WILSONVILLE, OR -- Mentor Graphics has extended its shareholder rights plan in advance of its planned expiration on Dec. 31, and increased the price per right by 25%, making a potential unwanted acquisition attempt more difficult.

The EDA company's amended shareholder rights plan extends the expiration date of the plan by 18 months to June 30, 2013, increases the exercise price per right from $50 to $65, and certain other changes such as the advance notice deadline for stockholder business proposals. Rights issue are typically offered only only to existing shareholders, who are given the right to purchase new shares before they are offered to the public, such as in the case of a stock split. Raising the price per right makes it more expensive for existing shareholders to accumulate the new shares.

The amended agreement was not adopted in response to any acquisition proposal, Mentor said in an SEC filing.

The amended and restated bylaws were adopted to update the advance notice provisions with respect to stockholder proposals to provide, among other things, greater flexibility for stockholders in response to concerns raised by stockholders in connection with the 2011 Annual Meeting.

Last year, Mentor engaged in a tactical battle with its largest single shareholder, Carl Icahn, over the semiconductor and printed circuit design software developer's maneuvers to keep its embattled board of directors intact. Mentor adopted a poison pill provision, moved up the date of its annual meeting, which Icahn said was an attempt to shut out alternative slates of board candidates, and also issued more than $220 million in new bonds, which Icahn said would dilute the company's stock.

The just-announced revisions change the advance notice deadline from 90 days before the anniversary of the prior year annual meeting to 30 days before the anniversary of the prior year proxy statement, which will give stockholders more time to submit business proposals or director nomination, Mentor said.

It also increases from 10 days to 30 days the minimum time period for stockholders to submit business proposals or director nominations after announcement of a change in its annual meeting date, and expands and modifies the disclosure stockholders must provide when submitting proposals and nominations at special or annual meetings.