Sugars promote graft union development in the heterograft of cucumber onto pumpkin

Li Miao,Xing Qin,Zhonghua Zhang,Sen Chai,Yansu Li,Tian-Shu Sun,Mintao Sun,Longqiang Bai,Xianchang Yu,Changlin Wang,Qing Li

doi:10.1038/s41438-021-00580-5

Abstract

The use of heterografts is widely applied for the production of several important commercial crops, but the molecular mechanism of graft union formation remains poorly understood. Here, cucumber grafted onto pumpkin was used to study graft union development, and genome-wide tempo-spatial gene expression at the graft interface was comprehensively investigated. Histological analysis suggested that resumption of the rootstock growth occurred after both phloem and xylem reconnection, and the scion showed evident callus production compared with the rootstock 3 days after grafting. Consistently, transcriptome data revealed specific responses between the scion and rootstock in the expression of genes related to cambium development, the cell cycle, and sugar metabolism during both vascular reconnection and healing, indicating distinct mechanisms. Additionally, lower levels of sugars and significantly changed sugar enzyme activities at the graft junction were observed during vascular reconnection. Next, we found that the healing process of grafted etiolated seedlings was significantly delayed, and graft success, xylem reconnection, and the growth of grafted plants were enhanced by exogenous glucose. This demonstrates that graft union formation requires the correct sugar content. Furthermore, we also found that graft union formation was delayed with a lower energy charge by the target of rapamycin (TOR) inhibitor AZD-8055, and xylem reconnection and the growth of grafted plants were enhanced under AZD-8055 with exogenous glucose treatment. Taken together, our results reveal that sugars play a positive role in graft union formation by promoting the growth of cucumber/pumpkin and provide useful information for understanding graft union healing and the application of heterografting in the future.

Highlights

Grafting is a widely used technique to improve horticultural crop product quality, quantity, and disease resistance, with effects on controlling tree vigor and fruit size[1,2]
Our results showed that phloem reconnection between the scion and rootstock was established two days earlier than xylem recovery, and heterografted plants started growing after xylem reconnection at the graft junction
The results showed that the process of healing is different in cucumber/cucumber, pumpkin/pumpkin, and cucumber/pumpkin

Summary

Introduction

Grafting is a widely used technique to improve horticultural crop product quality, quantity, and disease resistance, with effects on controlling tree vigor and fruit size[1,2]. Sufficient connection of functional vascular bundles between the scion and rootstock is essential for the graft union of woody trees[15,16]. Plasmodesmata provide an important channel for cell–cell communication at the graft junction, with higher plasmodesmal coupling observed between callus cells than between cortex cells in stem unions of Prunus spp.[18]. Despite these important findings, a more detailed study of the anatomy of graft union development is needed for many crops, especially in the field of horticulture

Methods

Results

Conclusion