Transcriptome analysis of the fungal pathogen Rosellinia necatrix during infection of a susceptible avocado rootstock identifies potential mechanisms of pathogenesis.

BACKGROUND: White root rot disease caused by Rosellinia necatrix is one of the most important threats affecting avocado productivity in tropical and subtropical climates. Control of this disease is complex and nowadays, lies in the use of physical and chemical methods, although none have proven to be fully effective. Detailed understanding of the molecular mechanisms underlying white root rot disease has the potential of aiding future developments in disease resistance and management. In this regard, this study used RNA-Seq technology to compare the transcriptomic profiles of R. necatrix during infection of susceptible avocado 'Dusa' roots with that obtained from the fungus cultured in rich medium. RESULTS: The transcriptomes from three biological replicates of R. necatrix colonizing avocado roots (RGA) and R. necatrix growing on potato dextrose agar media (RGPDA) were analyzed using Illumina sequencing. A total of 12,104 transcripts were obtained, among which 1937 were differentially expressed genes (DEG), 137 exclusively expressed in RGA and 160 in RGPDA. During the root infection process, genes involved in the production of fungal toxins, detoxification and transport of toxic compounds, hormone biosynthesis, gene silencing and plant cell wall degradation were overexpressed. Interestingly, 24 out of the 137 contigs expressed only during R. necatrix growth on avocado roots, were predicted as candidate effector proteins (CEP) with a probability above 60%. The PHI (Pathogen Host Interaction) database revealed that three of the R. necatrix CEP showed homology with previously annotated effectors, already proven experimentally via pathogen-host interaction. CONCLUSIONS: The analysis of the full-length transcriptome of R. necatrix during the infection process is suggesting that the success of this fungus to infect roots of diverse crops might be attributed to the production of different compounds which, singly or in combination, interfere with defense or signaling mechanisms shared among distinct plant families. The transcriptome analysis of R. necatrix during the infection process provides useful information and facilitates further research to a more in -depth understanding of the biology and virulence of this emergent pathogen. In turn, this will make possible to evolve novel strategies for white root rot management in avocado.

Nondestructive Detection of White Root Rot Disease in Avocado Rootstocks by Leaf Chlorophyll Fluorescence.

White root rot (WRR) disease caused by Rosellinia necatrix is one of the most important threats affecting avocado orchards in temperate regions. In this study, we monitored the progression of WRR disease at the leaf and root levels by the combination of nondestructive chlorophyll fluorescence measurements and confocal laser-scanning microscopy on avocado genotypes susceptible to R. necatrix. Leaf photochemistry was affected at early stages of disease development prior to the appearance of aboveground symptoms, made evident as significant decreases in the trapping efficiency of photosystem-II (Fv'/Fm') and in the steady-state of chlorophyll fluorescence yield (Fs) normalized to the minimal fluorescence yield (F0) (Fs/F0). Decreases in Fv'/Fm' and Fs/F0 were associated with different degrees of fungal penetration, primarily in the lateral roots but not in areas next to the main root collar. Aboveground symptoms were observed only when the fungus reached the root collar. Leaf physiology was also tracked in a tolerant genotype where no changes were observed during disease progression despite the presence of the fungus in the root system. These results highlight the usefulness of this technique for the early detection of fungal infection and the rapid removal of highly susceptible genotypes in rootstock avocado-breeding programs.