| Drought is a problem worldwide that threatens food supplies. To investigate the mechanism of plant response to drought stress and to improve the drought tolerance has been one of major considerations for a long time. Wheat (Triticum aestivum L.) is one of the important food crops. Polyamines (PAs) are biologically ubiquitous aliphatic amines with low molecular weight that are implicated in many aspects of growth and development. However, the studies on the relationship between polyamines and tolerance of plant to water stress remain disagreement, especially on the significance of great accumulation of putrescine (Put) under water stress, and at the same time the studies on relationship between conjugated polyamines and water stress are still lacking. The subcellular localization of conjugated polyamines in plants under osmotic stress and involvement of polyamine in water stress-induced signaling pathway have not yet been documented. Here, two wheat cultivars with different drought stress-tolerance (Yumai No. 18 cv., drought-tolerant; Yangmai No.9 cv., drought-sensitive) were used as experimental materials to study the changes in PA kinds and forms, cellular localization and function of polyamines in seedlings subjected to PEG-6000 osmotic stress. The results are as follow:PEG osmotic stress led to increases of three free polyamine level (fPut, fSpd and fSpm) in the leaves of two wheat cultivar seedlings. FSpd, fSpm and PISCC-Put (perchloric acid insoluble covalently conjugated Put) levels in leaves of Yumai No. 18 cv. (drought-tolerant) increased more markedly than those in Yangmai No.9 cv. (drought-sensitive) after polyethylene glycol (PEG) 6,000 treatment for 7 d, while fPut level has a more significant increase in Yangmai No.9. Thus Yumai No. 18 cv. gave a higher ratio of (fSpd + fSpm) / fPut than Yangmai No.9 cv. in leaves in response to osmotic stress. PEG osmotic stress also induced increases of perchloric acid soluble covalently conjugated polyamine ( PSCC-PA: PSCC-Put, PSCC-Spd and PSCC-Spm) levels in leaves, but such a increase has no aparently difference between the two cv. wheat seedlings. The results suggested that fSpd, fSpm and PISCC-Put in leaves enhance the adaptation of the seedlings to osmotic stres.This hypothesis has also been demonstrated by the results of experiments with exogenous polyamines and inhibitors and was consistent with the results of experiments on wheat coleoptiles.FPut level increased greatly in leaves of two cv. wheat seedlings with PEG treatment for 6 h, but fPut level in Yumai No. 18 cv. decreased sharply to a low level (200 nmol/g FW) after 6 h, while in Yangmai No.9 cv. a high level of fPut (300 nmol/g FW) was remained after 6 h. PEG osmotic stress affected slightly the fSpd and fSpm levels in leaves of two cv. wheat seedlings during PEG treatment for 6 h. However, after 6 h, the levels of fSpd and fSpm in leaves of Yumai No. 18 cv. increased greatly, while the increase in Yangmai No.9 cv. was slight. We also found that Yumai No. 18 cv. have greater increases of PISCC-Put than Yangmai No.9 cv. in leaves after PEG treatment for 12 h. These results suggested that tolerance of wheat seedlings to osmotic stress-induced injury is attributed not only to the great increase of fPut in leaves at prophase, but also to the conversion of fPut to fSpd, fSpm and PISCC-Put in time. This hypothesis has also been demonstrated by the results of experiments with exogenous polyamines and inhibitors.Under osmotic stress, significant increases of NCC-Spd, NCC-Spm and PISCC-Put levels and activity of H+-ATPase were observed in root plasma membrane (PM) of drought-tolerant Yumai No. 18 cv. wheat seedling, while no obvious change was found in drought-sensitive Yangmai No.9 cv. Furthermore, Yumai No. 18 gave more obvious increase of PISCC-Spd than Yangmai No. 9. The results suggested that the tolerance of the wheat seedlings to osmotic stress was associated with the activity of H+-ATPase and the contents of NCC-Spd and NCC-Spm and CC-Put and CC-Spd in PM of the seedling roots. |
