Iron Availability Affects Phosphate Deficiency-Mediated Responses, and Evidence of Cross-Talk with Auxin and Zinc in Arabidopsis.

Phosphate (Pi) is pivotal for plant growth and development. Pi deficiency triggers local and systemically regulated adaptive responses in Arabidopsis thaliana. Inhibition of primary root growth (PRG) and retarded development of lateral roots (LRs) are typical local Pi deficiency-mediated responses of the root system. Expression of Pi starvation-responsive (PSR) genes is regulated systemically. Here, we report the differential influence of iron (Fe) availability on local and systemic sensing of Pi by Arabidopsis. P-Fe- condition disrupted local Pi sensing, resulting in an elongated primary root (PR). Altered Fe homeostasis in the lpsi mutant with aberration in local Pi sensing provided circumstantial evidence towards the role of Fe in the maintenance of Pi homeostasis. Reporter gene assays, expression analysis of auxin-responsive genes (ARGs) and root phenotyping of the arf7arf19 mutant demonstrated the role of Fe availability on local Pi deficiency-mediated LR development. In addition, Fe availability also exerted a significant influence on PSR genes belonging to different functional categories. Together, these results demonstrated a substantial influence of Fe availability on Pi deficiency-mediated responses of ontogenetically distinct traits of the root system and PSR genes. The study also provided evidence of cross-talk between Pi, Fe and Zn, highlighting a complex tripartite interaction amongst them for maintaining Pi homeostasis.

Role of Fe-responsive genes in bioreduction and transport of ionic gold to roots of Arabidopsis thaliana during synthesis of gold nanoparticles.

Several studies have shown potassium chloroaurate (KAuCl4)-mediated synthesis of gold nanoparticles (AuNPs) by using extracts of different parts of diverse plant species. However, the mechanism underlying the formation of AuNPs in planta has far from being elucidated. Here, we report the molecular evidence towards the role of genes involved in iron (Fe) homeostasis during in planta synthesis of AuNPs in roots of Arabidopsis thaliana. Firstly, we examined the dosage-dependent effects of KAuCl4 treatment on primary root length (PRL), and meristematic activity of roots in transgenic CycB1;1::uidA. Compared to control seedling (0 ppm KAuCl4), PRL and meristematic activity of primary and lateral roots showed progressive attenuation in seedlings treated with higher concentrations of KAuCl4 (25 ppm or above). Therefore, subsequent studies on in planta synthesis of AuNPs, and molecular responses were carried out in roots of the seedlings treated with 10 ppm KAuCl4 for 7 d. TEM of KAuCl4-treated seedlings showed the presence of monodisperse AuNPs of different shapes and sizes in root biomatrix. There was a significant induction of FRO2 in KAuCl4-treated roots, and therefore its likely involvement in bioreduction of Au(3)(+) could be assumed. Elevated expression levels of Fe transporters IRT1 and IRT2 further suggested their potential role in transport of bioreduced Au(3+) across root membrane. Expression levels of other genes involved in Fe homeostasis, and also different members of zinc (Zn), phosphate (Pi), and potassium (K) transporter families remained unaffected by KAuCl4 treatment. An increased Au content in Fe-deprived roots further provided evidence towards the specific role of a subset of Fe-responsive genes during in planta synthesis of AuNPs.

Arabidopsis thaliana mutant lpsi reveals impairment in the root responses to local phosphate availability.

Phosphate (Pi) deficiency triggers local Pi sensing-mediated inhibition of primary root growth and development of root hairs in Arabidopsis (Arabidopsis thaliana). Generation of activation-tagged T-DNA insertion pools of Arabidopsis expressing the luciferase gene (LUC) under high-affinity Pi transporter (Pht1;4) promoter, is an efficient approach for inducing genetic variations that are amenable for visual screening of aberrations in Pi deficiency responses. Putative mutants showing altered LUC expression during Pi deficiency were identified and screened for impairment in local Pi deficiency-mediated inhibition of primary root growth. An isolated mutant was analyzed for growth response, effects of Pi deprivation on Pi content, primary root growth, root hair development, and relative expression levels of Pi starvation-responsive (PSR) genes, and those implicated in starch metabolism and Fe and Zn homeostasis. Pi deprived local phosphate sensing impaired (lpsi) mutant showed impaired primary root growth and attenuated root hair development. Although relative expression levels of PSR genes were comparable, there were significant increases in relative expression levels of IRT1, BAM3 and BAM5 in Pi deprived roots of lpsi compared to those of the wild-type. Better understanding of molecular responses of plants to Pi deficiency or excess will help to develop suitable remediation strategies for soils with excess Pi, which has become an environmental concern. Hence, lpsi mutant will serve as a valuable tool in identifying molecular mechanisms governing adaptation of plants to Pi deficiency.

Effects of deficiency and excess of zinc on morphophysiological traits and spatiotemporal regulation of zinc-responsive genes reveal incidence of cross talk between micro- and macronutrients.

Zinc (Zn) is an essential micronutrient which affects plant growth and development in deficiency and can be toxic when present in excess. In Arabidopsis thaliana , different families of cation transporters play pivotal roles in Zn homeostasis. In the present study, we evaluated the effects of Zn in its deficiency (0 µM; Zn-) and excess (75 µM; Zn++) on various morphophysiological and molecular traits. Primary root length was reduced in Zn- seedlings, whereas there were significant increases in the number and length of lateral roots under Zn- and Zn++ conditions, respectively. Concentration of various macro- and microelements showed variations under different Zn regimes and notable among them was the reduced level of iron (Fe) in Zn++ seedlings compared to Zn+. Certain members of the ZIP family (ZIP4, ZIP9, and ZIP12) showed significant induction in roots and shoots of the Zn- seedlings. Their suppression under Zn++ condition indicated their transcriptional regulation by Zn and their roles in the maintenance of its homeostasis. Zn-deficiency-mediated induction of HMA2 in roots and shoots suggested its role in effluxing Zn into xylem for long-distance transport. Attenuation in the expression of Fe-responsive FRO2 and IRT1 in Zn- roots and their induction in Zn++ roots provided empirical evidence toward the prevalence of a cross talk between Zn and Fe homeostasis. Variable effects of Zn- and Zn++ on the expression of subset of genes involved in the homeostasis of phosphate (Pi), potassium (K), and sulfur (S) further highlighted the prevalence of cross talk between the sensing and signaling cascades of Zn and macronutrients. Further, the inducibility of ZIP4 and ZIP12 in response to cadmium (cd) treatment could be harnessed by tailoring them in homologous or heterologous plant system for removing pollutant toxic heavy metals from the environment.

Increased accumulation and decreased catabolism of anthocyanins in red grape cell suspension culture following magnesium treatment.

Anthocyanins are the largest and best studied group of plant pigments. However, not very much is known about the fate of these phenolic pigments after they have accumulated in the cell vacuoles of plant tissues. We have previously shown that magnesium treatment of ornamentals during the synthesis of anthocyanins in the flowers or foliage caused an increase in the pigment concentration. In this study, we characterized the effect of magnesium on the accumulation of anthocyanin in red cell suspension originating from Vitis vinifera cv. Gamay Red grapes. Magnesium treatment of the cells caused a 2.5- to 4.5-fold increase in anthocyanin concentration, with no substantial induction of the biosynthetic genes. This treatment inhibited the degradation of anthocyanins occurring in the cells, and changed the ratio between different anthocyanins determining cell color, with an increase in the relative concentration of the less stable pigment molecules. The process by which magnesium treatment affects anthocyanin accumulation is still not clear. However, the results presented suggest at least part of its effect on anthocyanin accumulation stems from inhibition of the pigments' catabolism. When anthocyanin biosynthesis was inhibited, magnesium treatments prevented the constant degradation of anthocyanins in the cell suspension. Future understanding of the catabolic processes undergone by anthocyanins in plants may enable more efficient inhibition of this process and increased accumulation of these pigments, and possibly of additional phenolic compounds.