Abstract
Apple dwarfing rootstocks cause earlier shoot termination and reduced root and shoot mass. To identify physiological factors responsible for rootstock-induced growth restriction, we compared vascular-enriched gene expression between two dwarfing rootstocks (‘M27’ and ‘M9’) and the vigorous rootstock ‘M793’ using RNA sequencing and quantitative reverse transcriptase PCR. Differentially expressed genes common to both dwarfing rootstocks belonged to five main biological processes: (1) primary metabolism, (2) cell wall synthesis and modification, (3) secondary metabolism, (4) hormone signalling and response and (5) redox homeostasis. Genes promoting the biosynthesis of amino acids, lipids and cell walls were downregulated in dwarfing rootstocks, whereas genes promoting the breakdown of these compounds were upregulated. The only exception to this trend was the upregulation of starch synthesis genes in dwarfing rootstocks. Non-structural carbohydrate analysis demonstrated that starch concentrations in ‘M9’ roots, stems and grafted ‘Royal Gala’ (‘RG’) scions were double that of equivalent tissues from ‘RG’ homo-grafted trees (‘RG’/‘RG’). Fructose and glucose concentrations were much lower in all three tissues of the ‘RG’/‘M9’ trees. Together, these data indicate that dwarfing rootstocks are in a state of sugar depletion and reduced cellular activity despite having large starch reserves. Another significant finding was the over-accumulation of flavonoids and the downregulation of auxin influx transporters MdAUX1 and MdLAX2 in dwarfing rootstocks. We propose that both factors reduce polar auxin transport. The results of this study contribute novel information about the physiological state of dwarfing rootstocks.
Highlights
Dwarfing rootstocks are widely used in commercial apple production to reduce scion vigour, allowing high-density plantings and increased yield index
The ‘Royal Gala’ (RG)’ and ‘Malling 9’ (M9)’ stem samples used for analysis by liquid chromatography–high resolution accurate mass–mass spectrometry (LC–HRAM–MS) were the same as those used for the carbohydrate analysis
Lignin and cellulose biosynthesis pathways are highly downregulated in dwarfing rootstocks, consistent with reduced cell wall synthesis and modification, and possibly a contributing factor to the reduced root mass and hydraulic conductance observed in dwarfing rootstocks
Summary
Dwarfing rootstocks are widely used in commercial apple production to reduce scion vigour, allowing high-density plantings and increased yield index. Despite their utility and a century of research, the underlying mechanism of rootstock-induced dwarfing is still unknown. The most widely used and best-characterised dwarfing apple rootstock is ‘Malling 9’ (‘M9’), which was first phenotyped in the early twentieth century.[11] The ‘Malling’ series have been used to breed new rootstocks that confer a range of vigour control, including the strongly dwarfing ‘M27’. Rootstock-induced dwarfing is conferred by two major quantitative trait loci.[12,13,14,15] Most of the known dwarfing rootstocks carry markers linked to one or both loci, suggesting that there is one source of dwarfing rootstocks and they reduce scion vigour by the same underlying physiological mechanism.[14,15]
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