Transcriptome Analysis Revealed the Paternal Importance to Vegetative Growth Heterosis in Populus

Parental selection is important for heterosis formation during crossbreeding of <i>Populus</i>. However, in poplar hybrids, the effect of parents on vegetative growth heterosis is not well understood. In this study, one female parent (<i>P</i>. <i>simonii</i> XY4) and two male parents (<i>P. nigra</i> OH and <i>P. deltoides</i> × <i>P. nigra</i> BJLY3#) were used to produce two progenies (Hyb1 and Hyb2). Vegetative growth investigation showed that both Hyb1 and Hyb2 performed heterosis in plant growth and ground diameter. The vegetative growth of hybrids was strongly correlated with the male parents but not with the female parents. The gene expression levels in the hybrids were more biased toward the male parents. In Hyb1 and Hyb2, 51.93% and 45.03% of the expressed genes showed the non-additive effect, respectively, and over 65% of the non-additively expressed genes showed the dominant effect. It is noteworthy that genes of paternal expression dominant effect (ELD_♂) account for the majority of dominantly expressed genes, suggesting the paternal contribution to heterosis. KEGG enrichment analysis indicated that a large number of non-additively expressed genes were enriched in the plant hormone signal transduction pathway. WGCNA analysis showed that MEcyan was significantly correlated with the traits of hybrids, and 12 plant hormone signal transduction pathway genes were enriched in this module. Transcription factors (TFs) MYB88, LHY, and TCP4 may be involved in the regulation of these pathway genes. This finding supported that the male parents play an important role in the formation of vegetative growth heterosis of <i>Populus</i>. In addition, the non-additively expressed genes of the signal transduction pathway and the regulation of TFs related to these pathway genes may be one of the reasons for the generation of heterosis.