Lilium regale Wilson WRKY3 modulates an antimicrobial peptide gene, LrDef1, during response to Fusarium oxysporum

BACKGROUND

WRKY transcription factors (TFs) play vital roles in plant growth and development, secondary metabolite synthesis, and response to biotic and abiotic stresses. In a previous transcriptome sequencing analysis of Lilium regale Wilson, we identified multiple WRKY TFs that respond to exogenous methyl jasmonate treatment and lily Fusarium wilt (Fusarium oxysporum).

RESULTS

In the present study, the WRKY TF LrWRKY3 was further analyzed to reveal its function in defense response to F. oxysporum. The LrWRKY3 protein was localized in the plant cell nucleus, and LrWRKY3 transgenic tobacco lines showed higher resistance to F. oxysporum compared with wild-type (WT) tobacco. In addition, some genes related to jasmonic acid (JA) biosynthesis, salicylic acid (SA) signal transduction, and disease resistance had higher transcriptional levels in the LrWRKY3 transgenic tobacco lines than in the WT. On the contrary, L. regale scales transiently expressing LrWRKY3 RNA interference fragments showed higher sensitivity to F. oxysporum infection. Moreover, a F. oxysporum-induced defensin gene, Def1, was isolated from L. regale, and the recombinant protein LrDef1 isolated and purified from Escherichia coli possessed antifungal activity to several phytopathogens, including F. oxysporum. Furthermore, co-expression of LrWRKY3 and the LrDef1 promoter in tobacco enhanced the LrDef1 promoter-driven expression activity.

CONCLUSIONS

These results clearly indicate that LrWRKY3 is an important positive regulator in response to F. oxysporum infection, and one of its targets is the antimicrobial peptide gene LrDef1.

WRKY Transcription Factors Actively Respond to Fusarium oxysporum in Lilium regale

The WRKY transcription factors form a plant-specific superfamily important for regulating plant development, stress responses, and hormone signal transduction. In this study, many WRKY genes (LrWRKY1-35) were identified in Lilium regale, which is a wild lily species highly resistant to Fusarium wilt. These WRKY genes were divided into three classes (I to III) based on a phylogenetic analysis. The Class-II WRKY transcription factors were further divided into five subclasses (IIa, IIb, IIc, IId, and IIe). Moreover, the gene expression patterns based on a quantitative real-time PCR analysis revealed the WRKY genes were differentially expressed in the L. regale roots, stems, leaves, and flowers. Additionally, the expression of the WRKY genes was affected by an infection by Fusarium oxysporum as well as by salicylic acid, methyl jasmonate, ethephon, and hydrogen peroxide treatments. Moreover, the LrWRKY1 protein was localized to the nucleus of onion epidermal cells. The recombinant LrWRKY1 protein purified from Escherichia coli bound specifically to DNA fragments containing the W-box sequence, and a yeast one-hybrid assay indicated that LrWRKY1 can activate transcription. A co-expression assay in tobacco (Nicotiana tabacum) confirmed LrWRKY1 regulates the expression of LrPR10-5. Furthermore, the overexpression of LrWRKY1 in tobacco and the Oriental hybrid 'Siberia' (susceptible to F. oxysporum) increased the resistance of the transgenic plants to F. oxysporum. Overall, LrWRKY1 regulates the expression of the resistance gene LrPR10-5 and is involved in the defense response of L. regale to F. oxysporum. This study provides valuable information regarding the expression and functional characteristics of L. regale WRKY genes.

Recent Progress in Enhancing Fungal Disease Resistance in Ornamental Plants

Fungal diseases pose a major threat to ornamental plants, with an increasing percentage of pathogen-driven host losses. In ornamental plants, management of the majority of fungal diseases primarily depends upon chemical control methods that are often non-specific. Host basal resistance, which is deficient in many ornamental plants, plays a key role in combating diseases. Despite their economic importance, conventional and molecular breeding approaches in ornamental plants to facilitate disease resistance are lagging, and this is predominantly due to their complex genomes, limited availability of gene pools, and degree of heterozygosity. Although genetic engineering in ornamental plants offers feasible methods to overcome the intrinsic barriers of classical breeding, achievements have mainly been reported only in regard to the modification of floral attributes in ornamentals. The unavailability of transformation protocols and candidate gene resources for several ornamental crops presents an obstacle for tackling the functional studies on disease resistance. Recently, multiomics technologies, in combination with genome editing tools, have provided shortcuts to examine the molecular and genetic regulatory mechanisms underlying fungal disease resistance, ultimately leading to the subsequent advances in the development of novel cultivars with desired fungal disease-resistant traits, in ornamental crops. Although fungal diseases constitute the majority of ornamental plant diseases, a comprehensive overview of this highly important fungal disease resistance seems to be insufficient in the field of ornamental horticulture. Hence, in this review, we highlight the representative mechanisms of the fungal infection-related resistance to pathogens in plants, with a focus on ornamental crops. Recent progress in molecular breeding, genetic engineering strategies, and RNAi technologies, such as HIGS and SIGS for the enhancement of fungal disease resistance in various important ornamental crops, is also described.

Development, progress and future prospects in cryobiotechnology of Lilium spp

Lilium is one of the most popular flower crops worldwide, and some species are also used as vegetables and medicines. The availability of and easy access to diverse Lilium genetic resources are essential for plant genetic improvements. Cryopreservation is currently considered as an ideal means for the long-term preservation of plant germplasm. Over the last two decades, great efforts have been exerted in studies of Lilium cryopreservation and progress has been made in the successful cryopreservation of pollen, seeds and shoot tips in Lilium. Genes that exist in Lilium, including those that regulate flower shape, color and size, and that are resistant to cold stress and diseases caused by fungi and viruses, provide a rich source of valuable genetic resources for breeding programs to create novel cultivars required by the global floriculture and ornamental markets. Successful cryopreservation of Lilium spp. is a way to preserve these valuable genes. The present study provides updated and comprehensive information about the development of techniques that have advanced Lilium cryopreservation. Further ideas are proposed to better direct future studies on Lilium cryobiotechnology.