Localized gene expression changes during adventitious root formation in black walnut (Juglans nigra L.)

Cutting propagation plays a large role in the forestry and horticulture industries where superior genotypes need to be clonally multiplied. Integral to this process is the ability of cuttings to form adventitious roots. Recalcitrance to adventitious root development is a serious hurdle for many woody plant propagation systems including black walnut (Juglans nigra L.), an economically valuable species. The inability of black walnut to reliably form adventitious roots limits propagation of superior genotypes. Adventitious roots originate from different locations, and root induction is controlled by many environmental and endogenous factors. At the molecular level, however, the regulation of adventitious root formation is still poorly understood. In order to elucidate the transcriptional changes during adventitious root development in black walnut, we used quantitative real-time polymerase chain reaction to measure the expression of nine key genes regulating root formation in other species. Using our previously developed spatially explicit timeline of adventitious root development in black walnut softwood cuttings, we optimized a laser capture microdissection protocol to isolate RNA from cortical, phloem fiber and phloem parenchyma cells throughout adventitious root formation. Laser capture microdissection permitted high-resolution, site-specific analysis of gene expression that differentiated between participatory and non-participatory root progenitor cells. Results indicated mRNA abundance was altered in all nine rooting-related genes in response to auxin treatment in both juvenile and mature cuttings. SCARECROW LIKE-1 (SCL) had the greatest change in expression in juvenile rooting-competent cells at days 16 and 18, with a 24- and 23-fold increase relative to day 0, respectively. Tissues not linked to root organogenesis had little change in SCL expression at similar time points. AUXIN RESPONSE FACTOR (ARF)6 and ARF8 as well as SHORTROOT expression also increased 2- to 4-fold in rooting-competent tissue. The greatest transcript abundance in rooting-competent cuttings was restricted to root progenitor cells, while recalcitrant cuttings had a diffuse mRNA signal among tissue types.

Candidate gene association mapping for winter survival and spring regrowth in perennial ryegrass

Perennial ryegrass (Lolium perenne L.) is a widely cultivated cool-season grass species because of its high quality for forage and turf. Susceptibility to freezing damage limits its further use in temperate zones. The objective of this study was to identify candidate genes significantly associated with winter survival and spring regrowth in a global collection of 192 perennial ryegrass accessions. Significant differences in winter survival (WS), percentage of canopy green cover (CGC), chlorophyll index (Chl), and normalized difference vegetation index (NDVI) were found among accessions. After controlling population structure, LpLEA3 encoding a late embryogenesis abundant group 3 protein and LpCAT encoding a catalase were associated with CGC and Chl, while LpMnSOD encoding a magnesium superoxide dismutase and LpChl Cu-ZnSOD encoding a chlorophyll copper-zinc superoxide dismutase were associated with NDVI or Chl. Significant association was also discovered between C-repeat binding factor LpCBF1b and WS. Three sequence variations identified in LpCAT, LpMnSOD, and LpChl Cu-ZnSOD were synonymous substitutions, whereas one pair of adjacent single nucleotide polymorphisms (SNPs) in LpLEA3 and one SNP in LpCBF1b resulted in amino acid change. The results demonstrated that allelic variation in LpLEA3 and LpCBF1b was closely related to winter survival and spring regrowth in perennial ryegrass.

Agrobacterium-mediated genetic transformation and plant regeneration of the hardwood tree species Fraxinus profunda

This transformation and regeneration protocol provides an integral framework for the genetic improvement of Fraxinus profunda (pumpkin ash) for future development of plants resistant to the emerald ash borer. Using mature hypocotyls as the initial explants, an Agrobacterium tumefaciens-mediated genetic transformation system was successfully developed for pumpkin ash (Fraxinus profunda). This transformation protocol is an invaluable tool to combat the highly aggressive, non-native emerald ash borer (EAB), which has the potential to eliminate native Fraxinus spp. from the natural landscape. Hypocotyls were successfully transformed with Agrobacterium strain EHA105 harboring the pq35GR vector, containing an enhanced green fluorescent protein (EGFP) as well as a fusion gene between neomycin phosphotransferase (nptII) and gusA. Hypocotyls were cultured for 7 days on Murashige and Skoog (MS) medium with 22.2 μM 6-benzyladenine (BA), 4.5 μM thidiazuron (TDZ), 50 mg L(-1) adenine hemisulfate (AS), and 10 % coconut water (CW) prior to transformation. Hypocotyls were transformed using 90 s sonication plus 10 min vacuum infiltration after Agrobacterium was exposed to 100 μM acetosyringone for 1 h. Adventitious shoots were regenerated on MS medium with 22.2 μM BA, 4.5 μM TDZ, 50 mg L(-1) AS, 10 % CW, 400 mg L(-1) timentin, and 20 mg L(-1) kanamycin. Timentin at 400 and 20 mg L(-1) kanamycin were most effective at controlling Agrobacterium growth and selecting for transformed cells, respectively. The presence of nptII, GUS (β-glucuronidase), and EGFP in transformed plants was confirmed using polymerase chain reaction (PCR), while the expression of EGFP was also confirmed through fluorescent microscopy and reverse transcription-PCR. This transformation protocol provides an integral foundation for future genetic modifications of F. profunda to provide resistance to EAB.

Isolation and characterization of a TERMINAL FLOWER 1 homolog from Prunus serotina Ehrh

Flowering control is one of the several strategies for gene containment of transgenic plants. TERMINAL FLOWER 1 (TFL1) is known to be involved in the transcriptional repression of genes for inflorescence development. Two TFL1 transcripts with different 3' UTR were cloned from black cherry (Prunus serotina Ehrh.) using reverse transcription-polymerase chain reaction (RT-PCR) and rapid amplification of cDNA ends (RACE). Corresponding to the two TFL1 transcripts, two PsTFL1 gene sequences, 1248 bp and 1579 bp, were obtained and both contained the same 519 bp coding region which encoded a putative protein of 172 amino acid residues. The phylogenetic analysis of the amino acid sequences showed high identity of PsTFL1 to TFL1 orthologs of other Prunus species, including Yoshino cherry (Prunus × yedoensis Matsum.), peach (Prunus persica (L.) Batsch), apricot (Prunus armeniaca L.) and Japanese apricot (Prunus mume Sieb. et Zucc.). The real-time quantitative PCR detected a single copy of PsTFL1 gene sequences in the black cherry genome with two alleles. The gene expression of PsTFL1 was examined in several tissues including the stems, leaves, shoot tips, and vegetative and floral buds. The highest mRNA level was detected in shoot tips, and the lowest level in the leaves. Transgenic Arabidopsis thaliana (L.) Heynh. plants overexpressing PsTFL1 showed significantly delayed flowering. These plants also showed largely increased vegetative growth, plant height, number of nodes, trichome density, and the conversion of flower to shoot was observed at each node and shoot apex.

Association of candidate genes with drought tolerance traits in diverse perennial ryegrass accessions

Drought is a major environmental stress limiting growth of perennial grasses in temperate regions. Plant drought tolerance is a complex trait that is controlled by multiple genes. Candidate gene association mapping provides a powerful tool for dissection of complex traits. Candidate gene association mapping of drought tolerance traits was conducted in 192 diverse perennial ryegrass (Lolium perenne L.) accessions from 43 countries. The panel showed significant variations in leaf wilting, leaf water content, canopy and air temperature difference, and chlorophyll fluorescence under well-watered and drought conditions across six environments. Analysis of 109 simple sequence repeat markers revealed five population structures in the mapping panel. A total of 2520 expression-based sequence readings were obtained for a set of candidate genes involved in antioxidant metabolism, dehydration, water movement across membranes, and signal transduction, from which 346 single nucleotide polymorphisms were identified. Significant associations were identified between a putative LpLEA3 encoding late embryogenesis abundant group 3 protein and a putative LpFeSOD encoding iron superoxide dismutase and leaf water content, as well as between a putative LpCyt Cu-ZnSOD encoding cytosolic copper-zinc superoxide dismutase and chlorophyll fluorescence under drought conditions. Four of these identified significantly associated single nucleotide polymorphisms from these three genes were also translated to amino acid substitutions in different genotypes. These results indicate that allelic variation in these genes may affect whole-plant response to drought stress in perennial ryegrass.

Agrobacterium-mediated transformation of Fraxinus pennsylvanica hypocotyls and plant regeneration

A genetic transformation protocol for green ash (Fraxinus pennsylvanica) hypocotyl explants was developed. Green ash hypocotyls were transformed using Agrobacterium tumefaciens strain EHA105 harboring binary vector pq35GR containing the neomycin phosphotransferase (nptII) and beta-glucuronidase (GUS) fusion gene, and an enhanced green fluorescent protein gene. Pre-cultured hypocotyl explants were transformed in the presence of 100 microM acetosyringone using 90 s sonication plus 10 min vacuum-infiltration. Kanamycin at 20 mg l(-1) was used for selecting transformed cells. Adventitious shoots regenerated on Murashige and Skoog medium supplemented with 13.3 microM 6-benzylaminopurine, 4.5 microM thidiazuron, 50 mg l(-1) adenine sulfate, and 10% coconut water. GUS- and polymerase chain reaction (PCR)-positive shoots from the cut ends of hypocotyls were produced via an intermediate callus stage. Presence of the GUS and nptII genes in GUS-positive shoots were confirmed by PCR and copy number of the nptII gene in PCR-positive shoots was determined by Southern blotting. Three transgenic plantlets were acclimatized to the greenhouse. This transformation and regeneration system using hypocotyls provides a foundation for Agrobacterium-mediated transformation of green ash. Studies are underway using a construct containing the Cry8Da protein of Bacillus thuringiensis for genetic transformation of green ash.

Salicylate and catechol levels are maintained in nahG transgenic poplar

Metabolic profiling was used to investigate the molecular phenotypes of a transgenic Populus tremula x P. alba hybrid expressing the nahG transgene, a bacterial gene encoding salicylate hydroxylase that converts salicylic acid to catechol. Despite the efficacy of this transgenic approach to reduce salicylic acid levels in other model systems and thereby elucidate roles for salicylic acid in plant signaling, transgenic poplars had similar foliar levels of salicylic acid and catechol to that of non-transformed controls and exhibited no morphological phenotypes. To gain a deeper understanding of the basis for these observations, we analyzed metabolic profiles of leaves as influenced by transgene expression. Expression of nahG decreased quinic acid conjugates and increased catechol glucoside, while exerting little effect on levels of salicylic acid and catechol, the substrate and product, respectively, of the nahG enzyme. This suggests a biological role of elevated constitutive salicylic acid levels in Populus, in contrast to other plant systems in which nahG dramatically reduces salicylic acid levels.

Improving disease resistance of butternut (Juglans cinerea), a threatened fine hardwood: a case for single-tree selection through genetic improvement and deployment

Approaches for the development of disease-resistant butternut (Juglans cinerea L.) are reviewed. Butternut is a threatened fine hardwood throughout its natural range in eastern North America because of the invasion of the exotic fungus, Sirococcus clavigignenti-juglandacearum Nair, Kostichka and Kuntz, which causes butternut canker. Early efforts were made to identify and collect putatively resistant germ plasm, identify vectors and to characterize the disease. More recently, molecular techniques have been employed to genetically characterize both the pathogen and the resistant germ plasm. Much of the host resistance may originate from hybridization with a close Asian relative, Japanese walnut (Juglans ailanthifolia Carr.), and from a few natural phenotypic variants. Further genetic characterization is needed before classical breeding or genetic modification can be used to produce canker-resistant trees.