De Novo Transcriptome Assembly of Eucalyptus nitens and the Expression of R2R3-MYB Genes in Response to Cold Acclimation in Eucalyptus Spp.

Most of Eucalyptus species used in commercial forestry are frost sensitive. One of the few species with a higher frost tolerance is Eucalyptus nitens which can tolerate temperatures of - 15 degrees C, in contrast to Eucalyptus globulus, a frost-sensitive species, but an important species for cellulose production. Considering the excellent wood traits of E. globulus, relevant to the forestry industry, and the frost tolerance of E. nitens breeding programs has led to the generation of interspecific hybrids (Eucalyptus nitens x Eucalyptus globulus). MYB transcription factors (TFs) have been reported as key elements in the response to cold and freezing temperatures, especially the R2R3-MYB-type proteins involved in the biosynthesis of flavonoids, such as AtMYB12 and AtMYB111. In order to unravel the relation of MYB TFs and the cold acclimation (CA) process, a de novo transcriptome assembly from RNA-seq data was performed from non-acclimated, cold acclimated, and deacclimated plants of E. nitens. Twenty MYBs were identified by in silico analysis and ten of them were R2R3-type. MYB64 and MYB68, two R2R3-MYBs, showed the higher transcript abundance in response to CA. MYB64 and MYB68 were validated by qRT-PCR in samples from cold acclimated plants of a cold-tolerant species E. nitens, a cold-sensitive species, E. globulus, and a hybrid of E.nitens x E.globulus, characterized as a cold-tolerant genotype. MYB64, homologous to AtMYB12, showed a higher expression in the cold-tolerant species revealing a possible mechanism mediated by MYB TFs in response to CA in tolerant species of Eucalyptus spp.