Pollen Tip Growth by Gerhard Obermeyer & José Feijó

Author:Gerhard Obermeyer & José Feijó
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

9.6.2 Xyloglucan

Xyloglucan (XyG), the most abundant form of hemicellulose in dicots, interacts with cellulose microfibrils forming the load-bearing cellulose-xyloglucan network (Carpita and Gibeaut 1993). As it tethers adjacent cellulose microfibrils (Cosgrove 2005), it is expected that alterations in XyG might result in changes in mechanical properties of the cell walls and hence potential growth defects. Pena et al. (2012) reported that a unique acidic XyG is exclusively present in the Arabidopsis RH cell wall and that a loss-of-function mutation in ROOT HAIR SPECIFIC8/XYLOGLUCAN-SPECIFIC GALACTURONOSYLTRANSFERASE1 (RHS8/XUT1), catalysing the synthesis of this galacturonic acid-containing XyG, causes a short RH phenotype, indicating the importance of this form of XyG in polarized expansion. Furthermore, Cavalier et al. (2008) have shown that double mutants in xylosyltransferases (XXTs), xxt1/xxt2, that catalyse the addition of xylosyl to the glucan backbone of XyG lack detectable amounts of XyG and produce short RHs with swollen bases. The single mutant xxt5 and the triple mutant xxt1 xxt2 xxt5 have short and swollen RHs as well (Zabotina et al. 2008, 2012), but do not show rupturing tips. Selective degradation of XyG in growing RHs using xyloglucanase leads to the same conclusion that XyG absence results in reduced RH elongation, but not to rupture as was the case when cellulose was targeted (Park et al. 2011).

Besides altered synthesis, modifications of XyG can also affect cell wall mechanical properties. Xyloglucan endotransglucosylases/hydrolases (XTHs) are cell wall enzymes that cut XyG chains and reform bonds with water (hydrolase activity, XEH) and other xyloglucan acceptor substrates (endotransglycosylase activity, XET), potentially altering the distance between adjacent cellulose microfibrils and hence loosening cell walls (Nishitani and Vissenberg 2007; Van Sandt et al. 2007). High XET activity was indeed found in elongating epidermal cells and RHs at all stages of growth (Vissenberg et al. 2000, 2001, 2003), yet xth mutants with a detectable RH phenotype are still absent. Surprisingly, addition of recombinant XTH proteins leads to RH swelling, reduction and even cessation of RH growth (Maris et al. 2009), although one should take care since the XTH proteins are experimentally added from the outside of the wall, while under normal circumstances, they are secreted at the inside and younger part of the cell wall. Expansins are another class of proteins that interact with XyG and break their hydrogen bonds with cellulose at more acidic pH, thereby weakening the wall (Cosgrove 2005). A reduction of expansin A7 (EXPA7) expression resulted in shorter RHs (Lin et al. 2011a), confirming that expansin A7 likely performs active cell wall modification activities during the elongation of the hair tip.

In PTs, fucosylated XyG epitopes were uniform all along the cell wall of straight growing PT, with a slightly lower abundance at the tip compared with the distal region (Chebli et al. 2012). At the moment, no effects of XyG absence are described on PT and pollen development.

Taken together, XyG plays a role in the growth of RHs, but its absence does not lead to cell rupture, suggesting that cellulose is not only connected to XyG. Indeed, Dick-Pérez et al.

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