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Class I low-molecular-weight heat shock proteins (LMW HSPs) which are likely to promote thermotolerance in soybean seedlings are induced by heat shock treatment as well as azetidine-2-carboxylic acid (Aze), a proline analog. The amount of class I LMW HSPs induced by 10 mM Aze for 24 hours was equivalent to that induced by 40℃for 2 hours. However, these class I LMW HSPs induced by Aze treatment had no effect on thermotolerance of seedlings. When soybean seedlings, treated with 10 mM Aze for 6 hours, were then incubated with several different amino acids for 24 hours, respectively, all seedlings of each treatment accumulated certain amount of class I LMW HSPs. However, only the seedlings incubated with proline subsequently acquired thermotolerance. In these seedlings, about 40% of class I LMW HSPs were found in post-ribosomal supernatant (PRS) fraction of total protein.On the other hand, only around 10% of class I LMW HSPs induced by subsequent incubated with other amino acids (e.g., Gly, Phe, Cys and Gln) were found in PRS. Class I LMW HSP complex from the soybean seedlings treated with Aze and subsequently with proline reduced denaturation of PRS of soybean seedlings at high temperature. Analysis of amino acid composition indicated that Aze was not incorporated into class I LMW HSP (pI 6.2, MW 17.3), but was found in protein which was not inducible during heat shock.
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