| Polyamines (PAs), which are ubiquitous in all plant cells, participate in a wide range of physiological and biochemical processes. Nitric oxide (NO) is a physiologically reactive substance, as a plant cell signaling molecule, it play an important role in plant root growth and other physiological processes. It have been confirmed that both polyamines and NO promote the development of plant lateral roots, and that polyamines can induce NO production in plant tissues. However, there is no report whether NO formation occurs in the process of polyamines-induced lateral root development? Or whether NO mediates lateral root development induced by polyamines? In this study we choose a normal breed of lettuce seed (Lactuca sativa L. cv. Guasihong) as an experimental material to investigate the roles and interactions of polyamines and NO in the process of plant lateral root development and main root elongation. The results are as follows:Treatments with exogenous polyamines at low concentrations promoted main root elongation and lateral root development of lettuce seedlings. The optimal concentration of Spd and Spm for main root elongation was 0.5 mM, and Put had wide range of effective concentration from 0.1 mM to 1.0 mM. But for the development of lateral roots, the optimal concentrations of three kinds of exogenous PAs were 0.05 mM, and Spd even increased up to 1.0 mM. Consistently polyamines could significantly promote the fresh weight of roots. Among them, Spd was most effective, followed by Put, and Spm was least. However unlike PAs at low concentrations, PAs at high concentrations had less effect on lateral root development and main root elongation.In corresponding to the treatment with exogenous polyamines which could promote lateral root development and main root elongation, Polyamines inhibitor, methyl-glyoxal-bis-Guanyhyldrazone (MGBG, S-adenosyl-methionine decarboxylase inhibitor) had a strong inhibitory effect on lateral root development and main root elongation, thus resulting in decreases in root fresh weight. Differing from MGBG, cyclohexylamine (CHA, Spd synthase inhibitor) inhibited the development of lateral roots, but enhanced the elongation of main roots and root fresh weight.The results from determination of endogenous PA content showed that Put content in lettuce seedling roots was higher than Spd, and that Spm content was lowest. During root development, three kinds of PAs had different changes. Put content increased gradually, and declined after reaching peak at the sixth day, while contents of Spd and Spm gradually decreased, showing a similar way of their changes. Because Put content was much higher than the contents of Spd and Spm, the total content of PAs in the lettuce roots had the similar variation with Put. Moreover, treatment with exogenous PAs could increase endogenous polyamine contents in seedling roots. Spd and Spm contents markedly increased after two days of treatments with exdogenous PAs, reaching to the maximum value at the sixth day.Treatments with exogenous NO donor sodium nitroprusside (SNP) markedly restrained main root elongation. Differing from the effect, SNP significantly promoted lateral root development, with optimal concentration of 0.05 mM. SNP effect declined with its concentration increasing, exhibiting the similar effect as polyamines. When treated with SNP and PAs, the application of NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1,-oxyl-3-oxide (cPTIO) could prevented the SNP- or PAs- promotive effects on lateral roots. Fluorescent detection using the 1,2-diaminonanthraquinone (DAQ), a NO-sensitive fluorescent probe, showed that lateral root primordia treated with PAs or SNP emitted strong NO fluorescence, which could be eliminated by cPTIO, concomitantly with its inhibitory effect on lateral root emergence promoted by PAs or SNP.All together, these results suggest that NO is probably involved in PAs signaling pathway during the process of PAs- induced development of lateral roots in lettuce seedlings.
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