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

Sunita Kushwah,Mateusz Majda,Andras Gorzsas,Satoshi Endo,Simon Mcqueen-Mason,Björn Sundberg,Ewa J Mellerowicz,Alicja Banasiak,Vikash Kumar,Janet Braam,Nobuyuki Nishikubo,Marta Derba-Maceluch,Leonardo Gomez

doi:10.1104/pp.19.01529

Sunita Kushwah, Mateusz Majda + Show 11 more

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https://doi.org/10.1104/pp.19.01529

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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.

Highlights

Xyloglucan is the major hemicellulose of dicotyledon primary cell walls, affecting the load-bearing framework with the participation of xyloglucan endo-transglycosylase/hydrolases (XTHs)
Arabidopsis AtXTH4 and AtXTH9 genes known to be highly expressed in stems and seedlings (Yokoyama and Nishitani, 2001), similar to PtXTH34 and PtXTH35, respectively, were selected (Fig. 1A) for functional studies during secondary growth
Promoters of AtXTH4 and AtXTH9 were active in developing secondary vascular tissues in secondarily thickened hypocotyls and basal stems, where secondary growth occurs

Summary

Introduction

Xyloglucan is the major hemicellulose of dicotyledon primary cell walls, affecting the load-bearing framework with the participation of xyloglucan endo-transglycosylase/hydrolases (XTHs). These enzymes had positive effects on vessel element expansion and fiber intrusive growth They stimulated secondary wall thickening but reduced secondary xylem production. The number of secondary cell wall layers compared with the wild type was increased in xth and reduced in xth, 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. The plant cell wall is composed of cellulose microfibrils embedded in a matrix of hemicelluloses and pectins, structural glycoproteins, and, in some cell types, lignin.

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