Tissue and cellular localization of condensed tannins in poplar roots and potential association with nitrogen uptake

Abstract

Condensed tannins are common in vegetative tissues of woody plants, including in roots. In hybrid poplar (Populus tremula x alba; also known as P. x canescens) CT assays indicated they were most concentrated in younger white roots and at the root tip. Furthermore, CT-specific staining of embedded tissue sections demonstrated accumulation in root cap cells and adjacent epidermal cells, as well as a more sporadic presence in cortex cells. In older, brown roots as well as roots with secondary growth (cork zone), CT concentration was significantly lower. The insoluble fraction of CTs was greatest in the cork zone. To determine if CT accumulation correlates with nutrient uptake in poplar roots, a microelectrode ion flux measurement (MIFE™) system was used to measure flux along the root axis. Greatest NH₄ ⁺ uptake was measured near the root tip, but NO_3^- and Ca²⁺ did not vary along the root length. In agreement with earlier work, providing poplars with ample nitrogen led to higher accumulation of CTs across root zones. To test the functional importance of CTs in roots directly, CT-modified transgenic plants could be important tools.

Keywords: 4-dimethylaminocinnamaldehyde (DMACA); Populus; flavonoid; microelectrode ion flux measurement (MIFE); proanthocyanidin; root cap.

Figure 1

Images and cartoon of typical Populus tremula x alba roots after eight weeks of growth showing white zone and transition to brown zone. Black arrows indicates typical white zone; orange arrows show brown zone of roots prior to secondary growth. Brown arrow indicates the cork zone. Scale bar = 1 cm.

Figure 2

Mean soluble and insoluble CT concentrations in zones along the root axis in P. tremula x alba. Root zone samples were pooled individually by plant for each of five plants. Different letters indicated significant differences (Tukey HSD p<0.05). Error bars indicate SE (n=5) for total CT concentration.

Figure 3

Localization of condensed tannins in P. tremula x alba roots visualized by DMACA staining of longitudinal and cross root sections. CTs stain with reddish-brown color under these conditions. (A) Root tip (< 2 mm from root). (B) White zone/maturation zone (1cm back from root tip). (C) cork zone (> 20 cm back from root tip). Scale bar = 100µm.

Figure 4

Cellular localization of condensed tannins of P. tremula x alba. Longitudinal sections of roots 5-10mm from root tip in the white zone were embedded, sectioned and stained with DMACA as described under Materials and Methods. CTs stain with reddish-brown color under these conditions. Epidermal cells are indicated by ‘e’. Scale bars=25µm.

Figure 5

Mean influx of NH₄ ⁺, Ca²⁺, and NO₃ ^- ions into white zone roots of P. tremula x alba at defined distances from the root tip. Mean ion fluxes (+/- SE) were measured in young poplar roots at a defined distance from root tip using a MIFE system as described in Materials and Methods. *indicates significantly different means from other measurements for the same ion (p<0.001).

Figure 6

Total CT concentrations in distinct growth zones of P. tremula x P. tremuloides roots grown with 0.1, 1.0, or 10 mM N. Small letters indicate significantly different means (Tukey HSD, P<0.05). Bars show means ± SE (n=3). Letters above bars are Tukey pairwise comparisons for differing nitrogen treatments within each root zone. Letters within bars are pairwise comparison between different root sections with the same nitrogen treatment.