Helen H. Tai

Comparative transcriptional analysis provides new insights into the molecular basis of adventitious rooting recalcitrance in Eucalyptus

Adventitious rooting (AR) is essential in clonal propagation. Eucalyptus globulus is relevant for the cellulose industry due to its low lignin content. However, several useful clones are recalcitrant to AR, often requiring exogenous auxin, adding cost to clonal garden operations. In contrast, E. grandis is an easy-to-root species widely used in clonal forestry. Aiming at contributing to the elucidation of recalcitrance causes in E. globulus, we conducted a comparative analysis with these two species differing in rooting competence, combining gene expression and anatomical techniques. Recalcitrance in E. globulus is reversed by exposure to exogenous indole-3-acetic acid (IAA), which promotes important gene expression modifications in both species. The endogenous content of IAA was significantly higher in E. grandis than in E. globulus. The cambium zone was identified as an active area during AR, concentrating the first cell divisions. Immunolocalization assay showed auxin accumulation in cambium cells, further indicating the importance of this region for rooting. We then performed a cambium zone-specific gene expression analysis during AR using laser microdissection. The results indicated that the auxin-related genes TOPLESS and IAA12/BODENLOS and the cytokinin-related gene ARR1may act as negative regulators of AR, possibly contributing to the hard-to-root phenotype of E. globulus.

Decreased defense gene expression in tolerance versus resistance to Verticillium dahliae in potato

Verticillium dahliae Kleb., a soil-borne fungus that colonizes vascular tissues, induces wilting, chlorosis and early senescence in potato. Difference in senescence timing found in two diploid potato clones, 07506-01 and 12120-03, was studied and genetic variation in response to V. dahliae infection was identified as a causal factor. The clone, 07506-01, was infected with V. dahliae but did not develop symptoms, indicating tolerance to the pathogen. The other diploid clone, 12120-03 had low levels of pathogen with infection and moderate symptoms indicating partial resistance. 07506-01 was found to carry two susceptible alleles of the Ve2 gene and 12120-03 carried one Ve2 resistant and one susceptible allele. Infected leaves of the two clones were compared using gene expression profiling with the Potato Oligonucleotide Chip Initiative (POCI) microrarray. The results provide further evidence for differences in response of the two clones to infection with V. dahliae. Chlorophyll biosynthesis was higher in the tolerant 07506-01 compared to partially resistant 12120-03. On the other hand, expression of fungal defense genes, Ve resistance genes and defense phytohormone biosynthetic enzyme genes was decreased in 07506-01 compared to 12120-03 suggesting defense responses were suppressed in tolerance compared to resistance. Transcription factor gene expression differences pointed to the WRKY family as potential regulators of V. dahliae responses in potato.

Comparative metabolite profiling of Solanum tuberosum against six wild Solanum species with Colorado potato beetle resistance.

The Colorado potato beetle Leptinotarsa decemlineata (Say) (CPB) is a coleopteran herbivore that feeds on the foliage on Solanum species, in particular, potato. Six resistant wild Solanum species were identified, and two of these species had low levels of glycoalkaloids. Comparative analysis of the untargeted metabolite profiles of the foliage using UPLC-qTOF-MS was done to find metabolites shared between the wild species but not with Solanum tuberosum (L.) to identify resistance-related metabolites. It was found that only S. tuberosum produced the triose glycoalkaloids solanine and chaconine. Instead, the six wild species produced glycoalkaloids that shared in common tetrose sugar side chains. Additionally, there were non-glycoalkaloid metabolites associated with resistance including hydroxycoumarin and a phenylpropanoid, which were produced in all wild species but not in S. tuberosum.

The nitrogen responsive transcriptome in potato (Solanum tuberosum L.) reveals significant gene regulatory motifs

Nitrogen (N) is the most important nutrient for the growth of potato (Solanum tuberosum L.). Foliar gene expression in potato plants with and without N supplementation at 180 kg N ha−1 was compared at mid-season. Genes with consistent differences in foliar expression due to N supplementation over three cultivars and two developmental time points were examined. In total, thirty genes were found to be over-expressed and nine genes were found to be under-expressed with supplemented N. Functional relationships between over-expressed genes were found. The main metabolic pathway represented among differentially expressed genes was amino acid metabolism. The 1000 bp upstream flanking regions of the differentially expressed genes were analysed and nine overrepresented motifs were found using three motif discovery algorithms (Seeder, Weeder and MEME). These results point to coordinated gene regulation at the transcriptional level controlling steady state potato responses to N sufficiency.

Pathogen and Pest Responses Are Altered Due to RNAi-Mediated Knockdown of GLYCOALKALOID METABOLISM 4 in Solanum tuberosum

Steroidal glycoalkaloids (SGAs) are major secondary metabolites constitutively produced in cultivated potato Solanum tuberosum, and α-solanine and α-chaconine are the most abundant SGAs. SGAs are toxic to humans at high levels but their role in plant protection against pests and pathogens is yet to be established. In this study, levels of SGAs in potato were reduced by RNA interference (RNAi)-mediated silencing of GLYCOALKALOID METABOLISM 4 (GAME4)-a gene encoding cytochrome P450, involved in an oxidation step in the conversion of cholesterol to SGA aglycones. Two GAME4 RNAi lines, T8 and T9, were used to investigate the effects of manipulation of the SGA biosynthetic pathway in potato. Growth and development of an insect pest, Colorado potato beetle (CPB), were affected in these lines. While no effect on CPB leaf consumption or weight gain was observed, early instar larval death and accelerated development of the insect was found while feeding on leaves of GAME4 RNAi lines. Modulation of SGA biosynthetic pathway in GAME4 RNAi plants was associated with a larger alteration to the metabolite profile, including increased levels of one or both the steroidal saponins or phytoecdysteroids, which could affect insect mortality as well as development time. Colonization by Verticillium dahliae on GAME4 RNAi plants was also tested. There were increased pathogen levels in the T8 GAME4 RNAi line but not in the T9. Metabolite differences between T8 and T9 were found and may have contributed to differences in V. dahliae infection. Drought responses created by osmotic stress were not affected by modulation of SGA biosynthetic pathway in potato.

Ethylene is Involved but Plays a Limited Role in Tomato Chlorotic Dwarf Viroid-Induced Symptom Development in Tomato

Abstract To investigate whether ethylene is involved in tomato plants in response to the invasion of viroid, tomato cv . Pearson and its ethylene-insensitive mutant Never ripe ( Nr ) was challenged with Tomato chlorotic dwarf viroid (TCDVd). Both Pearson and Nr plants developed various symptoms including dwarf, bunch, reduced leaf size, leaf chlorosis, and necrosis at 3-wk-post-inoculation and thereafter even though the stunting in Nr was not as severe as in Pearson plants. The sizes of fruits were also significantly reduced in the viroid-infected plants. Treatments of Pearson with ethephon, an ethylenereleasing compound, mimicked TCDVd in induction of stunting and pathogenesis-related (PR) genes in the plants. Pretreatment of Pearson plants prior to TCDVd-inoculation or ethephon application with silver thiosulfate (STS), an ethylene action inhibitor, partially suppressed the viroid- or ethephon-induced symptoms as well as PR gene expression. In addition, Nr and STS-treated Pearson exhibited a slightly lower viroid titre in comparison to the control Pearson plants. These results suggest that ethylene is involved but plays a limited role in TCDVd-induced symptom development.

Verticillium dahliae Disease Resistance and the Regulatory Pathway for Maturity and Tuberization in Potato

Verticillium wilt resistance is linked to control of tuberization in potato. The StCDF1 tuberization gene is epistatic to the Ve2 resistance gene. eQTL can be used to examine gene networks for complex traits.