Cell wall modifications by α-XYLOSIDASE1 are required for control of seed and fruit size in Arabidopsis

Abstract

Cell wall modifications are of pivotal importance during plant development. Among cell wall components, xyloglucans are the major hemicellulose polysaccharide in primary cell walls of dicots and non graminaceous monocots. They can connect the cellulose microfibril surface to affect cell wall mechanical properties. Changes in xyloglucan structure are known to play an important role regulating cell growth. Therefore, the degradation of xyloglucan is an important modification that alters the cell wall. The α-XYLOSIDASE1 (XYL1) gene encodes the only α-xylosidase acting on xyloglucans in Arabidopsis thaliana. Here, we show that mutation of XYL1 strongly influences seed size, seed germination, and fruit elongation. We found that the expression of XYL1 is directly regulated in developing seeds and fruit by the MADS-box transcription factor SEEDSTICK (STK). We demonstrate that XYL1 complements the stk smaller seed phenotype. Finally, by Atomic Force Microscopy (AFM), we investigate the role of XYL1 activity in maintaining cell stiffness and growth, confirming the importance of cell wall modulation in shaping organs.

Xyloglucan remodelling enzymes and the mechanics of plant seed and fruit biology

“…Interestingly, while XyG-deficient A. thaliana xxt mutants exhibit only minor morphological phenotype changes, xyl1 mutants lacking α-xylosidase enzyme activity exhibit altered XyG side chains, free XyG oligosaccharide accumulation, and specific phenotypic defects during reproduction, seed dispersal, germination, and seedling growth. Di Marzo et al (2022) demonstrate that the expression of the XYL1 gene is directly regulated in developing seeds and fruits by the STK transcription factor.…”

“…Xyloglucan (XyG), the predominant hemicellulose, is composed of a β-1,4-glucan backbone that is consecutively substituted with α-1,6-linked xylosyl residues ( Frankova et al , 2013 ; Pauly et al , 2016 ). Di Marzo et al (2022) demonstrated that the MADS-box transcription factor SEEDSTICK (STK) specifically controls seed and fruit biology by α-xylosidase (XYL) mediated XyG remodelling.…”

“…Specific XyG remodelling by a battery of enzymes (A) has profound roles during reproduction, seed dispersal, and germination (B–E). The control of reproduction by the MADS-box transcription factor STK is achieved in part by αXYL-mediated cell wall remodelling (B) combined with other pathways which may differ between seed and fruit development (see cited references and figure 7 in Di Marzo et al , 2022 ). The control of silique growth (C) by STK, for example, requires XYL1 with a reduced silique size and increased valve cell wall stiffness in both the stk and the xyl1 mutant.…”

“…As for the xyl1 mutant, reduced silique elongation growth was also observed in the bgal10 mutant ( Sampedro et al , 2012 ); however, in contrast to the non-dormant xyl1 mutant seeds, the seeds of bgal10 mutants are dormant. The seeds of bgal6 ( mum2 ) ( Dean et al , 2007 ), stk ( Ezquer et al, 2016 ), and stk/xyl1 mutants are impaired in mucilage production (B), whereas xyl1 mutant seeds have wild-type (WT) phenotype and produce mucilage ( Di Marzo et al , 2022 ). As in the xyl1 mutant, increased cell wall stiffness (C) was also observed in developing seeds of the stk mutant ( Ezquer et al, 2016 ) and may lead to its smaller seed size as well as the defects in seed coat development in that stk , but not xyl1 , mutant seeds are impaired in mucilage production (B) and impaired seed abscission [D; from Balanza et al (2016) with permission ( https://doi.org/10.1242/dev.135202 )] required for seed dispersal ( Balanza et al , 2016 ).…”

“…Interestingly, the non-dormant xyl1 mutant seeds are thermoinhibition resistant (E) and have increased hypocotyl cell wall stiffness in creep-extension analysis ( Shigeyama et al , 2016 ). Altered XyG in cell walls and the accumulation of free XyG oligosaccharides (C, E) were associated with the altered fruit and seed phenotypes of the xyl1 ( Iglesias et al , 2006 ; Sampedro et al , 2010 ; Günl and Pauly, 2011 ; Sechet et al , 2016 ; Shigeyama et al , 2016 ; Di Marzo et al , 2022 ), bgal10 ( Sampedro et al , 2012 ), axy8 ( Günl et al , 2011 ), and bglc1 ( Sampedro et al , 2017 ) mutants. DAP, days after pollination.…”