Cadmium modulates tomato root architecture and root hair responses to ABA-metabolising rhizobacteria

Exogenous cadmium (Cd) stimulates abscisic acid (ABA) accumulation in planta, enhancing Cd tolerance by maintaining growth and limiting Cd accumulation. Since rhizobacteria that metabolise ABA may compromise Cd tolerance, tomato Ailsa-Craig plants were grown in vitro with or without 80 µM CdCl_2, and with or without ABA-metabolising Rhodococcus sp. P1Y and Novosphingobium sp. P6W and the Cd-tolerant, 1-aminocyclopropane-1-carboxylate (ACC) deaminase-containing Variovorax paradoxus 5 C-2 (negative control). Root colonisation of the ABA-metabolising strains was circa 50% and 2 orders of magnitude less than 5 C-2 without and with Cd respectively. While root Cd concentrations were independent of inoculation, only 5 C-2 decreased leaf Cd concentrations. P6W decreased root ABA concentrations without Cd, and leaf ABA concentrations irrespective of Cd treatment. Although P1Y didn't affect root ABA concentrations, it decreased and increased leaf ABA concentrations with and without Cd respectively. With Cd, both ABA-metabolising strains inhibited primary root length and P1Y increased lateral root length. Without Cd, 5 C-2 increased primary root length and lateral root number, while all strains increased root hair length and density. Cd increased these root hair traits in uninoculated seedlings, with bacterial addition causing further increments. To further investigate how ABA affects root (hair) traits, the ABA-deficient flacca mutant and its wild-type (WT) were compared at varying (0-100 µM) ABA concentrations. Although WT plants tended to have longer primary roots, flacca root hair length was circa 29% longer irrespective of exogenous ABA concentration. Higher ABA concentrations decreased primary and lateral root length and lateral root number, had no effect on root hair length but increased root hair density similarly in both genotypes. That both ABA-metabolising strains increased root hair length and density of flacca suggests ABA-independent regulation of these root hair traits. Despite stimulating root hair development, the ABA-metabolizing strains didn't enhance Cd concentrations in planta but they compromised Cd tolerance by inhibiting primary root length.

The online version contains supplementary material available at 10.1007/s12298-025-01689-w.