Abstract
As a vital antioxidant, L-ascorbic acid (AsA) affects diverse biological processes in higher plants. Lack of AsA in cell impairs plant development. In the present study, we manipulated a gene of GDP-mannose pyrophosphorylase which catalyzes the conversion of D-mannose-1-P to GDP-D-mannose in AsA biosynthetic pathway and found out the phenotype alteration of tomato. In the tomato genome, there are four members of GMP gene family and they constitutively expressed in various tissues in distinct expression patterns. As expected, over-expression of SlGMP3 increased total AsA contents and enhanced the tolerance to oxidative stress in tomato. On the contrary, knock-down of SlGMP3 significantly decreased AsA contents below the threshold level and altered the phenotype of tomato plants with lesions and further senescence. Further analysis indicated the causes for this symptom could result from failing to instantly deplete the reactive oxygen species (ROS) as decline of free radical scavenging activity. More ROS accumulated in the leaves and then triggered expressions of defence-related genes and mimic symptom occurred on the leaves similar to hypersensitive responses against pathogens. Consequently, the photosynthesis of leaves was dramatically fallen. These results suggested the vital roles of AsA as an antioxidant in leaf function and defence response of tomato.
Highlights
In higher plants, reactive oxygen species (ROS) are produced as by-products in most energy-generating processes, such as photosynthesis and respiration
Blast results showed that GMP1 and GMP3 are both located on chromosome 3, and GMP2 and GMP4 are located on chromosome 6 and 9, respectively
Expression of yeast-derived GDP-D-mannose pyrophosphorylase (GMP) gene in tomato was found to enhance ascorbic acid (AsA) levels in leaves, green and red fruits [36], indicating biotechnological manipulation of AsA biosynthesis in tomato can be achieved through increasing GMP activity
Summary
Reactive oxygen species (ROS) are produced as by-products in most energy-generating processes, such as photosynthesis and respiration. ROS production in plant cells is low under optimal growth conditions, but increases dramatically when plants are subjected to abiotic stresses and pathogen attack. Unfavourable environmental conditions, such as cold, heat, drought, and salt, limit the rate of carbon fixation, which results in an increase in photoinhibition and overproduction of superoxide radicals and H2O2 [1]. The levels of ROS are strictly regulated by an efficient battery of enzymatic and non-enzymatic antioxidants [2]. Chloroplasts are potentially the major site for the generation of ROS, in which AsA is present at a high level, at concentration of 20 mM or more
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