Auxin accumulation and transport in Arabidopsis thaliana (L.) Heynh adventitious roots are modified by cadmium and arsenic

Cadmium (Cd) and arsenic (As) are very toxic elements for all organisms and the environment. They are not essential elements for plants but can be easily absorbed by the plant root system provoking damages in tissues of sensitive plants, e.g. Arabidopsis thaliana. Damages to the root system have negative consequences on plant growth and productivity. It is known that the quiescent centre (CQ) of the root apical meristem controls the root development in the primary root (PR), lateral roots and adventitious roots (ARs). The QC definition and maintenance depend on polar auxin transport and accumulation in the stem cell niche surrounding the QC. The membrane carriers LAX3 and PIN1 are involved in root polar auxin influx and efflux, respectively. DR5::GUS line is a useful system for monitoring auxin localization at cell and tissue levels. Our investigations, based on morphological analyses of Columbia (Col) plantlets and histochemical GUS analyses on ARs of QC25::GUS (i.e., QC identity marker) transgenic plantlets, grown in the same experimental conditions of this research, revealed that these pollutants affected AR development, either increasing AR formation (60 μM CdSO4, combined or not with 100 μM Na2HAsO4.7H2O) or inhibiting formation and growth (400 μM Na2HAsO4.7H2O). In all cases both pollutants altered the QC definition and maintenance. These alterations prompted us to investigate whether the two pollutants may cause anomalous IAA levels/distribution during AR development. To the aim, we investigated auxin levels in the wild type, and auxin localization in the DR5::GUS transgenic line, under in vitro growth in the presence/absence of 60 μM CdSO4 or 400 μM Na2HAsO4.7H2O or in the presence of both salts. The plantlets were cultured in continuous darkness for nine days and then transferred to the light (long-day exposure) for additional seven days. The effects of Cd and/or As on auxin transport were also investigated in the ARs by analyses on PIN1::GUS and LAX3::GUS lines.Results show that no signal was detected in the QC and surrounding stem cell niche of the ARs in DR5::GUS plantlets treated with 400 μM As, indicating that the semimetal compromised the regular establishment of the auxin maximum necessary for the correct definition and maintenance of the stem cell niche. Cadmium, combined or not with As, either caused a significant reduction in the number of ARs with a normally localized DR5::GUS signal with respect to control treatment, or induced a weak signal often dislocated to the columella cells. These results indicated that also the metal altered the auxin maximum necessary to the correct stem cell niche organization and functioning. Cd and As also interfered with the expression patterns of PIN1::GUS and LAX3::GUS, especially in AR primordia. Cadmium alone negatively affected auxin efflux by PIN1 causing its unexpression in the most of the primordia. Arsenic, alone or combined with Cd, exhibited minor effects in inhibiting PIN1 expression. The auxin influx by LAX3 was reduced in AR primordia in the same way by both pollutants, alone or combined. In conclusion, Cd and As alter auxin apical accumulation by affecting the influx and efflux auxin carriers, thus negatively disturbing stem cell niche and QC functioning in the ARs.