Apple dwarfing rootstocks exhibit an imbalance in carbohydrate allocation and reduced cell growth and metabolism

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.

MdWRKY9 overexpression confers intensive dwarfing in the M26 rootstock of apple by directly inhibiting brassinosteroid synthetase MdDWF4 expression

“…To further identify how MdWRKY9 induces this dwarfing phenotype, we measured the concentrations of the following four hormones that are believed to have roles in plant dwarfing: IAA, CTK, BR and GA. It is widely accepted that IAA, GA and CTK are key factors affecting apple dwarfing (Lockard & Schneider, ); changes in their synthesis, transportation and signaling can cause the dwarfing phenotype (Werner et al ., , ; Yang et al ., ; Foster et al ., ). However, we did not detect any significant differences in IAA, CTK and GA concentrations between transgenic lines and wild‐type control plants.…”

“…Dwarfing rootstocks are widely used in modern apple production for high-density planting and efficient mechanical management of orchards. Despite their utility and a century of research, the underlying molecular mechanism that mediates plant dwarfing is unknown (Foster et al, 2017).…”

“…Changes in both cell division and cell elongation lead to the dwarfing phenotype, and these processes are regulated by cytokinin (CTK), auxin (IAA), GA and brassinosteroid (BR) (Ma et al, 2016). IAA, CTK and GA are widely considered to have crucial roles in plant dwarfing, and several studies have investigated their synthesis, transport and signaling in dwarfing apple rootstock (Soumelidou et al, 1994;Michalczuk, 2002;Ma et al, 2016;Foster et al, 2017). A recent study reported that BR also has an important role in apple dwarfing.…”

A Dwarf Phenotype Identified in Breadfruit (Artocarpus altilis) Plants Growing on Marang (A. odoratissimus) Rootstocks

“…For example, peach trees growing on dwarfing rootstocks had a lower level of glucose and fructose in leaves [27]. Levels of glucose and fructose are much lower in apple scions on dwarfing rootstocks, although sucrose and galactose concentrations remain unchanged [28]. For temperate species, the lower soluble sugar levels in the above-ground portion of trees growing on dwarfing rootstocks during the late autumn and dormancy periods have been associated with the low availability of carbohydrate reserve that reduces vegetative growth in the following spring season [26,29].…”

Role of phytohormones in the formation of dwarf and tall Siberian crabapple (Malus bаccata L. Borkh.)

“…In their later work, Lordan and his colleagues (2017) showed a higher ABA content in the xylem exudates of the dwarfing stocks (G.11 and B.9) as compared to that in the taller forms (B.72020 and G.814). At the same time, it was found that the activity of ABA gene response and biosynthesis was decreased in the dwarfing stocks (Foster et al, 2017).…”