Is callose a barrier for lead ions entering Lemna minor L. root cells?

Abstract

Plants have developed a range of strategies for resisting environmental stresses. One of the most common is the synthesis and deposition of callose, which functions as a barrier against stress factor penetration. The aim of our study was to examine whether callose forms an efficient barrier against Pb penetration in the roots of Lemna minor L. exposed to this metal. The obtained results showed that Pb induced callose synthesis in L. minor roots, but it was not deposited regularly in all tissues and cells. Callose occurred mainly in the protoderm and in the centre of the root tip (procambial central cylinder). Moreover, continuous callose bands, which could form an efficient barrier for Pb penetration, were formed only in the newly formed and anticlinal cell walls (CWs); while in other CWs, callose formed only small clusters or incomplete bands. Such an arrangement of callose within root CWs inefficiently protected the protoplast from Pb penetration. As a result, Pb was commonly present inside the root cells. In the light of the results, the barrier role of callose against metal ion penetration appears to be less obvious than previously believed. It was indicated that induction of callose synthesis is not enough for a successful blockade of the stress factor penetration. Furthermore, it would appear that the pattern of callose distribution has an important role in this defence strategy.

Fig. 1

Occurrence and distribution of lead (Pb) and callose in Lemna minor L. cells of the root tip; CW cell wall; a, b detection of callose by aniline blue staining (green colour) and autofluorescence of chlorophyll (red colour). Root cells of L. minor contain chloroplasts in the root (like other water plants), even in the root tip. The occurrence of chloroplasts and the absence of proplastids in meristematic cells—unusual for vascular plants (Landolt 1986). a control cells containing callose only in the CW of the root cap, b callose in the protoderm (green arrow) and in the procambial central cylinder (green arrowhead) in lead-treated plant. c–j Detection of callose and Pb by TEM in plant exposed to lead, callose indicated by immunogold (black particles), Pb visible as the electron-dense, greyish deposits, c, f callose and Pb detected by the X-ray mapping microanalysis (callose-green cross, Pb-red cross), c,d callose which forms continuous bands localized in newly formed CW in its central part (green arrow and green cross) and near the border between the newly formed CW and the mother CW (green arrowhead). Pb deposits in CW (red arrow and red cross), e callose visible in the outer tangential CW of the protoderm (green arrow) and f in the radial CW (green arrowhead) as small clusters or incomplete band. Pb deposits localized mostly in the CW (red arrowhead), much lower amount of Pb detected in the protoplast than in the CW (red arrow), g callose which forms a continuous band in anticlinal CW of root tip (green arrow)—Pb not visible in this case, h–j localization and distribution of callose and Pb in radial CW. Callose indicated in the area of plasmodesmata (green arrowhead) and in the CW thickening (green arrow), Pb localized mostly in the CW, much lower amount of Pb in the protoplast than in the CW (red arrow), a, b Bars 25 μm, c–j Bars 200 nm