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

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 o f growing roots lost viability within 1-2 h following exposure to low pH, b ut non-growing roots showed no damage under the same treatment. Low-pH dama ge in growing roots was irreversible after 1 h incubation at pH 4.5 as judg ed by regrowth in growing medium at pH 5.6, Growing lateral roots also lost viability in the same treatment, whereas non-growing lateral roots remaine d 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, growin g roots required 25 muM of calcium to maintain elongation, and at pH 4.8 an d 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+ g reater than or equal to Ca2+ > Mg2+, The monovalent cation K+ showed no ame liorative effect, but berate showed a strong ameliorative effect with Ca2+, These results indicate that the primary target of proton toxicity may be l inked to a disturbance of the stability in the pectic polysaccharide networ k, where calcium plays a key role in plant roots.