Differential Al resistance and citrate secretion in the tap and basal roots of common bean seedlings

Abstract

The common bean root system is composed of several types of root (e.g. tap, basal, and lateral roots), whose physiological functions may be of great difference. However, we do not know if the root system of common bean differs in organic acid secretion and thus aluminium (Al) resistance. In the present study, the tap and basal roots of three common bean genotypes (i.e. G19842, SQ12 and BAT881) from different origins were compared for their citrate secretion and Al resistance. Grown in a simple solution containing 30 µM Al3+ for 24 h, genotype G19842 maintained 75% relative tap root length [RTRL = (tap root length with Al)/(tap root length without Al)], 48% relative basal root length [RBRL = (basal root length with Al)/ (basal root length without Al)], genotype SQ12 maintained 62% RTRL and 57% RBRL, while BAT881 only maintained 31% RTRL and 19% RBRL, indicating differential sensitivity of bean genotypes and root types to Al stress. The amounts of Al‐induced citrate secretion by the tap/basal roots were 9.8/5.1, 8.2/5.9 and 5.4/4.1 nmol cm−2 FR (fresh root) [12 h]−1 for G19842, SQ12 and BAT881, respectively, indicating that both bean genotypes and root types differ in organic acid secretion. In G19842, the root surface area was 25% higher in tap root apex than that in basal root apex, and the amounts of citrate secretion per unit surface area and per root apex were 29 and 62% higher in tap root apex than those in basal root apex, respectively, suggesting that the higher citrate secretion in the tap root apex could be attributed to the larger surface area and the higher secretion activity. Stronger inhibition of Al‐induced citrate secretion in the basal than tap roots by anthracene‐9‐carboxylic acid, an inhibitor of anion channel and K‐252a, a broad range inhibitor of protein kinase may also imply the differences in the activities of anion channels and K‐252a‐sensitive protein kinases on the plasma membrane between the tap and basal roots, resulting in differential citrate secretion. We propose that the higher Al resistance in the tap root than in basal roots might be attributed to both greater number and higher activity of the anion channels in the former, thus allowing more citrate secretion in this root type.