## 3 Results
### Transcriptional Profile Analysis of AsA-GSH Pathway  
The nonenzymatic antioxidant pool of AsA-GSH pathway undergoes turnover along with the redox function metabolism in plants, therefore the biosynthesis, degradation, and recycling of these compound not only maintained the reduced/oxidized ratio, but also the total buffer pool [8]. We next set to explore the transcriptional variation between Jing72464 and JingX201. Gene expression in biosynthesis, degradation, and recycling of AsA-GSH pathway was detected using semi-quantitative RT-PCR. AsA is produced via the D-mannose/L-galactose pathway, in which GGP catalyzes the conversion of GDP-L-galactose to L-galactose 1-P and is the first committed step in this pathway [21]. Transcription level of ZmVTC2, GGP encoding gene in maize, was significantly increased in response to LT stress. At NT condition, transcripts level of ZmVTC2 in Jing72464 was about 4.3-fold higher in Jing72464 than JingX201 (\(p<0.001\)), indicating the enhanced production of AsA pool. Other genes encoding catalyzing enzymes of this pathway, including ZmGME, ZmVTC4, ZmGalDH, and ZmGalLDH were not significantly changed between two genotypes (Figure S2). Transcription level of GSH biosynthesis genes, including ZmECS, ZmGSH1, and ZmGSH2 were induced at different degree in response to LT, while there was no significant difference between the two inbred lines (Figure S2). Transcript levels of genes encoding enzymes in recycling of AsA-GSH pathway were analyzed. Among the four antioxidant enzymes in AsA-GSH recycle pathway, major difference came from ZmAPX, which use AsA as an electron donor to detoxify H\({}_{2}\)O\({}_{2}\). Transcription level of ZmAPX1 and ZmAPX2 were significantly higher in Jing72464 than that of JingX201 (\(p<0.001\)) (Figure 5a). Furthermore, ascorbate oxidase (AO) catalyzes the oxidation of AsA to MDHA, with the concomitant reduction of molecular oxygen to water. Transcription level of ZmAO was 4.1-fold higher in Jing72464 than that of JingX201 (\(p<0.001\)) (Figure 5a). The other three antioxidant enzymes in AsA-GSH recycle pathway, including ZmMDHAR, ZmDHAR, and ZmGR, also increased transcription in response to LT, but no significant differences were observed between two genotypes (Figure S2). Transcription of SOD, CAT, and GST were also analyzed. Majority of the analyzed genes increased in response to LT. For example, transcripts levels of ZmSODA.3, ZmSODB, and ZmGST2 were much higher in Jing72464 than JingX201 under LT condition, whereas transcripts level of ZmSOD4 was lower in Jing72464 (Figure S2). Collectively, our results suggested that enhancement of the GSH and AsA redox pools improved LT tolerance of maize seedlings, which might be conferred by the coordination of VTC2-mediate AsA biosynthesis and APX-mediate antioxidants recycling (Figure 5a). The ratios of AsA to DHA and GSH to GSSG may signal the plant with appropriate response to abiotic stress, which coordinate the actions of multiple signaling pathways in regulation of plant development. As illustrated in Figure 2, LT-tolerant inbred Jing72464 maintain the considerable post-germinative growth and normal photosynthesis. Compared with that, LT-sensitive inbred JingX201 had strong response to LT stress, leading to the impairment of seedling establishment and photosynthesis in subsequent growth. Consequently, Jing72464 maintained considerable plant height, biomass, and production of seeds at mature stage, whereas JingX201 was significantly suppressed on these physiological indexes (Figures 6 and S3). Figure 5: Transcription profile revealed the enhanced AsA biosynthesis and recycling in LT-tolerant inbred Jing72464. (**a,b**) Transcript levels of key enzymatic components in AsA biosynthesis (**a**) and AsA-GSH recycle pathway (**b**) in Jing72464 and JingX201. Transcription is measured relative to that of JingX201 seedlings under NT condition (set to 1). Data are mean \(\pm\) s.e.m. (\(n=3\)), n.s. means no significance, * means \(p<0.05\). (**c**) Schematic representation of AsA biosynthesis and AsA-GSH recycle pathway.