Background and aims: Plants are able to adapt to the environment dynamically through regulation of their growth and development. Excess copper (Cu(2+)), a toxic heavy metal, induces morphological alterations in plant organs; however, the underlying mechanisms are still unclear. With this in mind, the multiple signalling functions of nitric oxide (NO) in plant cells and its possible regulatory role and relationship with auxin were examined during Cu(2+)-induced morphological responses.
Methods: Endogenous auxin distribution was determined by microscopic observation of X-Gluc-stained DR5::GUS arabidopsis, and the levels of NO, superoxide and peroxynitrite were detected by fluorescence microscopy. As well as wild-type, NO-overproducer (nox1) and -deficient (nia1nia2 and nia1nia2noa1-2) arabidopsis plants were used.
Key results: Cu(2+) at a concentration of 50 µm resulted in a large reduction in cotyledon area and hypocotyl and primary root lengths, accompanied by an increase in auxin levels. In cotyledons, a low Cu(2+) concentration promoted NO accumulation, which was arrested by nitric oxide synthase or nitrate reductase inhibitors. The 5-μm Cu(2+)-induced NO synthesis was not detectable in nia1nia2 or nia1nia2noa1-2 plants. In roots, Cu(2+) caused a decrease of the NO level which was not associated with superoxide and peroxynitrite formation. Inhibition of auxin transport resulted in an increase in NO levels, while exogenous application of an NO donor reduced DR5::GUS expression. The elongation processes of nox1 were not sensitive to Cu(2+), but NO-deficient plants showed diverse growth responses.
Conclusions: In plant organs, Cu(2+) excess results in severe morphological responses during which the endogenous hormonal balance and signal transduction are affected. Auxin and NO negatively regulate each other's level and NO intensifies the metal-induced cotyledon expansion, but mitigates elongation processes under Cu(2+) exposure.