Salt (NaCl) is a common physiological stressor of plants. To better understand how germinating seeds respond to salt stress, we examined the changes that occurred in the proteome of maize seeds during NaCl-treated germination. Phenotypically, salt concentrations less than 0.2 M appear to delay germination, while higher concentrations disrupt development completely, leading to seed death. The identities of 96 proteins with expression levels altered by NaCl-incubation were established using 2-DE-MALDI-TOF-MS and 2-DE-MALDI-TOF-MS/MS. Of these 96 proteins, 79 were altered greater than twofold when incubated with a 0.2 M salt solution, while 51 were altered when incubated with a 0.1 M salt solution. According to their functional annotations in the Swiss-Prot protein-sequence databases, these proteins are mainly involved in seed storage, energy metabolism, stress response, and protein metabolism. Notably, the expression of proteins that respond to abscisic acid signals increased in response to salt stress. The results of this study provide important clues as to how NaCl stresses the physiology of germinating maize seeds.

De-etiolation consists of a series of developmental and physiological changes that a plant undergoes in response to light. During this process light, an important environmental signal, triggers the inhibition of mesocotyl elongation and the production of photosynthetically active chloroplasts, and etiolated leaves transition from the “sink” stage to the “source” stage. De-etiolation has been extensively studied in maize (Zea mays L.). However, little is known about how this transition is regulated. In this study, we described a quantitative proteomic and phosphoproteomic atlas of the de-etiolation process in maize. We identified 16,420 proteins in proteome, among which 14,168 proteins were quantified. In addition, 8746 phosphorylation sites within 3110 proteins were identified. From the combined proteomic and phosphoproteomic data, we identified a total of 17,436 proteins. Only 7.0% (998/14,168) of proteins significantly changed in abundance during de-etiolation. In contrast, 26.6% of phosphorylated proteins exhibited significant changes in phosphorylation level; these included proteins involved in gene expression and homeostatic pathways and rate-limiting enzymes involved in photosynthetic light and carbon reactions. Based on phosphoproteomic analysis, 34.0% (1057/3110) of phosphorylated proteins identified in this study contained more than 2 phosphorylation sites, and 37 proteins contained more than 16 phosphorylation sites, indicating that multi-phosphorylation is ubiquitous during the de-etiolation process. Our results suggest that plants might preferentially regulate the level of posttranslational modifications (PTMs) rather than protein abundance for adapting to changing environments. The study of PTMs could thus better reveal the regulation of de-etiolation.

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47De-etiolation consists of a series of developmental and physiological changes that a 48 plant undergoes in response to light. During this process light, an important 49 environmental signal, triggers the inhibition of mesocotyl elongation and the 50 production of photosynthetically active chloroplasts, and etiolated leaves transition 51 from the "sink" stage to the "source" stage. De-etiolation has been extensively studied 52 in maize (Zea mays L). However, little is known about how this transition is regulated. 53In this study, we describe a quantitative proteomic and phosphoproteomic atlas of the 54 de-etiolation process in maize. We identified 16,420 proteins and quantified 14,168. In 55 addition, 8,746 phosphorylation sites within 3,110 proteins were identified. From the 56 proteomic and phosphoproteomic data combined, we identified a total of 17,436 57 proteins, 27.6% of which are annotated protein coding genes in the Zea_mays 58AGPv3.28 database. Only 6% of proteins significantly changed in abundance during 59 de-etiolation. In contrast, the phosphorylation levels of more than 25% of 60 phosphoproteins significantly changed; these included proteins involved in gene 61 expression and homeostatic pathways and rate-limiting enzymes involved in 62 photosynthesis light and carbon reactions. Based on phosphoproteomic analysis, 34% 63(1,057) of all phosphoproteins identified in this study contained more than three 64 phosphorylation sites, and 37 proteins contained more than 16 phosphorylation sites, 65 which shows that multi-phosphorylation is ubiquitous during the de-etiolation process. 66Our results suggest that plants might preferentially regulate the level of PTMs rather 67 than protein abundance for adapting to changing environments. The study of PTMs 68 could thus better reveal the regulation of de-etiolation. 69 70