Molecular and physiological characterization of the effects of auxin-enriched rootstock on grafting

Longmei Zhai,Wei Li,Zhenhai Han,Qi Qi,Huseyin Yer,Dan Tang,Richard Mcavoy,Xiaomin Wang,Xiangning Jiang,Yi Li

doi:10.1038/s41438-021-00509-y

Abstract

sGrafting is a highly useful technique, and its success largely depends on graft union formation. In this study, we found that root-specific expression of the auxin biosynthetic gene iaaM in tobacco, when used as rootstock, resulted in more rapid callus formation and faster graft healing. However, overexpression of the auxin-inactivating iaaL gene in rootstocks delayed graft healing. We observed increased endogenous auxin levels and auxin-responsive DR5::GUS expression in scions of WT/iaaM grafts compared with those found in WT/WT grafts, which suggested that auxin is transported upward from rootstock to scion tissues. A transcriptome analysis showed that auxin enhanced graft union formation through increases in the expression of genes involved in graft healing in both rootstock and scion tissues. We also observed that the ethylene biosynthetic gene ACS1 and the ethylene-responsive gene ERF5 were upregulated in both scions and rootstocks of the WT/iaaM grafts. Furthermore, exogenous applications of the ethylene precursor ACC to the junction of WT/WT grafts promoted graft union formation, whereas application of the ethylene biosynthesis inhibitor AVG delayed graft healing in WT/WT grafts, and the observed delay was less pronounced in the WT/iaaM grafts. These results demonstrated that elevated auxin levels in the iaaM rootstock in combination with the increased auxin levels in scions caused by upward transport/diffusion enhanced graft union formation and that ethylene was partially responsible for the effects of auxin on grafting. Our findings showed that grafting success can be enhanced by increasing the auxin levels in rootstocks using transgenic or gene-editing techniques.

Highlights

Grafting is an ancient but essential technique used worldwide in the agricultural industry that involves combining plants of the same or different species to continue their growth and development[1]
We conducted a microscopic analysis of the graft unions of both the WT/WT and WT/ iaaM grafts, and the results showed complete healing in the WT/iaaM grafts, whereas obvious gaps were observed between the scion and rootstock in the WT/WT grafts (Fig. 1E, F)
Reduced callus production at the WT/WT graft junction might lead to graft failure, whereas our results suggest that rapid callus formation in the iaaM rootstock might improve graft union formation

Summary

Introduction

Grafting is an ancient but essential technique used worldwide in the agricultural industry that involves combining plants of the same or different species to continue their growth and development[1]. In the horticultural and silvicultural industries, grafting is widely used to increase the yield, enhance biotic and abiotic stress resistance, and modify the scion architecture[2,3]. Grafting has been used for millennia in agriculture, the mechanisms underlying the graft healing process are largely unclear. Various plant hormones are involved in the graft healing process. Auxin accumulates above the graft junction and is depleted at the bottom of the junction[5]. Exogenous auxin application can lead to successful graft formation during tissue culture[6]. Hypocotyl graft unions are inhibited in response to treatment with an auxin transport inhibitor[7]. Many Arabidopsis thaliana mutants with impaired auxin perception or auxin responses show delayed phloem reconnection during hypocotyl grafting[4]

Methods

Results

Conclusion