Relationship between osmotic stress and the levels of free, conjugated and bound polyamines in leaves of wheat seedlings

The changes in content and form of polyamines (PAs) in the leaves of two wheat (Triticum aestivum L.) cultivar seedlings, differing in drought tolerance, were investigated under osmotic stress. Yumai No. 18 cv. (drought-tolerant) showed marked increases of free-spermidine (free-Spd) and free-spermine (free-Spin) levels in leaves after polyethylene glycol (PEG) 6000 treatment for 7 days, while Yangmai No. 9 cv. (drought-sensitive) showed a significant increase of free-putrescine (free-Put) level. Thus, Yumai No. 18 cv. gave a higher ratio of (free-Spd + free-Spm)/free-Put than Yangmai No. 9 cv. in leaves in response to osmotic stress. Exogenous treatment of seedlings with methylglyoxal-bis-(guanyl-hydrazone) (MGBG), an S-adenosylmethionine decarboxylase (SAMDC) inhibitor, resulted in reduction of free-Spd and -Spm levels in leaves and aggravation of PEG-induced injury to Yumai No. 18 cv. seedlings, while exogenous Spd treatment resulted in an increase of free-Spd + free-Spin in leaves, in concert with an alleviation of PEG-induced injury to Yangmai No. 9 cv. seedlings. There was no statistically significant difference between the content.,; of perchloric-acid soluble conjugated PAs (PS-conjugated PAs) in leaves of both cultivars under PEG osmotic stress, suggesting that the level Of PS-conjugated PAs in leaves has no relationship with the osmotic stress tolerance of wheat seedlings. In contrast, osmotic stress induced a significant increase of perchloric acid insoluble bound putrescine (PIS-bound Put) level in seedling leaves of Yumai No. 18 cv., whereas osmotic stress affected slightly the PIS-bound Put level in Yangmai No. 9 cv. seedlings. Exogenous treatment of seedlings with phenanthrolin (o-Phen), an inhibitor of transglutaminase (TGase), also aggravated PEG-induced injury to Yumai No. 18 cv. seedlings, coupled with a decrease of PIS-bound Put level. The results suggested that free-Spd, -Spm and PIS-bound Put facilitated the osmotic stress tolerance of wheat seedlings. (C) 2003 Elsevier Ireland Ltd. All rights reserved.