mostly its substitution for S in biochemical key processes.

As Dr. Schnug said, Selenium replaced Sulfur in the bio-synthesis of the amino acid methionine, which renders it dysfunctional and perturbs cellular function and the translation of proteins.

More details can be found here:

http://onlinelibrary.wiley.com/doi/10.1111/j.1469-185X.1982.tb00364.x/abstract

I hope this helps and is not redundant.

Thanks Dr. Schnug and Dr. Chidiac, 

I have gone through the literature and found 5 basis of Se induced stress in plants;

1. formation of toxic selenoproteins

2. alteration of metabolisms of plant nutrients like S, N, P, Fe and Ca

3. Production of Reactive oxygen species

4. Glutathione-mediated reduction of Selenite may effect photosynthetic activity

5. most recently proposed production of reactive nitrogen species such as nitric oxide and peroxinitrite 

All this I explore after putting this question at the forum. I invite you to discuss on  above mentioned basis of Se induced stress that how much these factors contribute and associted with eachother.

Dear Nadeem

I hope your primary question is replied by the scientists. The subsidiary questions are good for discussion.