Significance of putrescine conversion in filling grain embryos of wheat plants subjected to drought stress

Background and AimsPolyamines (PAs) play important roles in drought tolerance, but the physiological significance of putrescine (Put) conversion into other forms of PAs is not clear in filling grain embryos of wheat (Triticum aestivum L.) plants under drought stress. MethodsThe changes in the levels of three main free PAs, Put, spermidine (Spd), and spermine (Spm), covalently conjugated PAs (perchloric acid-soluble), and covalently bound PAs (perchloric acid-insoluble), were investigated in embryos of filling grains, in two wheat cultivars, Longmai No. 079 (drought-tolerant) and Wanmai No. 52 (drought-sensitive). Furthermore, the activities of arginine decarboxylase, S-adenosylmethionine decarboxylase, and transglutaminase, which affect free Put biosynthesis, Spd and Spm biosynthesis from Put, and conversion of free PAs into bound PAs, respectively, were also determined. Exogenous PAs and PA biosynthesis inhibitors were also applied in the experiment. ResultsUnder drought stress, the levels of conjugated PAs did not significantly differ between the two cultivars. However, the levels of free Spd, free Spm and bound Put increased more markedly (p < 0.05) in drought-tolerant Longmai No. 079 than in drought-sensitive Wanmai No. 52, suggesting that free Spd and Spm, and bound Put, which were converted from free Put, were possibly involved in drought tolerance. Exogenous Spd treatment enhanced the drought-induced increases in endogenous Spd and Spm contents in drought-sensitive Wanmai No. 52, and increased drought tolerance, as judged by the decrease in flag leaf relative plasma membrane permeability and increases in flag leaf relative water content, 1000-grain weight and grain number per spike. Methylglyoxal-bis guanylhydrazone and o-phenanthrolin inhibited drought-induced increases in free Spd, Spm, and bound-Put contents in drought-tolerant Longmai No. 079, and decreased in drought tolerance. ConclusionsCollectively, the conversions of free Put into free Spd, Spm, and bound Put in filling grain embryos enhanced the tolerance of wheat plants to drought stress.