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Plants adapt to challenging growth conditions, such as the scarcity of nutrients or exposure to toxic metals, via changes in root system architecture. Chromium (Cr) is a non-essential element that when supplied in sublethal concentrations inhibits primary root growth through decreasing meristematic activity and affects photosynthesis. Here, we show that sucrose reverses the inhibitory effects of Cr(VI) on plant growth and development. Sucrose supplementation reactivated primary root growth under repressing Cr(VI) concentrations by restoring cell division and auxin distribution at the root meristem, keeping stem cell niche functioning. Analysis of the growth of Arabidopsis wild-type and mutant seedlings defective in auxin transport or signaling further revealed a critical role of auxin in mediating the effects of sucrose to protect plants from Cr(VI) toxicity. The results suggest that sucrose acts as a regulator in the maintenance of root meristem activity to overcome the stress generated by sublethal Cr(VI) concentrations.

Morphological root plasticity optimizes nutrient and water uptake by plants and is a promising target to improve tolerance to metal toxicity. Exposure to sublethal chromate [Cr(VI)] concentrations inhibits root growth, decreases photosynthesis and compromises plant development and productivity. Despite the increasing environmental problem that Cr(VI) represents, to date, the Cr tolerance mechanisms of plants are not well understood, and it remains to be investigated whether root architecture remodelling is important for plant adaptation to Cr(VI) stress. In this report, we analysed the growth response of Arabidopsis thaliana seedlings to concentrations of Cr(VI) that strongly repress primary and lateral root growth. Interestingly, adventitious roots started developing, branched and allowed seedlings to grow under highly growth-repressing Cr(VI) concentrations. Cr(VI) negatively regulates auxin transport and response gene expression in the primary root tip, as evidenced by decreased expression of auxin-related reporters DR5::GFP, DR5::uidA and PIN1::PIN1::GFP, and then, another auxin maximum is established at the site of adventitious root initiation that drives adventitious root organogenesis. Both primary root growth inhibition and adventitious root formation induced by high Cr(VI) levels are blocked by a gain-of-function mutation in the SOLITARY-ROOT/IAA14 gene of Arabidopsis. These data provide evidence that suggests a critical role for auxin transport and signalling via IAA14/SLR1 in the developmental program linking Cr(VI) to root architecture remodelling.

Soil contamination by hexavalent chromium [Cr(VI) or chromate] due to anthropogenic activities has become an increasingly important environmental problem. Mineral nutrients such as phosphate (Pi), sulfate and nitrate have been reported to attenuate Cr(VI) toxicity, but the underlying mechanisms remain to be clarified. Here, we show that chromate activates the expression of low-Pi inducible reporter genes AtPT1 and AtPT2 in Arabidopsis thaliana transgenic seedlings. Primary-root growth was inhibited by 60 % in AtPT2::uidA-expressing seedlings upon exposure to 140-μM Cr(VI). However, increasing the Pi and sulfate supply to the seedlings that were experiencing Cr(VI) toxicity completely and partially restored the root growth, respectively. This effect correlated with the Cr(VI)-induced AtPT2::uidA expression being completely reversed by addition of Pi. To evaluate whether the nutrient supply may affect the endogenous level of Cr in plants grown under toxic Cr(VI) levels, the contents of Cr were measured (by ICP-MS analyses) in seedlings treated with Cr and with or without Pi, sulfate or nitrate. It was found that Cr accumulation increases tenfold in plants treated with 140-μM Cr(VI) without modifying the phosphorus concentration in the plant. In contrast, the supply of Pi specifically decreased the Cr content to levels similar to those found in seedlings grown in medium without chromate. Taken together, these results show that in A. thaliana seedlings the uptake of Cr(VI) is reduced by Pi. Moreover, our data indicate that Pi and sulfate supplements may be useful in strategies for handling Cr-contaminated soils.

Soil contamination by hexavalent chromium [Cr(VI) or chromate] due to anthropogenic activities has become an increasingly important environmental problem. To date few studies have been performed to elucidate the signaling networks involved on adaptive responses to (CrVI) toxicity in plants. In this work, we report that depending upon its concentration, Cr(VI) alters in different ways the architecture of the root system in Arabidopsis thaliana seedlings. Low concentrations of Cr (20–40 µM) promoted primary root growth, while concentrations higher than 60 µM Cr repressed growth and increased formation of root hairs, lateral root primordia and adventitious roots. We analyzed global gene expression changes in seedlings grown in media supplied with 20 or 140 µM Cr. The level of 731 transcripts was significantly modified in response to Cr treatment with only five genes common to both Cr concentrations. Interestingly, 23 genes related to iron (Fe) acquisition were up-regulated including IRT1, YSL2, FRO5, BHLH100, BHLH101 and BHLH039 and the master controllers of Fe deficiency responses PYE and BTS were specifically activated in pericycle cells. It was also found that increasing concentration of Cr in the plant correlated with a decrease in Fe content, but increased both acidification of the rhizosphere and activity of the ferric chelate reductase. Supply of Fe to Cr-treated Arabidopsis allowed primary root to resume growth and alleviated toxicity symptoms, indicating that Fe nutrition is a major target of Cr stress in plants. Our results show that low Cr levels are beneficial to plants and that toxic Cr concentrations activate a low-Fe rescue system.

Instituto de Investigaciones Agropecuarias y Forestales. Facultad de Biología. Facultad de Medicina Veterinaria y Zootecnia. Facultad de Agrobiología. Facultad de Químico Farmacobiología. Programa Institucional de Maestría en Ciencias Biológicas The increment of the chromium (Cr) concentrations in the atmosphere due to activities antropogénicas have transformed it into an environmental serious pollutant. Cr (VI) is the most toxic form in Cr, due to the generation of free radicals during their reduction to Cr (III) in the cells. It is known that Cr is transported to inclination of the membrane for an active mechanism that involves transporting of essential anions in bacterias and mushrooms, but you lacks information conclusive about the reception processes, mobilization and accumulation that they happen in the plants. El incremento de las concentraciones de cromo (Cr) en el ambiente debido a actividades antropogénicas lo ha convertido en un serio contaminante ambiental. El Cr (VI) es la forma más tóxica del Cr, debido a la generación de radicales libres durante su reducción a Cr (III) en las células. Se sabe que el Cr se transporta a través de la membrana por un mecanismo activo que involucra acarreadores de aniones esenciales en bacterias y hongos, pero se carece de información concluyente sobre los procesos de captación, movilización y acumulación que ocurren en las plantas.

Instituto de Investigaciones Agropecuarias y Forestales. Facultad de Biología. Facultad de Medicina Veterinaria y Zootecnia. Facultad de Agrobiología. Facultad de Químico Farmacobiología. Programa Institucional de Maestría en Ciencias Biológicas Root growth in Arabidopsis thaliana begins with the primary root derived from the embryo, later lateral roots derived from the pericycle are formed and occasionally adventitious roots can be formed from the hypocotyl region; these roots together with the radical hairs constitute the architecture of the root and determine the anchoring and absorption of water and nutrients from the soil or culture medium. The root has a post-embryonic growth that depends on endogenous factors such as hormones and gene expression, and on the other hand exogenous factors given by the environmental conditions and signals, among which are the shortage of nutrients and the high concentrations of essential and non-essentials elements. In this work, the growth at in vitro conditions of the root of A. thaliana ecotype Col-0 and the mutant line iaa14/slr1 under different conditions of Cr(VI) and sucrose was analyzed, finding that: i) A decrease in the transport of auxin in the primary root by the effect of Cr(VI) is correlated with an inhibition of the growth of the primary root, with the loss of cell division and markers of meristem identity, ii) The sucrose supplementation restores the primary root growth, iii) Cr(VI) induces the formation of lateral roots in both A. thaliana Col-0 and in the mutant line iaa14/slr1, with normal patterns of auxin distribution and cellular division. In addition, the effect of low phosphate and sucrose on the formation of lateral roots was analyzed, finding that the stress generated by low phosphate sensitizes or predisposes the plants to form more lateral roots by the effect of sucrose. El crecimiento de la raíz en Arabidopsis thaliana se inicia con la raíz primaria derivada del embrión, posteriormente se forman raíces laterales derivadas del periciclo y ocasionalmente se pueden formar raíces adventicias a partir de la región del hipocótilo; estas raíces junto con los pelos radicales constituyen la arquitectura de la raíz y determinan el anclaje y la absorción de agua y nutrientes del suelo o medio de cultivo. La raíz tiene un crecimiento post-embrionario que depende de factores endógenos como hormonas y expresión de genes, y por otra parte factores exógenos dado por las condiciones y señales ambientales, entre las que se encuentran la escasez de nutrientes y las altas concentraciones de elementos esenciales y no esenciales. En este trabajo se analizó el crecimiento en condiciones in vitro de la raíz de A. thaliana ecotipo Col-0 y la línea mutante iaa14/slr1 bajo diferentes condiciones de Cr(VI) y sacarosa, encontrando: i) Una disminución en el transporte de la auxina en la raíz primaria por efecto del Cr(VI) se correlaciona con una inhibición del crecimiento de la raíz primaria, con la pérdida de división celular y de marcadores de identidad del meristemo, ii) la suplementación con sacarosa restablece el crecimiento de la raíz primaria, iii) el Cr(VI) induce la formación de raíces laterales tanto en A. thaliana Col-0 y en la línea mutante iaa14/slr1, con patrones normales de distribución de auxina y de división celular. Además, se analizó el efecto de bajo fosfato y sacarosa en la formación de raíces laterales, encontrando que el estrés generado por bajo fosfato sensibiliza o predispone a las plantas a formar más raíces laterales por efecto de la sacarosa.