Dynamics of intracellular mannan and cell wall folding in the drought responses of succulent Aloe species

Abstract

Plants have evolved a multitude of adaptations to survive extreme conditions. Succulent plants have the capacity to tolerate periodically dry environments, due to their ability to retain water in a specialized tissue, termed hydrenchyma. Cell wall polysaccharides are important components of water storage in hydrenchyma cells. However, the role of the cell wall and its polysaccharide composition in relation to drought resistance of succulent plants are unknown. We investigate the drought response of leaf-succulent Aloe (Asphodelaceae) species using a combination of histological microscopy, quantification of water content, and comprehensive microarray polymer profiling. We observed a previously unreported mode of polysaccharide and cell wall structural dynamics triggered by water shortage. Microscopical analysis of the hydrenchyma cell walls revealed highly regular folding patterns indicative of predetermined cell wall mechanics in the remobilization of stored water and the possible role of homogalacturonan in this process. The in situ distribution of mannans in distinct intracellular compartments during drought, for storage, and apparent upregulation of pectins, imparting flexibility to the cell wall, facilitate elaborate cell wall folding during drought stress. We conclude that cell wall polysaccharide composition plays an important role in water storage and drought response in Aloe.

Keywords: Aloe; CoMPP; adaptation; drought; hydrenchyma; leaf anatomy; morphology; plant cell walls; polysaccharides; succulence.

Figure 1

Effect of drought on two Aloe species. (table) The calculated relative and saturated water content for Aloe helenae and Aloe vera and measured before (control) and after 6 weeks of drought. A. helenae showed visible signs of drought stress. Pictures (a) and (b) show A. helenae (a) before and (b) after the drought period

Figure 2

Comparative morphology and anatomy of Aloe helenae (a, b, d, e, and f), Aloe vera (a, c, g, h, and i), and Aloe somaliensis (j and k). Transverse sections (marked by dashed squares) of hydrenchyma tissue stained with toluidine blue showing typical leaf tissue arrangement in Aloe species with hydrenchyma, (h) surrounded by an outer photosynthetic chlorenchyma, (c) epidermis and cuticle (d, e, g and h). Convoluted folding of hydrenchyma cell walls in drought‐stressed plants (f and i) indicated by arrows. Aloe somaliensis (j and k) from the Kew slide collection also showing folds in the hydrenchyma cell walls

Figure 3

Heatmap representation of hydrenchyma polysaccharides of drought stressed, watered, and control treatments of Aloe helenae and Aloe vera: (a) all data from each extraction; (b) summed data from comprehensive microarray polymer profiling. Sums were made using the measured quantifications and are not the sums of the calculated data shown in (a). XG, xyloglucan; Ab, antibody; R, rat; M, mouse; RGI, rhamnogalacturonan‐I. Data not shown here are available in the Supporting information

Figure 4

In situ detection of homogalacturonan in sections of Aloe vera leaves using monoclonal antibodies JIM7 (binding highly methylated homogalacturonan), JIM5 (binding homogalacturonan with a low degree of esterification), and COS488 (binding highly deesterified homogalacturonan); images overlayed with Calcofluor White signal (blue) highlighting the cell walls. Micrographs showing (a–c) watered and (d–f) drought‐stressed specimens show remarkable changes in cell wall polysaccharide composition and folding. Detail is shown in (g–j) high magnification micrographs. Arrowheads indicate (g and h) middle lamella and (i) intracellular accumulation

Figure 5

In situ detection of mannan epitopes in Aloe vera hydrenchyma using three monoclonal antibody probes. (a) Colocalization of Calcofluor White (blue signal), LM21 (red signal) recognizing β‐(1,4)‐mannan/galactomannan/glucomannan, and CCRCM‐170 (green signal) recognizing acetylated mannan; (b) colocalization of Calcofluor White (blue signal), LM21 (red signal), and BS‐400‐4 recognizing β‐(1,4)‐galactomannan (green signal); cell wall is marked with arrowheads

Figure 6

Detection of mannan using three different antimannan monoclonal antibodies on sections of Aloe helenae (a, b) and Aloe vera (c, d) hydrenchyma in (a, c) well‐watered, and (b,\c) drought‐stressed samples. LM21 binds (1,4)‐β‐D‐mannan/galactomannan/glucomannan, CCRCM170 binds acetylated mannan, and BS‐400‐4 binds (1,4)‐β‐D‐mannan. Calcofluor White (staining β‐(1,4) glucans) is used on all sections. The cell wall signal is marked by closed arrowhead and the intracellular signal with open arrowheads

Figure 7

Changes in cell wall‐associated mannan in watered and droughtstressed leaf hydrenchyma tissue of Aloe helenae. The sections are probed with LM21, CCRC‐M170, and BS‐400‐4 in (a) watered and (b) drought‐stressed specimens. Note the disappearance of the CCRCM‐170 and partial disappearance of LM21 in hydrenchyma of drought‐stressed specimen