BASEL, SWITZERLAND — The board of directors of Syngenta has rejected a proposal from Monsanto Co. that would have seen the St. Louis-based company acquire Syngenta for 449 Swiss francs per share ($486.20), or about $45 billion.

Syngenta said the proposal “fundamentally undervalues Syngenta’s prospects and underestimates the significant execution risks, including regulatory and public scrutiny at multiple levels in many countries.”

Michel Demaré, chairman of Syngenta, said, “Syngenta is the world leader in crop protection, the No. 3 in seeds and the first company to introduce integrated solutions for growers. Monsanto’s proposal does not reflect the outstanding growth prospects of Syngenta’s integrated strategy and the significant future value potential of the company’s crop-focused innovation and market leading positions.

“While Syngenta’s valuation is currently affected by short-term currency and commodity price movements, the business outlook is strong, with emerging markets accounting for over 50% of our sales. Our integrated strategy has been particularly successful in these markets, which in 2014 registered double-digit growth rates for the fifth consecutive year, and which represent a major part of the future growth potential for our industry. Recently launched new products are achieving rapid sales growth globally as growers demand the latest technologies, and we have a strong pipeline of innovative crop protection products in development, which have total peak sales potential of over $3 billion.”

Mr. Demaré said Syngenta is on track in 2015 to achieve the first $265 million of savings from its Accelerating Operational Leverage Program, and is targeting savings of $1 billion in 2018.

“This will allow us to realize the full benefits of the integrated strategy and will ensure that increases in profitability are sustained for the benefit of Syngenta’s shareholders,” he said.

The combination of Syngenta and Monsanto would create an agricultural company with combined annual revenue of approximately $31 billion. Syngenta specializes in chemicals used to protect crops and enhance yield, while Monsanto makes bioengineered crops.

In making its case to acquire Syngenta, Monsanto said its offer would provide Syngenta shareholders with “a very attractive premium and significant further value creation through ongoing ownership in the combined company.”

Monsanto said it has long respected and followed Syngenta’s business and believes combining the two companies would deliver significant value to all stakeholders, including shareholders.

“Creating a new company from the combination of Syngenta’s strengths and leadership in crop protection chemicals and Monsanto’s leadership in seeds, traits and information technology would form an integrated global leader in agriculture with comprehensive and complementary product portfolios, and an ag-focused organization with enhanced abilities to develop and accelerate innovative solutions for growers,” Monsanto said. “Monsanto believes the combined company would be uniquely positioned to deliver a comprehensive suite of integrated solutions to farmers around the world and to accelerate technological innovation through precision agriculture and advanced research and development capabilities aimed at increasing the world’s food supply in a sustainable fashion.”