Based on the experiments performed on the AtHMA4 – expressing tobacco plants (AtHMA4 encodes protein that exports Zn to the apoplast), it was proposed that apoplast may be regarded as a site of perception of the Zn status in cells. It was presumed that the apoplastic Zn concentration might contribute to the generation of a signal of Zn excess leading to loading of Zn to leaf areas consisting of groups of cells in which the exceptional ability to accumulate Zn is initiated. Up to now the question about the nature of the signal inducing loading Zn into these specific cells remain unanswered.
Among the proteins associated with the cell wall, only the WAKs (Wall-Associated Kinases) have been implicated in a plant response to metals. It is known that the members of WAKs family have extracellular domain binding pectins. From the variety of cell wall components, pectins are among of the molecules that may play a signaling role. Furthermore, the exposure to different metals concentrations changes the level of the methylesteryfication of the cell wall pectins.
The expression analysis of NtWAKs/NtWAKLs genes showed an increase in the transcript level of NtWAK2 in the leaves from high Zn-treated plants in comparison to the control ones. To analyze the differences in pectin localization between control and Zn-treated plants, the immunolocalization of pectin fractions was performed on the leaf cross-sections. Only one epitope of rhamnogalacturonan I was present exclusively in the Zn-exposed tobacco. Analysis showed that the level of rhamnogalacturonan I increased with time of exposure to high Zn concertation. Furthermore, this fraction of pectins was localized preferentially near the veins, and its distributions were correlated with the pattern of high concentrations of Zn in the groups of mesophyll cells localized within the same regions. Performed research suggest that signal inducting responsible for formation of the accumulating cells in the mesophyll of tobacco leaves is generated by rhamnogalacturonan I and transduced into the cell.
Take Away Notes:
• Wall associated kinases 2 are probably involved in perception of zinc level in tobacco leaf cells.
• During the prolonged exposure plants for high zinc concentration in medium, the level of rhamnogalacturonan I in the cell wall is increasing.
• Probably RG I generated a signal about zinc level in apoplast which is most likely received by NtWAK2.