Auxin jasmonate crosstalk in Oryza sativa L. root system formation after cadmium and/or arsenic exposure

Soil pollutants may affect root growth through interactions among phytohormones like auxin and jasmonates. Rice is frequently grown in paddy fields contaminated by cadmium and arsenic, but the effects of these pollutants on jasmonates/auxin crosstalk during adventitious and lateral roots formation are widely unknown. Therefore, seedlings of Oryza saliva cv. Nihonmasari and of the jasmonate-biosynthetic mutant coleoptile photomorphogenesis2 were exposed to cadmium and/or arsenic, and/or jasmonic acid methyl ester, and then analysed through morphological, histochemical, biochemical and molecular approaches. In both genotypes, arsenic and cadmium accumulated in roots more than shoots. In the roots, arsenic levels were more than twice higher than cadmium levels, either when arsenic was applied alone, or combined with cadmium. Pollutants reduced lateral root density in the wild-type in every treatment condition, but jasmonic acid methyl ester increased it when combined with each pollutant. Interestingly, exposure to cadmium and/or arsenic did not change lateral root density in the mutant. The transcript levels of OsASA2 and OsYUCCA2, auxin biosynthetic genes, increased in the wild-type and mutant roots when pollutants and jasmonic acid methyl ester were applied alone. Auxin (indole-3-acetic acid) levels transiently increased in the roots with cadmium and/or arsenic in the wild-type more than in the mutant. Arsenic and cadmium, when applied alone, induced fluctuations in bioactive jasmonate contents in wild-type roots, but not in the mutant. Auxin distribution was evaluated in roots of OsDR5::GUS seedlings exposed or not to jasmonic acid methyl ester added or not with cadmium and/or arsenic. The DR5::GUS signal in lateral roots was reduced by arsenic, cadmium, and jasmonic acid methyl ester. Lipid peroxidation, evaluated as malondialdehyde levels, was higher in the mutant than in the wild-type, and increased particularly in As presence, in both genotypes. Altogether, the results show that an auxin/jasmonate interaction affects rice root system development in the presence of cadmium and/or arsenic, even if exogenous jasmonic acid methyl ester only slightly mitigates pollutants toxicity.