Cadmium and Arsenic alter auxin homeostasis during adventitious root formation in Arabidopsis thaliana L. (Heynh)

The metalloid arsenic (As) and the heavy metal cadmium (Cd) are environmental pollutants with a wide-spread and persistent presence in all ecosystems. Cadmium toxicity affects plant development, in particular altering root growth and differentiation (Brunetti et al., 2011, J Experiment Bot 62:5509; Zanella et al., 2016, Planta 243:605). Arsenic is a metalloid present in the environment in inorganic and organic forms. The inorganic ones are more toxic. It was reported that Cd and As mainly localize in the root meristems (Feng et al, 2013, Environ Sci Pollut Res 20:5449; Ko-pittke et al, 2012, Plant Physiol 159:1149). Arabidopsis thaliana (L.) Heynh exhibits a root system composed of primary (PR), lateral (LRs) and adventitious roots (ARs). Indole-3-acetic acid (IAA) is the main auxin in plants, and its homeostasis is regulated by an integrated and coordinated action between synthesis and polar transport, both essential for a proper root formation and development (Blilou et al. 2005, Nature 433:39; Della Rovere et al.,2013, Ann Bot 112:1395). YUCCA6 gene is involved in the tryptophan-dependent IAA biosynthesis, with an important role in root formation and development (Kim et al., 2007, Plant Physiol 145:722). Influx and efflux membrane proteins regulate IAA shoot-to-root polar transport. Among them, key roles in root development have been described for PIN1, member of the PIN-FORMED fami-ly of auxin efflux carriers (Adamowski and Friml 2015, Plant Cell 27:20) and for LAX3, member of the AUXIN1/LIKE-AUX1 (AUX/LAX) family of auxin influx carriers (Swarup et al., 2008, Nat Cell Bio 10:946). The coordinated auxin efflux/influx activities, generating the IAA gradients and maxima, are required for LR and AR initiation and development. Stress caused by toxic met-als alters growth and development by interfering with auxin levels and homeostasis (Potters et al., 2009 Plant Cell Environ 32:158; Sofo et al., 2013, Physiol Plant 149:487). However, less is known about the combined effect of Cd and As on IAA biosynthesis, levels and transport in ARs and LRs. The study’s aim was to determine if Cd and/or As affected root formation/development by alter-ing IAA biosynthesis and/or transport. To the aim, the expression pattern of the quiescent centre (QC) marker QC25::GUS, YUCCA6 transcript levels, the expression patterns of PIN1 and LAX3, detection of IAA levels, and auxin localization monitored by the DR5::GUS system, were investi-gated in Arabidopsis seedlings exposed to Cd and/or As. The results show that Cd and/or As significantly inhibited PR and hypocotyl growth. The mean density of LRs and ARs were significantly increased in the seedlings exposed to either Cd alone or to both pollutants, while root density was reduced by As alone. Cd and As disrupted QC formation, and stem cell niche maintenance over time in both LRs and ARs. Cadmium in-creased total auxin levels through an overexpression of YUCCA6. The toxic elements altered the expression pattern of PIN1 and LAX3, negatively affecting IAA accumulation in ARs and LRs. In conclusion, our results show that the most severe damages that Cd and/or As cause in Arabidopsis root system are due to a strong alteration of auxin biosynthesis, transport and accumulation in LRs and ARs.