Transcriptome Analysis of Eucalyptus grandis Implicates Brassinosteroid Signaling in Defense Against Myrtle Rust (Austropuccinia psidii)

Eucalyptus grandis, in its native Australian range, varies in resistance to Austropuccinia psidii (syn. Puccinia psidii). The biotrophic rust fungus, A. psidii is the causal agent of myrtle rust and poses a serious threat to Australian biodiversity. The pathogen produces yellow pustules of urediniospores on young leaves and shoots, resulting in shoot tip dieback, stunted growth, and death. Dissecting the underlying mechanisms of resistance against this pathogen will contribute to improved breeding and control strategies to mitigate its devastating effects. The aim of this study was to determine the molecular dialogue between E. grandis and A. psidii, using an RNA-sequencing approach. Resistant and susceptible E. grandis seedlings grown from seed collected across its natural range were inoculated with the pandemic biotype of A. psidii. The leaf tissue was harvested at 12-h post inoculation (hpi), 1-day post inoculation (dpi), 2-dpi and 5-dpi and subjected to RNA-sequencing using Illumina 50 bp PE reads to a depth of 40 million reads per sample. Differential gene expression and gene ontology enrichment indicated that the resistant seedlings showed controlled, coordinated responses with a hypersensitive response, while the susceptible seedlings showed no systemic response against myrtle rust. Brassinosteroid signaling was apparent as an enriched term in the resistant interaction at 2-dpi, suggesting an important role of this phytohormone in defense against the pathogen. Brassinosteroid mediated signaling genes were also among the candidate genes within two major disease resistance loci (Puccinia psidii resistance), Ppr3 and Ppr5. While brassinosteroids have been tagged as positive regulators in other plant disease resistance interactions, this is the first report in the Eucalyptus – Austropuccinia psidii interaction. Furthermore, several putative resistance genes, underlying known resistance loci and implicated in the interaction have been identified and highlighted for future functional studies. This study provided further insights into the molecular interactions between E. grandis and A. psidii, contributing to our understanding of this pathosystem.

Modificaciones histológicas foliares causadas por organismos fúngicos en tres especies de Myrtaceae.

La familia Myrtaceae es una de las más numerosas en Argentina. En la provincia de Tucumán habitan 11 especies nativas, de las cuales Blepharocalyx salicifolius (Kunth) O. Berg, Eugenia uniflora L. y Myrcianthes pungens (O.Berg) D. Legrand presentan importancia medicinal, alimenticia y forestal. La información sobre las especies fúngicas y las modificaciones histológicas foliares que ellos producen es escasa. Por lo tanto, el objetivo de la presente investigación fue identificar las especies de hongos en hojas de las tres especies arbóreas y describir las modificaciones anatómicas causadas por la presencia de los mismos. Se trabajó con material fresco al que se aplicó técnicas y tinciones convencionales para su estudio. Se identificaron 12 especies fúngicas, 10 de ellas se citan por primera vez para la Argentina, en los tres hospedantes arbóreos estudiados. Puccinia psidii y Alternaria sp. se encontraron con mayor frecuencia. Los tejidos foliares más afectados fueron el parénquima esponjoso y el parénquima en empalizada. La lesión más significativa en la lámina foliar se observó en E. uniflora, producida por Microsphaerospis arundinis. Las especies fúngicas identificadas amplían el conocimiento de la diversidad de hongos asociados a hojas en especies arbóreas nativas de la provincia de Tucumán.

Seleção de genótipos de eucalipto resistentes à ferrugem (Puccinia psidii) através de parâmetros monocíclicos

RESUMO A variabilidade genética dentro do gênero Eucalyptus para resistência à doenças e a fatores abióticos estão presentes, particularmente, com a ferrugem do eucalipto (Puccinia psidii WINTER), cuja primeira medida de controle é evitar o plantio comercial de materiais suscetíveis. A busca de caracteres quantitativos da resistência pode levar a genótipos superiores com resistência mais durável. Por isso, este estudo objetivou identificar parâmetros monocíclicos de avaliação da ferrugem em eucalipto, mais proximamente relacionados à resistência genética e estabelecer um ranking com os genótipos de maior resistência à ferrugem para compor um programa de melhoramento genético, além de relacionar a resposta do hospedeiro às vias metabólicas de defesa. Para tanto, foi realizado um estudo comparativo entre a avaliação da ferrugem no campo, com auxilio de escala qualitativa de severidade da doença com parâmetros monocíclicos quantitativos da doença em ambiente controlado. O predomínio de indivíduos suscetíveis indicou possíveis distorções de segregação nos cruzamentos, corroborando com hipóteses de estudos anteriores. Os resultados demonstraram, que o gene de resistência presente nesta população é dependente da ação de outros genes indiretamente relacionados à resistência e que participam das vias metabólicas de defesa das plantas.

Beenken, L. (2017) Austropuccinia: a new genus name for the myrtle rust Puccinia psidii placed within the redefined family Sphaerophragmiaceae (Pucciniales) Phytotaxa 297 (1): 053–061.

N/A

Austropuccinia: a new genus name for the myrtle rust Puccinia psidii placed within the redefined family Sphaerophragmiaceae (Pucciniales)

The myrtle rust Puccinia psidii originates from South America but is nowadays a very important and invasive pathogen on several genera of Myrtaceae outside of its native area, and especially in Australia, a biodiversity hotspot of Myrtaceae. To date, the taxonomic position of P. psidii has been unclear. The present phylogenetic analyses as well as several former studies show that it does not belong to the genus Puccinia and appears outside of the family Pucciniales. It is, however, closely related to the genera Dasyspora, Puccorchidium, Sphenorchidium and Sphaerophragmium. Consequently, the new genus Austropuccinia has been erected and placed in the newly circumscribed family Sphaerophragmiaceae.