GDP‐d‐mannose 3,5‐epimerase (GME) plays a key role at the intersection of ascorbate and non‐cellulosic cell‐wall biosynthesis in tomato

Abstract

SUMMARYThe GDP-D-mannose 3,5-epimerase (GME, EC 5.1.3.18), which converts GDP-D-mannose to GDP-L-galactose, is generally considered to be a central enzyme of the major ascorbate biosynthesis pathway in higher plants, but experimental evidence for its role in planta is lacking. Using transgenic tomato lines that were RNAi-silenced for GME, we confirmed that GME does indeed play a key role in the regulation of ascorbate biosynthesis in plants. In addition, the transgenic tomato lines exhibited growth defects affecting both cell division and cell expansion. A further remarkable feature of the transgenic plants was their fragility and loss of fruit firmness. Analysis of the cell-wall composition of leaves and developing fruit revealed that the cell-wall monosaccharide content was altered in the transgenic lines, especially those directly linked to GME activity, such as mannose and galactose. In agreement with this, immunocytochemical analyses showed an increase of mannan labelling in stem and fruit walls and of rhamnogalacturonan labelling in the stem alone. The results of MALDI-TOF fingerprinting of mannanase cleavage products of the cell wall suggested synthesis of specific mannan structures with modified degrees of substitution by acetate in the transgenic lines. When considered together, these findings indicate an intimate linkage between ascorbate and non-cellulosic cell-wall polysaccharide biosynthesis in plants, a fact that helps to explain the common factors in seemingly unrelated traits such as fruit firmness and ascorbate content.

Allelic Variation in Paralogs of GDP-l-Galactose Phosphorylase Is a Major Determinant of Vitamin C Concentrations in Apple Fruit

“…In cherry tomatoes, fruit firmness and AsA concentrations have been reported to be closely correlated (Gilbert et al, 2009), and here, we show that firm apples also generally contain higher flesh AsA concentrations. Flesh AsA-totAsA concentrations and percentage DHA are also significantly correlated with fruit flesh soluble solids content, suggesting a link to the supply of sugar substrates for AsA biosynthesis.…”

“…In tomato (Ioannidi et al, 2009) and kiwifruit (Bulley et al, 2009), fruit AsA concentrations were also not correlated with GMP expression. GME has been proposed to be critical for the regulation of plant AsA concentrations by several groups (Wolucka and Van Montagu, 2007;Gilbert et al, 2009), and in tomato, SlGME1 colocates with a cluster of stable fruit AsA-QTL (Stevens et al, 2007), while in apple, MdGME transcript levels are highly correlated with AsA concentrations during apple fruit development (Li et al, 2011). However, the data presented here support the results obtained in peach (Prunus persica; Imai et al, 2009) and tomato (Ioannidi et al, 2009), which suggest that this step is not a major point of control in mature apple fruit, at least under our field conditions.…”

“…Genes involved in several of these mechanisms have been proposed to be key regulators of fruit AsA concentrations, including GDP-L-Gal phosphorylase (GGP) or vitamin c defective2 (VTC2) in kiwifruit (Actinidia deliciosa; Bulley et al, 2009Bulley et al, , 2012 as well as GDP-Man-3,5-epimerase (GME; Gilbert et al, 2009), L-Gal-1-Pphosphatase (GPP or VTC4; Ioannidi et al, 2009), and MDHAR (Stevens et al, 2007) in tomato. However, apart from GGP (Bulley et al, 2012), overexpression of these structural genes has to date had limited success in altering the fruit AsA pool (Agius et al, 2003;Bulley et al, 2009;Haroldsen et al, 2011).…”

Transcriptomics of induced defense responses to greenbug aphid feeding in near isogenic wheat lines

“…Two transcripts similar to GDP-D-mannose 3,5-epimerase 1 (GME1) involved in ascorbate biosynthetic pathway were induced in SB at both 24 and 48 hai (Fig. 6C) suggesting increased ascorbate content for ROS detoxification [73]. Also, up-regulation of two transcripts encoding to glutathione synthase 2 (GHS2) involved in glutathione biosynthesis [74] in SB consistent with other antioxidant related transcripts suggest severe feeding-induced stress responses.…”

KONJAC1 and 2 Are Key Factors for GDP-Mannose Generation and Affect l-Ascorbic Acid and Glucomannan Biosynthesis in Arabidopsis

“…The physiological importance of GDP-Man as the starting substrate has also been shown. In tomato, the RNAi silencing of a GDP-Man 3,5-epimerase generating GDP-L-Gal in the Man/LGal pathway results in the altered structure of the pectin rhamnogalacturonan II, which causes a growth defect (Gilbert et al, 2009;Voxeur et al, 2011). As mentioned above, GDP-Glc likely serves as a donor substrate in the synthesis of glucomannan.…”