Evolutionary Expansion of WRKY Gene Family in Banana and Its Expression Profile during the Infection of Root Lesion Nematode, Pratylenchus coffeae.

The WRKY family of transcription factors orchestrate the reprogrammed expression of the complex network of defense genes at various biotic and abiotic stresses. Within the last 96 million years, three rounds of Musa polyploidization events had occurred from selective pressure causing duplication of MusaWRKYs with new activities. Here, we identified a total of 153 WRKY transcription factors available from the DH Pahang genome. Based on their phylogenetic relationship, the MusaWRKYs available with complete gene sequence were classified into the seven common WRKY sub-groups. Synteny analyses data revealed paralogous relationships, with 17 MusaWRKY gene pairs originating from the duplication events that had occurred within the Musa lineage. We also found 15 other MusaWRKY gene pairs originating from much older duplication events that had occurred along Arecales and Poales lineage of commelinids. Based on the synonymous and nonsynonymous substitution rates, the fate of duplicated MusaWRKY genes was predicted to have undergone sub-functionalization in which the duplicated gene copies retain a subset of the ancestral gene function. Also, to understand the regulatory roles of MusaWRKY during a biotic stress, Illumina sequencing was performed on resistant and susceptible cultivars during the infection of root lesion nematode, Pratylenchus coffeae. The differential WRKY gene expression analysis in nematode resistant and susceptible cultivars during challenged and unchallenged conditions had distinguished: 1) MusaWRKYs participating in general banana defense mechanism against P.coffeae common to both susceptible and resistant cultivars, 2) MusaWRKYs that may aid in the pathogen survival as suppressors of plant triggered immunity, 3) MusaWRKYs that may aid in the host defense as activators of plant triggered immunity and 4) cultivar specific MusaWRKY regulation. Mainly, MusaWRKY52, -69 and -92 are found to be P.coffeae specific and can act as activators or repressors in a defense pathway. Overall, this preliminary study in Musa provides the basis for understanding the evolution and regulatory mechanism of MusaWRKY during nematode stress.

Transcriptional analysis of distant signaling induced by insect elicitors and mechanical wounding in Zea mays.

When plants are under insect herbivore attack defensive measures are activated not only locally, but also in distant and systemic tissues. While insect elicitors (IE) abundant in the oral secretions of the attacking herbivore are essential in the regulation of induced defenses, little is known about their effects on systemic defense signaling in maize (Zea mays). The goal of this study was therefore to identify genetic markers that can be used to further characterize local and systemic signaling events induced by IE or mechanical wounding (MW). We selected genes for this study based on their putative involvement in signaling (allene oxide synthase), regulation of gene expression (transcription factor MYC7), and in direct defenses (ribosome inactivating protein) and analyzed their expression in different sections of the treated leaf as well as in systemic parts of the same plant. We found the most significant transcript accumulation of the selected genes after treatment with insect elicitors in those parts with increased JA levels. Additionally, treatment with IE did also induce the accumulation of MYC7 transcripts in basal parts of the treated leaf and systemically. MW, in contrast, did induce RIP and AOS only locally, but not MYC7. This local suppression of MYC7 was further studied by adding glutathione (GSH) as an electron donor to MW plants to quench putative α, ß-unsaturated carbonyls, which build up to significant levels around the damage site. Indeed, GSH-treated MW plants accumulated MYC7 at the damage site and also produced more volatiles, suggesting a putative redox-regulatory element being involved in the suppression of MYC7. The results presented herein provide evidence for the specific induction of distant signaling events triggered by IE, most likely through electric signaling. Additionally, a putative role for MW-induced α, ß-unsaturated carbonyls in the transcriptional regulation of defense genes was discovered.

Low concentrations of salicylic acid stimulate insect elicitor responses in Zea mays seedlings.

Salicylic acid (SA) generally is thought to suppress jasmonic acid (JA) related signaling events. However, when we treated the roots of corn seedlings overnight with low physiological concentrations of SA (50 µM), we found a priming effect of this pretreatment on typical insect elicitor (IE)-induced responses in the leaves of these plants. IE-induced JA was more than 2-fold up regulated in SA-pretreated plants. Consequently, IE-induced volatile organic compounds (VOC) release also was significantly increased. In contrast, when corn seedlings were treated with SA overnight and then mechanically damaged, we found no significant differences in JA accumulation. We also found that the application of even lower concentrations of SA (5 µM) had no significant effect on IE-induced responses, while higher concentrations (500 µM) inhibited IE-induced JA accumulation. Likewise, shorter exposure to SA did not affect subsequent JA accumulation induced by IE or mechanical wounding. These results provide evidence for the existence of non-compatible defense priming by signaling molecules that usually are involved in a conflictive defense signaling pathway and suggests common elements in the regulation of priming plant defense responses.