Brief exposure to low‐pH stress causes irreversible damage to the growing root in Arabidopsis thaliana: pectin–Ca interaction may play an important role in proton rhizotoxicity

Abstract

The viability of Arabidopsis thaliana (strain Landsberg) roots exposed to a low pH (4.5 or 4.7) solution that contained 100 muM CaCl2 was examined by staining with fluorescein diacetate-propidium iodide, The elongation zone of growing roots lost viability within 1-2 h following exposure to low pH, but non-growing roots showed no damage under the same treatment. Low-pH damage in growing roots was irreversible after 1 h incubation at pH 4.5 as judged by regrowth in growing medium at pH 5.6, Growing lateral roots also lost viability in the same treatment, whereas non-growing lateral roots remained viable during and after the treatment. The low-pH damage was ameliorated by the simultaneous application of calcium, indicating the involvement of a calcium-requiring process in overcoming proton toxicity. At pH 5.0, growing roots required 25 muM of calcium to maintain elongation, and at pH 4.8 and pH 4.5 more than 250 muM and 750 muM, respectively. The low-pH damage was ameliorated by divalent cations in the Order of Ba2+ approximate to Sr2+ greater than or equal to Ca2+ > Mg2+, The monovalent cation K+ showed no ameliorative effect, but berate showed a strong ameliorative effect with Ca2+, These results indicate that the primary target of proton toxicity may be linked to a disturbance of the stability in the pectic polysaccharide network, where calcium plays a key role in plant roots.