Scion control of miRNA abundance and tree maturity in grafted avocado

Abstract

Background Grafting is the common propagation method for avocado and primarily benefits orchard production by reducing the time to tree productivity. It also allows use of scions and rootstocks specifically selected for improved productivity and commercial acceptance. Rootstocks in avocado may be propagated from mature tree cuttings (‘mature’), or from seed (‘juvenile’). While the use of mature scion material hastens early bearing/maturity and economic return, the molecular factors involved in the role of the scion and/or rootstock in early bearing/reduced juvenility of the grafted tree are still unknown. Results Here, we utilized juvenility and flowering associated miRNAs; miR156 and miR172 and their putative target genes to screen pre-graft and post-graft material in different combinations from avocado. The abundance of mature miR156, miR172 and the miR156 target gene SPL4 , showed a strong correlation to the maturity of the scion and rootstock material in avocado. Graft transmissibility of miR156 and miR172 has been explored in annual plants. Here, we show that the scion may be responsible for grafted tree maturity involving these factors, while the rootstock maturity does not significantly influence miRNA abundance in the scion. We also demonstrate that the presence of leaves on cutting rootstocks supports graft success and contributes towards intergraft signalling involving the carbohydrate-marker TPS1 . Conclusion Here, we suggest that the scion largely controls the molecular ‘maturity’ of grafted avocado trees, however, leaves on the rootstock not only promote graft success, but can influence miRNA and mRNA abundance in the scion. This constitutes the first study on scion and rootstock contribution towards grafted tree maturity using the miR156- SPL4 -miR172 regulatory module as a marker for juvenility and reproductive competence. Electronic supplementary material The online version of this article (10.1186/s12870-019-1994-5) contains supplementary material, which is available to authorized users.

Mechanisms Underlying Graft Union Formation and Rootstock Scion Interaction in Horticultural Plants

“…Maturity in grafted avocado was found to be controlled by scion whereas rootstock encourages the successful union formation along with its influence on miRNA and mRNA profusion in the scion. The large amount of miR172, miR156 and the miR156 target gene SPL4, was directly associated with the scion and rootstock maturity in avocado ( Ahsan et al, 2019 ). By comparing the expression of miRNAs in leaves and roots of homografts of cucumber seedlings, it was found that the expression of most miRNAs in the leaves and roots of heterografted seedlings altered vigorously: prompted under regular conditions and down regulated after some time of drought stress, and then again up regulated after 24 h of drought stress.…”

MicroRNA156‐mediated changes in leaf composition lead to altered photosynthetic traits during vegetative phase change

“…For example, grafting old shoots to young roots is often used to determine if a trait is dependent on plant size. However, miR156 is a mobile microRNA and can move across a graft junction (Marin‐Gonzalez & Suarez‐Lopez, 2012; Bhogale et al ., 2014; Ahsan et al ., 2019; Fouracre & Poethig, 2019), so this approach does not necessarily eliminate the effect of this key regulator of vegetative identity. Similarly, the methods that are typically used to induce vegetative rejuvenation ( in vitro culture, pruning) affect both the level of miR156 (Irish & Karlen, 1998; Li et al ., 2012) and plant size.…”

Epigenetic Changes and Transcriptional Reprogramming Upon Woody Plant Grafting for Crop Sustainability in a Changing Environment

“…Results demonstrated that scion age significantly influenced grafted tree maturity through the control of miR156- SPL4 -miR172 regulatory network. They also showed that the existence of leaves on cutting rootstocks provided grafting success and influence the abundance of miR156 and miR172 in the scion ( Ahsan et al, 2019 ).…”

“…Similarly, "Tahiti acid lime" (TAL) scions grafted on RL and SM exhibited hypermethylation and hypomethylation, respectively, upon water deficit treatments, however, physiological responses were similar (Santos et al, 2020). Ahsan et al (2019) provided the first evidence of avocado inter-graft regulation of miR156 and miR172 that control transition from juvenile to flowering phase by mediating the expression of the SQUAMOSA promoter binding protein-like (SPL) gene using different combinations (scions from shoot of young seedlings or mature scion wood grafted onto both seedlings or clonal rootstocks). Results demonstrated that scion age significantly influenced grafted tree maturity through the control of miR156-SPL4-miR172 regulatory network.…”