Reduced content of homogalacturonan does not alter the ion-mediated increase in xylem hydraulic conductivity in tobacco

Xylem hydraulic conductivity (Ks) in stems of tobacco (Nicotiana tabacum) 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. Ks measured with deionized water was compared to that with KCl solutions at increasing concentrations (ΔKs, %). Plants transformed for HG degree of neutrality showed a dwarfed phenotype, but ΔKs did not differ among the three experimental groups. All the three tobacco lines showed ΔKs of around +12.5% and +17.0% when perfused with 10 and 25 mM KCl, respectively. Because modification of HG content did not influence ion-mediated hydraulic enhancement, we suggest that pectin components other than HG, like rhamnogalacturonan-I and/or rhamnogalacturonan-II, might play important roles in the hydrogel behavior of pit membranes.