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Published on February 16, 2007; 10.1104/pp.106.091827

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Received October 23, 2006
Accepted January 27, 2007

A Reduced Content of Homogalacturonan Does Not Alter the Ion-mediated Increase in Xylem Hydraulic Conductivity in Tobacco

Andrea Nardini *, Antonio Gascó , Felice Cervone , and Sebastiano Salleo

Dipartimento di Biologia, Università di Trieste, 34127 Trieste, Italia; Dipartimento di Biologia Vegetale, Università di Roma La Sapienza, 00185 Roma, Italia

* Corresponding author; email: nardini{at}units.it.

Xylem hydraulic conductivity (Ks) in stems of tobacco wild type SR1 was compared to that of PG7 and PG16, two transgenic lines with increased levels of expression of the gene encoding the Aspergillus niger endopolygalacturonase (AnPGII). Activity of AnPGII removes in planta blocks of homogalacturonan (HG) with deesterified carboxyls, thus increasing the degree of neutrality (DE) of pectins. The effect was tested of K+ in increasing stem Ks using model plants with more neutral polysaccharides in primary walls and, hence, in intervessel pit membranes. Ks measured with deionized water was compared to that with KCl solutions at increasing concentrations ({Delta}Ks, %). Plants transformed for HG-DE showed a dwarfed phenotype but {Delta}Ks did not differ among the three experimental groups. The ion-mediated hydraulic effect saturated at a KCl concentration of 25 mM in SR1 plants. All the three tobacco lines showed {Delta}Ks of around +12.5% and +17.0% when perfused with 10 and 25 mM KCl, respectively. Because the modification of HG content did not influence the ion-mediated hydraulic enhancement, we suggest that pectin components other than HG (like RGI and/or RGII) might play important roles in the hydrogel behaviour of pit membranes.




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