To understand the molecular factors underlying these 

 mechanisms in higher plants, cultures of plant cells were 

 established from metal-tolerant and -sensitive plants. Analysis of 

 the cells demonstrated that part of the mechanism of tolerance 

 involves the de novo synthesis of glutathione-derived, metal- 

 binding polypeptides that have high binding affinities for Cd, Cu, 

 Zn, Hg, and other metal ions. In metal-tolerant cells, these 

 peptides are normally undetectable. However, de novo synthesis can 

 be detected within 5 minutes of exposure to metal ions, and these 

 peptides reach 6% of the total protein in the cells within 

 12 hours. 



Polypeptide synthesis results from the catalysis of a reaction 

 by the enzyme gamma-glutamylcysteinyl dipeptidyl transpeptidase 

 which consumes glutathione. This enzyme is constitutively present 

 in all plant tissues, and is directly activated by the binding of 

 Cd or Cu (and, perhaps, other ions) directly to the enzyme. 



At least 34 different genes are up- or down-regulated or 

 differentially expressed in tolerant vs. sensitive cells (and 

 plants) upon exposure to metal ions. Some genes are specific to Cd 

 or Cu exposure, but others also are expressed in response to other 

 stresses. In addition, seven enzymes are involved in the 

 biosynthesis of metal-binding polypeptides and their precursors. 

 Each enzyme is encoded by one or more genes, which may be directly 

 or indirectly regulated by metal exposure. 



Preliminary experiments demonstrate that DNA probes for such 

 genes can be used to determine whether or not they are active in 

 plants growing in the environment. The presence of a metal-induced 

 gene product (mRNA) may, therefore, indicate the presence of the 

 metal ion in the soil. Moreover, because the metal ion must be 

 taken up by the plant for expression to occur, information is 

 provided about the mobility of the metal ion within the biosphere. 



Temperature Limitation 



Temperature is a fundamental environmental factor which 

 geographically controls many ecologically important activities and 

 ranges of species. Sudden or gradual temperature changes, or 

 extreme temperature perturbations can result in new or modified 

 activity, reflecting either temperature- induced changes in 

 metabolism or altered gene expression. 



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