| Soil salinization is an important global problem affecting agricultural production and ecological environment and is a siginificant cause of agricultural production decline. The expanding area of secondary salinization soil causing heavy losses to agricultural production is due to industrial pollution, agricultural irrigation and the heavy use of chemical fertilizers. Corn with rapid growth, large biomass and high sugar content is a high-yielding crop. Corn is one of major grain crops occupying an important position in the national economy. Soil salinization has seriously affected the growth and yield of maize. Seedling stage of corn is the most sensitive to salt stress. Seedling protection is an important part of cultivation. Thus, the growth and production of salt-tolerant corn in seedling stage may be improved in saline.In our study, maize nnbreds including maizes from salt screening of 113 inbred lines provided by breeding units, representative inbred lines in stress and inbred lines widely used in breeding reseach were screened out high salt-tolerant TL1317 and salt-sensitive SL1303 on the base of morphology phenotype under 150mmol/L NaCl. We confirmed the difference of salt tolerance between TL1317 and SL1303 germination and seedling stages by three levels of salt stress tests including germination, nutrient solution hydroponic seedling and soil culture seedling stages. The salt tolerance of TL1317 is much higher than other other published maize inbreds. Compared with wheat, camellia sinensis and so on, salt-tolerant variety also be existence in maize.The responsible for salt stress includes the osmotic and ion-specific phases. We studied the changes of chlorophyll content (SPAD), maximum photochemical efficiency (Fv/Fm), photochemical efficiency (Pn), stomatal conductance (Gs), membrane stability index (MSI), relative water content (RWC), shoot and root fresh/dry weight, proline and water soluble sugar content of TL1317 and SL1303 seedlings under salt stress. The results revealed obvious difference between TL1317 and SL1303 in the osmotic phase of salt-tolerant mechanism. All the physiological indexes of TL1317 and SL1303 had provided that TL1317 was more tolerant than SL1303 in osmotic phase. This study promoted our understanding of the osmotic stress of corn in salt stress and provided a theoretical basis of corn salt-tolerant cultivation.Ion-specific phase of plant response to salinity when salt accumulates to toxic concentrations. To analysis the mechanism of ion toxicity, we treated the two mazie inbreds with isotonic 25% PEG-6000 and 150mmol/L NaCl separately. The results showed that the time of SL1303 death under PEG-6000 delayed for 10 days than that under NaCl. The results revealed that ion toxicity would siginificantly hasten death of SL1303. To understand the difference in ion-specific phase between TL1317 and SL1303, we measured the ion contents in roots, stems, young leaves and old leaves of the two maize inbreds. Compared with SL1303, salt-tolerant TL1317 had the complex mechanism of Na+exclusion to maintain the lower Na+concentration in various issues except old leaf. The old leaf of TL1317 fell off slowly with more Na+ accumulation. While the old leaf of SL1303 fell off fast without Na+accumulation. We speculated that TL1317 had the ability to exclude excessive Na+by Na+accumulation in old leaf. The mechanism has not yet been document in maize inbreds. Our study found the changes of K+ content in various issues was siginificantly different between the two maizes, especially in roots. The results emphasized that the ability of K+uptake and retention was the key determinant of TL1317 salt tolerance. While K+concentration in root of SL1303 shows an obvious decline. The interference of K+homeostasis may lead to a series of metabolic disorders.In oeder to compare the salt response of TL1317 and SL13013 in molecular mechanism, RNA-seq analysis was performed using treatment of the two maize seedlings with 150 mmol/L NaCl. The numbers of up-regulated gene were obvious more than that of down-regulated gene in the two maize inbreds. We presumed that physiological metabolism of maize inbreds may be inhibited by salt stress. In response to salt stress, the differential regulation of a relatively larger number of genes was observed in SL1303 compared withTL1317. This, together with more dramatic accumulation of proline, suggested that SL1303 is more responsive to salt stress signals.Combining physiological and biochemical results with RNA-seq results, we took use of qRT-PCR as verification method with a part of genes. The genes of high affinity K+transporter, Ca2+transporter, LRR protein kinase, aquaporin and some unknown protein induced by salt stress had a siginificant difference between TL1317 and SL1303. The results showed that the genes were related to salt stress. Functional studies of these genes can promote understanding of salt tolerant mechanism in maize inbreds and new salt-tolerant genes. In this study, new genetic elements and polcies can be provided to improve the quality of maize. |
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