Shares of Syngenta AG (NYSE:SYT) soared more than 12 percent Friday after the Swiss insecticide maker rejected an unsolicited $45 billion takeover offer from U.S. rival Monsanto Company. Syngenta said the bid “undervalues” the company. A deal with St. Louis Missouri-based Monsanto Co., the world’s largest seed company with a market capitalization of $58 billion, would combine two of the largest makers of insecticides and other crop protection products. 

Following the announcement, shares of Monsanto Company (NYSE:MON) gained 2.7 percent to $122.36 in morning trading. Meanwhile, Syngenta's shares in Switerland surged 20 percent to a 52-week high of 411.80 Swiss francs. 

Monsanto on Friday confirmed the private bid for Syngenta, adding that a combined company would deliver "significant value to shareholders of both companies" and generate "substantial synergies." Monsanto made a private proposal to acquire the company at 449 Swiss francs a share, or about $490, in a combination of cash and stock, valuing Syngenta at more than $45 billion.

Basel, Switzerland-based Syngenta, which has a market value of $37 billion, said Monsanto’s proposal does not reflect Syngenta's growth prospects, undervaluing the potential future value of the company’s crop-focused positions. “The offer fundamentally undervalues Syngenta’s prospects and underestimates the significant execution risks, including regulatory and public scrutiny at multiple levels in many countries,” the company said in a statement Friday. 

A deal would create a combined company with more than $30 billion in annual sales.

Syngenta employs 29,000 people in more than 90 countries. Its U.S. shares are listed on the New York Stock Exchange and have gained around 33 percent this year, while shares of Monsanto have edged up just over 2 percent.