Arabidopsis thaliana as a model species for xylem hydraulics: does size matter?

Abstract

While Arabidopsis thaliana has been proposed as a model species for wood development, the potential of this tiny herb for studying xylem hydraulics remains unexplored and anticipated by scepticism. Inflorescence stems of A. thaliana were used to measure hydraulic conductivity and cavitation resistance, whereas light and electron microscopy allowed observations of vessels. In wild-type plants, measured and theoretical conductivity showed a significant correlation (R (2) = 0.80, P < 0.01). Moreover, scaling of vessel dimensions and intervessel pit structure of A. thaliana were consistent with structure-function relationships of woody plants. The reliability and resolution of the hydraulic methods applied to measure vulnerability to cavitation were addressed by comparing plants grown under different photoperiods or different mutant lines. Sigmoid vulnerability curves of A. thaliana indicated a pressure corresponding to 50% loss of hydraulic conductance (P 50) between -3 and -2.5MPa for short-day and long-day plants, respectively. Polygalacturonase mutants showed a higher P 50 value (-2.25MPa), suggesting a role for pectins in vulnerability to cavitation. The application of A. thaliana as a model species for xylem hydraulics provides exciting possibilities for (1) exploring the molecular basis of xylem anatomical features and (2) understanding genetic mechanisms behind xylem functional traits such as cavitation resistance. Compared to perennial woody species, however, the lesser amount of xylem in A. thaliana has its limitations.

Keywords: Arabidopsis thaliana; bordered pit; cavitation resistance; hydraulic conductivity; inflorescence stem; xylem..

Fig. 1.

Anatomical features of the xylem in the inflorescence stem of A. thaliana. (A) Transverse sections showing xylem organization in vascular bundles: the closed arrow shows a vascular bundle with secondary xylem dominance and the open arrow shows a vascular bundle with metaxylem dominance. (B) Detail of a vascular bundle with metaxylem and vessels marked by asterisks: toluidine blue stains vessels in blue and fibres and parenchyma cells in purple. (C) Transverse section of a vascular bundle from a stem injected with silicone: conductive vessels are filled with silicone mixed with the fluorescent dye Uvitex. (D) Transverse section of xylem vessels observed by TEM: the closed arrow shows an intervessel pit and the open arrow shows a vessel-fibre pit. (E) Intervessel pit observed with TEM: arrows show vestures. (F–H) SEM images of inner vessel wall with four vestured pit apertures (F), pit membrane (G), and pit chamber bearing vestures near the outer aperture (H): closed arrows show vestures and the open arrow shows a small part of the aspirated pit membrane. F, fibre; IPA, inner pit aperture; PB, pit border; Phl, phloem; Proxyl, protoxylem; V, vessel. Bars, 100 μm (A), 10 μm (B), 50 μm (C), 1 μm (D–H).

Fig. 2.

Vessel diameter and vessel length distribution for inflorescence stem of A. thaliana. (A) Vessel diameter distribution analysed from eight plants. (B) Vessel length distribution analysed from three plants. Data are means ± SE. Solid line indicates the average frequency of vessel length (Sperry et al., 2005).

Fig. 3.

Hydraulic conductivity of the inflorescence stem of A. thaliana. (A) Plot of the experimental (K _hs) versus theoretical (K _hts) hydraulic specific conductivity. K _hs was measured on 1-cm-long basal inflorescence stems from eight plants, then K _hts was calculated by measuring vessel diameters and applying the Hagen–Poiseuille law. Values are for each plant. The black line represents a linear correlation between K _hts and K _hs (K _hts = 1.0043K _hs + 0.0048; R ² = 0.8044) and the grey line represents the 1:1 line. (B) Hydraulic conductivity (K _h) versus segment length. K _h was measured on 5-cm-long basal inflorescence stems that were perfused basipetally and shortened every 0.5cm. Data are means ± SE from six plants. There was no significant difference between means (ANOVA, P < 0.05).

Fig. 4.

Xylem vulnerability to cavitation of A. thaliana plants depending of the photoperiod or the genetic background. (A) Vulnerability curves of WT Col-0 plants grown under short days (SD, open circles) show a significant difference (Student’s t-test, P < 0.05) with that of plants cultivated under long days (LD, closed circles). (B–D) Vulnerability curves of WT plants and mutants grown under long days. Vulnerability curves of WT (closed circles) show a significant difference (Student’s t-test, P < 0.05) with PG mutants (open circles) (B), but no significant differences were found with PME3 (C) and PME5 (D) mutants (open circles). Values are means ± SE from 2–8 stem samples, and 15–24 plants were required for each curve. Grey lines represent regression curves according to Pammenter and Vander Willigen (1998). WT, wild type.