Patch budding (bud grafting) is a commonly used method for pecan reproduction; however, the grafting survival rate varies with cultivars. Clarifying the underlying mechanisms of successful grafting is pivotal for graft technique improvement. Here, weighted gene co-expression network analysis (WGCNA) was conducted to dissect the key pathways and genes related to the successful grafting of pecan. Based on the transcriptome data of two contrasting cultivars (an easy-to-survive cultivar ‘Pawnee’ and a difficult-to-survive cultivar ‘Jinhua’) in response to budding, all the genes with variable transcripts were grouped into 18 modules. There were two modules that were significantly correlated with the trait of different cultivars. Enrichment analysis showed that several enriched gene ontology (GO) terms were related to oxidative detoxification and genes associated with hormone signaling pathway occupied a high ratio for the two modules. A total of 52 hub genes were identified, and 48 showed promoter polymorphisms between the two cultivars. Our study suggested that oxidative detoxification and hormone signaling were probably the key pathways for the successful grafting of pecan. The 48 hub genes identified here might be the key genes that led to the divergence of graft survival rates among different pecan cultivars. Our results will lay a foundation for future graft technique improvement in pecan.

Drought severely affects the growth and production of pecan (Carya illinoinensis), while genes conferred drought adaptation are yet to be fully elucidated. Here, an in-depth exploration of the two different RNA-seq projects regarding drought stress (designated as P1 and P2) was performed via weighted gene co-expression network analysis. For the two projects, there existed one pair of modules (P1 turquoise module and P2 blue module) that was probably associated with drought resistance, as the paired modules both exhibited an increased expression profile with increasing water shortage stress and were annotated to be involved in oxidative stress response and the signaling pathways of abscisic acid and jasmonic acid. There were 441 and 1258 hub genes in the P1 turquoise module and P2 blue module, respectively, among which, 140 were overlapped and thus were recognized as core drought responsive genes. An additional drought stress experiment was conducted for RT-qPCR validation, and the results showed that the 20 core genes selected for detection were highly responsive to water deficit. Together, our results will be helpful for understanding the molecular mechanism of drought response and improving drought resistance in pecan.

Response regulator (RR) is the core component of cytokinin (CK) signaling, and it regulates the expression of numerous downstream CK-responsive genes. However, the knowledge regarding the pecan RR (CiRR) gene family is still limited. In this study, we first monitored trans-zeatin riboside (tZR) content in the graft union 0, 7, 14, and 32 days after grafting and then conducted genome-wide analysis and expression profiling of the CiRR gene family using an available genome sequence and RNA-seq dataset, aiming to better understand the roles of CK during pecan grafting. The dynamic contents of tZR showed an increased trend during the specific period for both the scion and rootstock. There were 20 CiRRs in the pecan genome, including 12 type A CiRRs, 5 type B members, and 3 type C genes. All members contained a receiver domain and type B CiRRs possessed an additional Myb-like DNA-binding domain. Promoter analysis showed that the CiRR gene family contained cis-elements associated with growth and development, hormones, and stress. A total of 10 genes, including CiRR18/9/4a/14a/12c/5/12b/14b/2b/2a, were abundantly expressed in the samples of different tissues, drought stress, and kernel development. There were 12 genes (CiRR5/18/4a/12b/2b/12c/14b/2a/14a/4b/9/11a) showing active expressions during grafting, and weighted gene co-expression network analysis (WGCNA) grouped them into six modules. Among them, CiRR14a and CiRR12b were the hub genes for the turquoise and brown modules, respectively. Functional annotation indicated that the turquoise module was associated with gene transcription and translation, while the brown module was related to cell proliferation. Our results suggest that the CiRR gene family central to CK signaling is probably involved in callus formation during pecan grafting.