Differential transcription pathways associated with rootstock-induced dwarfing in breadfruit (Artocarpus altilis) scions

Yuchan Zhou,Steven J R Underhill

doi:10.1186/s12870-021-03013-6

Abstract

BackgroundBreadfruit (Artocarpus altilis) is a traditional staple tree crop throughout the tropics. Through interspecific grafting, a dwarf phenotype with over 50% reduction in plant height was identified when marang (Artocarpus odoratissimus) rootstocks were used. However, the molecular mechanism underlying the rootstock-induced breadfruit dwarfing is poorly understood.ResultsAn RNA-sequencing study of breadfruit scions at 22 months after grafting identified 5409 differentially expressed genes (DEGs) of which 2069 were upregulated and 3339 were downregulated in scion stems on marang rootstocks compared to those on self-graft. The DEGs were predominantly enriched for biological processes involved in carbon metabolism, cell wall organization, plant hormone signal transduction and redox homeostasis. The down-regulation of genes encoding vacuolar acid invertases and alkaline/neutral invertases, was consistent with the decreased activity of both enzymes, accompanying with a higher sucrose but lower glucose and fructose levels in the tissues. Key genes of biosynthetic pathways for amino acids, lipids and cell wall were down regulated, reflecting reduction of sucrose utilisation for stem growth on dwarfing rootstocks. Genes encoding sugar transporters, amino acid transporters, choline transporters, along with large number of potassium channels and aquaporin family members were down-regulated in scion stems on marang rootstocks. Lower activity of plasma membrane H+-ATPase, together with the predominance of genes encoding expansins, wall-associated receptor kinases and key enzymes for biosynthesis and re-modelling of cellulose, xyloglucans and pectins in down-regulated DGEs suggested impairment of cell expansion. Signalling pathways of auxin and gibberellin, along with strigolacton and brassinosteroid biosynthetic genes dominated the down-regulated DEGs. Phenylpropanoid pathway was enriched, with key lignin biosynthetic genes down-regulated, and flavonoid biosynthetic genes upregulated in scions on marang rootstocks. Signalling pathways of salicylic acid, jasmonic acid, ethylene and MAPK cascade were significantly enriched in the upregulated DEGs.ConclusionsRootstock-induced disruption in pathways regulating nutrient transport, sucrose utilisation, cell wall biosynthesis and networks of hormone transduction are proposed to impair cell expansion and stem elongation, leading to dwarf phenotype in breadfruit scions. The information provides opportunity to develop screening strategy for rootstock breeding and selection for breadfruit dwarfing.

Highlights

Breadfruit (Artocarpus altilis) is a traditional staple tree crop throughout the tropics
As the performance of a grafted plant and the commercial application of the rootstock depends on the genotype of scion and rootstock, and the compatibility of the rootstock with scion [16,17,18], it is necessary to examine the graft combination of breadfruit/marang for graft compatibility
Consistent with the results, a previous monthly measurement of chlorophyll by a nondestructive method showed no difference in total chlorophyll of scion leaves on different rootstocks at any time point [3]

Summary

Introduction

Breadfruit (Artocarpus altilis) is a traditional staple tree crop throughout the tropics. A dwarf phenotype with over 50% reduction in plant height was identified when marang (Artocarpus odoratissimus) rootstocks were used. Transition toward high-density commercial planting, as well as tree loss due to intense tropical windstorm in the regions has driven an interest in small size breadfruit trees [2]. Rootstock of marang (Artocarpus odoratissimus Blanco) was identified as a size-controlling rootstock conferring over 50% reduction in breadfruit scion size, with over 70% shorter internode length compared to those on self-graft and the standard, non-grafted breadfruit plants [3, 4]. Breadfruit plants on marang rootstocks grows normally except for being dwarf, providing a potential solution for tree vigour control in commercial cultivation [3]. Under the same genus of Artocarpus, marang is a large tropical fruit tree to 25 m, no dwarf phenotype has been identified. Little is known about the intriguing interactive processes by which marang greatly reduces the tree size of its grafted scions when used as rootstocks

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Results

Discussion

Conclusion