Modulation Of Polyamines In Seed Embryos And Seedlings Of Maize And Its Relationship With Chilling Tolerance

Chilling stress affects seed germination and seedling growth of mazie (Zea mays L.), finally causes loss of maize yield. To investigate the mechanism of plant response to chilling stress and improve chilling tolerance is one of major considerations for a long time worldwide. Polyamines (PAs) are biologically ubiquitous aliphatic amines with low molecular weight that involved in the response of plants to environmental stress. In present research, the chilling-tolerant inbred line (HuangC) and chilling-sensitive inbred line (Mo17), selected from nine maize inbred lines, were used to investigate the modulation of polyamines in seed embryos and seedlings of maize and its relationship with chilling tolerance. The main results were as follows:The chilling tolerance of nine maize inbred lines at germination and seedling growth stages were investigated by determining traits of germination characteristics and seedling quality at normal temperature (25℃) and low temperature (11℃) stress. The ratios (relative values) of trait values at low temperature to those at normal temperature were used to assess chilling tolerance. The results showed that the shoot of maize was more sensitive than root to low temperature, dry weight of root and seedling in chilling tolerance lines were not reduced by low temperature stress. The cluster analysis was used to solve assessing difficult of chilling tolerance due to small differences among relative values in maize inbred lines. Maize inbred lines were clustered into three groups: chilling tolerance, intermediate chilling tolerance and chilling sensitivity. The correlation analysis showed that the relative values of germination percentage, dry weight of seedling, shoot and root were significantly correlated with each other; they were suitable parameters for evaluating chilling tolerance. HuangC and Mo17 were selected as chilling-tolerant and chilling-sensitive inbred lines.The relationship between the imbibitional chilling tolerance of seeds and changes of polyamines in embryos under imbibitional chilling stress were investigated. During imbibitional chilling stress at 5°C for 48 h, the content of putrescine (Put) was reduced, and the contents of spermidine (Spd), spermine (Spm) and malondialdehyde (MDA) were increased in embryos of Huang and Mo17. At the five stages (0, 12,24, 36, and 48 h) of imbibition under chilling stress, the embryonic Spd and Spm contents of HuangC were higher than those of Mo17, whereas the contents of Put and MDA in embryos of HuangC were lower than those of Mo17. There were higher ratio of (Spd+Spm)/Put and lower ratio of Put/PAs in HuangC than those of Mol7 in response to imbibitional chilling stress. The values of (Spd+Spm)/Put and Put/PAs were correlated negatively and positively with the contents of MDA, respectively. Seed treatment by 0.5 mM exogenous Put had no significant effects on embryonic MDA content and the germination ability (germination percentage, energy of germination and mean germination time (MGT)) of HuangC and Mo17. Seed treatment by 0.5 mM exogenous Spd and Spm significantly reduced MDA content, increased germination ability with various degrees in two maize inbreds. 2.0 mM MGBG (Methylglyoxal-bis-(guanylhydrazone), inhibitor of Spd and Spm biosynthesis) treatment resulted in increase of MDA content, reduction of germination percentage and energy of germination and delay of germination in the two maize inbreds. The results suggestedthat Spd and Spm could improve the imbibitional chilling tolerance of maize seeds.The relationship between the chilling tolerance of maize seedlings and changes of polyamines in seedlings under chilling stress were studied. After suffering chilling stress of 5°C, the activities of polyamine biosynthesis emzymes, arginine decarboxylase (ADC) and ornithine decarboxylase (ODC) increased, the activity of ODC was higher than ADC, the content of Put increased, the contents of Spd, Spm decreased, MDA content and membrane permeability increased in seedlings of the two maize inbred. The activities of ADC and ODC and polyamine contents decreased after the seedlings were recovery growth for three days under normal temperature. There were higher ratio of (Spd+Spm)/Put and lower ratio of Put/PAs in seedlings of HuangC than those of Mo17 in response to chilling stress. The values of (Spd+Spm)/Put and Put/PAs were correlated negatively and positively with the contents of MDA and membrane permeability, respectively. Seedlings treated by 0.5 mM exogenous Put before chilling stress had no significant effects on MDA content and the permeability of HuangC and Mol7. Treatment with 0.5 mM exogenous Spd and 0.5 mM Spm significantly reduced MDA content and the permeability of maize seedlings. 2.0 mM MGBG treatment resulted in the increase of MDA content and permeability. The results suggested that the changes of polyamines closely correlated with chilling tolerance of maize seedlings, and a higher value of (Spd+Spm)/Put was one of the important physiological characters in chilling-tolerant maize inbred lines.The effects of exogenous Put, Spd and Spm on the antioxidant enzyme activities, the contents of MDA and proline (Pro), the growth of seedlings in maize inbred lines grown hydroponically under chilling stress were investigated. The results showed that after chilling stress (5℃), the activities of catalase (CAT), peroxidase (POD), superoxide dismutase (SOD) and ascorbate peroxidese (APX), the content of Pro were higher in seedlings of HuangC than those of Mo17, however the MDA content was lower than that of Mo17. Seedlings treated with 0.2 mM exogenous Put had no significant effects on activities of antioxidant enzymes, the contents of Pro and MDA, and dry weight of seedlings, however reduced seedling length. Exogenous 0.2 mM Spd and 0.2 mM Spm treatment significantly increased the activities of CAT, POD, SOD and APX and Pro content, reduced MDA content and length of maize seedlings, increased seedling dry weight. It is supposed that exogenous Spd and Spm improved the chilling tolerance of maize seedlings by increasing activities of antioxidant enzymes.The effects of seed priming with Spd and Spm on imbibitional chilling tolerance of embryos and seed germinability were investigated. After seeds were primed with 0.25 mM of Spd and Spm for 72 h under 15℃, they were imbibed at 5℃for different time (12, 24, 36 and 48 h). The results showed that both Spd and Spm priming increased the contents of Put, Spd and Spm, enhanced the activities of POD, CAT, SOD and APX of seed embryos, increased the contents of Pro and soluble protein in embryos and reduced nucleic acid content from embryo leachate. The priming treatments also increased germination percentage and shortened the mean germination time after suffering imbibitional chilling stress for different periods. After seeds imbibed for 48 h under chilling stress, Spd and Spm priming increased germination percentage by 7%～30% and 3%～25%, and shortened mean germination time by about 1.2 d and 0.96 d～1.33 d as compared to unprimed control, respectively. During the imbibition under chilling stress, the activities of POD, CAT, SOD and APX, Pro and soluble protein contents in embryos and germination percentage of HuangC were higher than those of Mo17, whereas the content of nucleic acid and mean germination time in HuangC were lower than those of Mo17. The results suggested that seed priming with Spd and Spm could improve the imbibitional chilling tolerance of embryos, alleviate the chilling-induced imbibition injury and enhance the germinability in maize seeds.The relationship between changes of polyamines in embryos and seedlings of primed maize seeds and chilling tolerance of seeds and seedlings were investigated by seed priming with sand, water, salicylic acid (SA), priming combination A (SA + D-Arg (ADC inhibitor) + DFMO (Difluoromethylomithine, ODC inhibitor ) and priming combination B (SA + MGBG). After seeds were primed with sand, water and 0.2 mM SA for 72 h, they were imbibed at 5℃for 48 h. The results showed that sand, water and SA priming increased the contents of embryonic Put, Spd and Spm of HuangC and Mo17 with various degree, the treatments decreased MDA content and membrane permeability of embryos, increased germination percentage and shortened MGT. Both priming combination A (0.2 mM SA + 2.0 mM D-Arg + 2.0 mM DFMO) and combination B (0.2 mM SA + 2.0 mM MGBG) significantly decreased the contents of Put, Spd and Spm of two maize inbred lines. However, combination B increased Put content. Both combination A and B treatments significantly increased embryonic MDA content and membrane permeability of the two maize inbred lines, reduced germination percentage and prolonged MGT. The SA priming had the best effects of improving chilling tolerance of seeds and seedlings among those priming treatments demonstrated by higher the contents of polyamines in embryos and germination percentage, lower MDA content and membrane permeability and shorter MGT. After primed seeds were imbibed under 5℃ for 48 h, then grew at 25℃ for 7 days, sand priming increased the contents of Put, Spd and Spm in shoots of two maize inbred lines with various degree. Priming treatments with sand, water and SA significantly increased the root length, shoot height, root and shoot dry weight of maize seedlings, increased the contents of chlorophyll a, carotenoid and total chlorophyll. The combination A and B treatments decreased root length, shoot height, root and shoot dry weight of seedlings, reduced the contents of chlorophyll a, chlorophyll b, carotenoid and total chlorophyll in both maize inbred lines. The combination A and B treatments had a lower the contents of polyamines in maize embryos accompanied decline of chilling tolerance of seeds and seedlings. The resulted indicated that priming with sand, water and SA might induce the chilling tolerance of maize seeds and seedlings by increased the polyamine contents.