Authors: Derek G Gray
Publish Date: 2014/08/03
Volume: 21, Issue: 5, Pages: 3181-3191
Abstract
Leaf stalks petioles are critical components of the vascular system that conducts water from the roots to the photosynthetic apparatus of most green plants Helical coiled cellulosic microfibrils that reinforce the tracheary elements in plant leaf petioles were isolated by a gentle treatment with alkali and acid chlorite from celery and from a number of tree species including sugar maple London plane horse chestnut tulip tree paulownia and ginko Analysis of the hydrolysate of celery coils gave glucose as the main product but significant quantities of xylose and other sugars were also detected Polarized light microscopy was used to determine the location dimensions and handedness of the coils The coils were made up of single or multiple parallel strands in contact ranging up to 35 μm in diameter and several cm in length The strand structure was chiral significantly only lefthanded helices were observed An attempt is made to rationalize the lefthanded structure by comparison with the spontaneous handedness observed in vitro for cellulose nanocrystals suspensions The ubiquitous presence of coiled thickenings in petiole vascular elements indicates their importance in plant functionsPreliminary reports described the isolation of long tightlycoiled cellulosic filaments from the tracheary elements of celery apium graveolens L Ulkem and Gray 2006 and some tree species Gray and Lucate 2009 Spiral thickening of the xylem secondary wall is of course wellknown but we were interested to note that the helical coils all appeared to be lefthanded This behaviour contrasts with that of the coils produced by in vitro electrospinning of cellulose derivatives where both left and righthanded coils were observed Godinho et al 2009 Manifestations of the molecular chirality of cellulosic materials have been reported at the levels of individual cellulosederived polymers Werbowyj and Gray 1976 Harkness and Gray 1994 cellulose nanocrystal suspensions Revol et al 1992 thin wood sections Gray and Kam 1997 and paper Gray 1989 The chiral twisting of wood and paper was attributed to the righthanded twist of the cellulose microfibrils observed in the S2 layer of virtually all wood cells Meylan and Butterfield 1978a bThe helical orientation of the cellulose microfibrils is thought to be controlled during deposition by the orientation of cortical microtubules Abe et al 1995 and the helical winding and fibril angle are important in determining the mechanical properties of wood Barnett and Bonham 2004 and wood pulp fibres Page and ElHosseiny 1983 The helical secondary wall thickenings often observed in waterconducting cells or tracheary elements in the primary xylem of nonwoody plants allow for cell extension during elongation and growth of the surrounding tissue during plant development and may strengthen the cell walls against collapse due to the negative pressure in the plant vascular system In general the orientation of cellulose microfibrils in the cell wall governs many plant functions Burgert and Fratzl 2009 Again there is much evidence that the major factor in controlling microfibril orientation during cell growth is the orientation of the cortical microtubules but some other factor must also be involved Baskin 2001Whatever the mechanism or mechanisms that result in the orientation of cellulose microfibrils it is evident that the arrangement of microfibrils in plant cell walls is often chiral A chiral physical interaction between microfibrillar elements of cellulose has also been demonstrated in vitro Revol et al 1992 and this raises the general question whether and under what circumstances this chiral interaction might also influence the processes that are involved in cellulose deposition In this paper we describe the isolation of cellulosic coils from tracheary elements of leaf petioles the methods we used to determine coil handedness and we comment on the possible significance of the observationsa Sketch of celery petiole crosssection b Sketch of vascular bundle crosssection Ph phloem Xy xylem Te tracheary elements c Sketch of coiled secondary wall thickening of tracheary element d Celery petiole showing orientation of vascular bundles The coils are aligned with their long axis parallel to the long axis of the petioleLeaves were collected in summer from trees on the campus of McGill University Montreal or in the vicinity thereof The leaf petioles were cut into ~1 cm lengths and as much as possible of the petiole epidermis was removed with a razor blade and discarded The remains were sliced lengthwise Cellulose was isolated from the petiole slices by a sequence of chemical treatments with rinsing in water between each step First the samples were boiled for 1 h in distilled water then in aqueous sodium dodecyl sulfate 15–2 for 1 h This was followed by a basic treatment in 5 NaOH for 1 h at room temperature or in 35 aq NaOH at 85–100 °C for 45–55 min depending on the sample Finally after thorough rinsing in water until neutrality the fibrous mass was bleached in a 8–10 soln of sodium chlorite NaClO2 in 04 M acetic acid for 3 h at ambient temperature For some samples it was necessary to repeat the bleaching step After rinsing the petiole structure was then partially disrupted by a brief ultrasonic treatment in water
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