Transcriptomic Signatures of Ash (Fraxinus spp.) Phloem

Ash (Fraxinus spp.) is a dominant tree species throughout urban and forested landscapes of North America (NA). The rapid invasion of NA by emerald ash borer (Agrilus planipennis), a wood-boring beetle endemic to Eastern Asia, has resulted in the death of millions of ash trees and threatens billions more. Larvae feed primarily on phloem tissue, which girdles and kills the tree. While NA ash species including black (F. nigra), green (F. pennsylvannica) and white (F. americana) are highly susceptible, the Asian species Manchurian ash (F. mandshurica) is resistant to A. planipennis perhaps due to their co-evolutionary history. Little is known about the molecular genetics of ash. Hence, we undertook a functional genomics approach to identify the repertoire of genes expressed in ash phloem.Using 454 pyrosequencing we obtained 58,673 high quality ash sequences from pooled phloem samples of green, white, black, blue and Manchurian ash. Intriguingly, 45% of the deduced proteins were not significantly similar to any sequences in the GenBank non-redundant database. KEGG analysis of the ash sequences revealed a high occurrence of defense related genes. Expression analysis of early regulators potentially involved in plant defense (i.e. transcription factors, calcium dependent protein kinases and a lipoxygenase 3) revealed higher mRNA levels in resistant ash compared to susceptible ash species. Lastly, we predicted a total of 1,272 single nucleotide polymorphisms and 980 microsatellite loci, among which seven microsatellite loci showed polymorphism between different ash species.The current transcriptomic data provide an invaluable resource for understanding the genetic make-up of ash phloem, the target tissue of A. planipennis. These data along with future functional studies could lead to the identification/characterization of defense genes involved in resistance of ash to A. planipennis, and in future ash breeding programs for marker development

Variations in plant metallothioneins: the heavy metal hyperaccumulator Thlaspi caerulescens as a study case

Plant metallothioneins (MTs) are extremely diverse and are thought to be involved in metal homeostasis or detoxification. Thlaspi caerulescens is a model Zn/Cd hyperaccumulator and thus constitutes an ideal system to study the variability of these MTs. Two T. caerulescens cDNAs (accession: 665511; accession: 665515), that are highly homologous to type 1 and type 2 Arabidopsis thaliana MTs, have been isolated using a functional screen for plant cDNAs that confer Cd tolerance to yeast. However, TcMT1 has a much shorter N-terminal domain than that of A. thaliana and so lacks Cys motifs conserved through all the plant MTs classified as type 1. A systematic search in plant databases allowed the detection of MT-related sequences. Sixty-four percent fulfil the criteria for MT classification described in Cobbett and Goldsbrough (2002) and further extend our knowledge about other conserved residues that might play an important role in plant MT structure. In addition, 34% of the total MT-related sequences cannot be classified strictly as they display modifications in the conserved residues according to the current plant MTs' classification. The significance of this variability in plant MT sequences is discussed. Functional complementation in yeast was used to assess whether these variations may alter the MTs' function in T. caerulescens. Regulation of the expression of MTs in T. caerulescens was also investigated. TcMT1 and TcMT2 display higher expression in T. caerulescens than in A. thaliana. Moreover, their differential expression patterns in organs and in response to metal exposure, suggest that the two types of MTs may have diverse roles and functions in T. caerulescens. © Springer-Verlag 2005.Journal ArticleResearch Support, Non-U.S. Gov'tSCOPUS: ar.jinfo:eu-repo/semantics/publishe

Zinc accumulation and distribution over tissues in Noccaea сaerulescens in nature and in hydroponics: a comparison

Aims: Zinc distribution at the tissue level is studied almost exclusively in lab-grown plants. It is essential to establish to what extent the patterns observed in lab-grown plants are corresponding with those in nature. To this end, we compared Zn localization in Noccaea caerulescens growing in its natural environment, a zinc/lead mine tailing, with that in hydroponically grown plants of the same origin. Methods: Zinc concentrations in plants and soil were determined by flame AAS and Zn localization in leaf tissues was studied using Zn indicators Zincon and Zinpyr-1. Results: The mean Zn concentration in plants at the mine tailings was around 15,000 mg/kg DW, which corresponded well with the Zn concentration in the leaves of plants grown at 1600 μM Zn in the nutrient solution. The Zn distribution patterns in leaves of plants sampled from the mine and plants grown in hydroponics were identical. Zn-dependent staining was the most intensive in water-storage epidermal cells, guard cells and vascular bundles, and less intensive in subsidiary and mesophyll cells. Conclusions: Zinc distribution in hydroponically grown plants is representative for plants in nature. Preferential Zn sequestration in leaves, particularly in water-storage epidermal cells, restricts metal accumulation in mesophyll and contributes to Zn hypertolerance