9 displayed an intermediate number of layers. These changes correlated with specific Raman signals from the walls, indicating changes in lignin and cellulose. Secondary walls were affected also in the interfascicular fibers, where neither XTH4 nor XTH9 was expressed, indicating that these effects were indirect. Transcripts involved in secondary wall biosynthesis and cell wall integrity sensing, including THESEUS1 and WALL ASSOCIATED KINASE2, were highly induced in the mutants, indicating that deficiency in XTH4 and XTH9 triggers cell wall integrity signaling, which, we propose, stimulates xylem cell production and modulates secondary wall thickening. Prominent effects of XTH4 and XTH9 on secondary xylem support the hypothesis that altered xyloglucan affects wood properties both directly and via cell wall integrity sensing. cell wall composition, mutant growth, and gene expression and wrote the article; A.B. analyzed hypocotyl phenotypes and xylem cell morphology in mutants, discovered secondary wall phenotypes, and carried out immunolocalization; N.N. isolated and purified mutants and created OE plants; M.D.-M. and M.M. analyzed cell walls by in situ Raman and in situ FTIR; S.E. created GUS reporter lines; V.K. analyzed Populus XTH genes and their expression in Asp-Wood; L.G. and S.M.-M. carried out saccharification analyses; A.G. interpreted FTIR and Raman data and helped with spectroscopic analyses; J.B. analyzed XET activity; B.S. and E.J.M. conceived and coordinated the project; E.J.M. finalized the article with contribution from all authors.[OPEN]Articles can be viewed without a subscription. www.plantphysiol.org/cgi/

Genome-wide identification and expression analysis of the xyloglucan endotransglucosylase/hydrolase gene family in poplar

“…For example, Arabidopsis XTH19 and XTH23 (a homologous gene of PtrXTH19 ) were regulated by BES1 to participate in the development of lateral roots through brassinosteroid signaling pathway, contributing to salt adaptation of lateral roots [ 39 ]. The deficiency of XTH9 (a homologous gene of PtrXTH40 ) in Arabidopsis regulated the secondary wall thickening by triggering integrity signal of cell wall and stimulating the production of xylem cells [ 40 ].…”

High-order mutants reveal an essential requirement for peroxidases but not laccases in Casparian strip lignification

“…Indeed, endodermal lignification occurs in the cell walls between two endodermal cells that amount to a total thickness of barely 200 nm ( 3 , 31 ), very different in dimension from lignified secondary walls, which in the case of interfascicular fibers can be 10-fold thicker (ca. 2 µm) ( 50 , 51 ). Beyond this, primary and secondary walls also differ in composition and abundance of their cellulose, hemicellulose, and pectin components ( 52 , 53 ).…”

The Role of Mechanoperception in Plant Cell Wall Integrity Maintenance

“…The next steps will be to determine to what extent CWI maintenance exists in both crop and non-model plants and investigate CWI maintenance processes in secondary cell walls [4]. Our knowledge is very limited, but it suggests that interesting research opportunities exist and will feed scientific discussion in the future [127,128]. While research into food and energy crop plants has the obvious attractions of potentially leading to results with translational potential, investigating plants surviving in extreme environments may yield results with more pronounced impact in the long term [129].…”