Abstract
BackgroundGymnosporangium spp. are fungal plant pathogens causing rust disease and most of them are known to infect two different host plants (heteroecious) with four spore stages (demicyclic). In the present study, we sequenced the transcriptome of G. japonicum teliospores on its host plant Juniperus chinensis and we performed comparison to the transcriptomes of G. yamadae and G. asiaticum at the same life stage, that happens in the same host but on different organs.ResultsFunctional annotation for the three Gymnosporangium species showed the expression of a conserved genetic program with the top abundant cellular categories corresponding to energy, translation and signal transduction processes, indicating that this life stage is particularly active. Moreover, the survey of predicted secretomes in the three Gymnosporangium transcriptomes revealed shared and specific genes encoding carbohydrate active enzymes and secreted proteins of unknown function that could represent candidate pathogenesis effectors. A transcript encoding a hemicellulase of the glycoside hydrolase 26 family, previously identified in other rust fungi, was particularly highly expressed suggesting a general role in rust fungi. The comparison between the transcriptomes of the three Gymnosporangium spp. and selected Pucciniales species in different taxonomical families allowed to identify lineage-specific protein families that may relate to the biology of teliospores in rust fungi. Among clustered gene families, 205, 200 and 152 proteins were specifically identified in G. japonicum, G. yamadae and G. asiaticum, respectively, including candidate effectors expressed in teliospores.ConclusionsThis comprehensive comparative transcriptomics study of three Gymnosporangium spp. identified gene functions and metabolic pathways particularly expressed in teliospores, a stage of the life cycle that is mostly overlooked in rust fungi. Secreted protein encoding transcripts expressed in teliospores may reveal new candidate effectors related to pathogenesis. Although this spore stage is not involved in host plant infection but in the production of basidiospores infecting plants in the Amygdaloideae, we speculate that candidate effectors may be expressed as early as the teliospore stage for preparing further infection by basidiospores.
Highlights
IntroductionGymnosporangium spp. are fungal plant pathogens causing rust disease and most of them are known to infect two different host plants (heteroecious) with four spore stages (demicyclic)
Gymnosporangium spp. are fungal plant pathogens causing rust disease and most of them are known to infect two different host plants with four spore stages
Telia formation is a relatively well conserved feature within Gymnosporangium spp. [6], unlike the two other Gymnosporangium species commonly found on J. chinensis, G. yamadae and G. asiaticum, telia formed by G. japonicum are mainly found on hard tissues such as trunks and branches [2]
Summary
Gymnosporangium spp. are fungal plant pathogens causing rust disease and most of them are known to infect two different host plants (heteroecious) with four spore stages (demicyclic). G. japonicum, Gymnosporangium asiaticum and Gymnospo rangium yamadae are the three most widespread Gymnosporangium species in Asia and the two later species are the causal agents of Japanese apple rust and Japanese pear rust diseases, respectively [6]. They all share J. chinensis as a telial host, but the parasitic symptoms are formed on different organs of the tree (Fig. 1). There is no molecular data available for G. japonicum
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