By Shaimaa Fayed

CAIRO, Aug 12 (Reuters) - Egypt's Nour Party said on Monday it could join the assembly writing a new constitution, adding Islamist support to the military's political transition plan following its overthrow of President Mohamed Mursi last month.

Nour, the second largest Islamist party after Mursi's Muslim Brotherhood, said it had reservations about the fact that the constitutional amendments would take place under an appointed interim president as opposed to an elected one.

It also wanted certain clauses in the constitution retained.

"But in spite of that, the party does not object to taking part in the committee of 50 to defend the right of the nation in protecting its constitution," Nour said in a statement.

The development is a significant one as Egypt struggles through a political crisis heightened by Mursi's overthrow.

It adds Islamist legitimacy to the military's "road map" leading to new elections in nine months, and further isolates the Muslim Brotherhood.

Mursi became Egypt's first freely elected president in June, 2012, but faced a wave of popular discontent which led the army to intervene and replace him with an interim government.

Mursi has been arrested, but his supporters have made a stand in two protest camps in Cairo demanding his reinstatement.

Nour had earlier rejected a call by the head of the army, General Abdel Fattah al-Sisi, to take part in mass rallies which he said would give him a mandate to crack down on what he called terrorism.

Though agreeing to take part in the constituent assembly drawing up the new constitution, Nour said in its statement:

"The party objects to the principle of the constitutional amendments taking place under an appointed president and through a committee whose work is reigned over by individuals appointed by that same president."

It voiced other reservations including what it said was an under-representation of political parties in the 50-member committee that would be tasked with writing the document. (Reporting by Shaimaa Fayed; Editing by Angus MacSwan and Mike Collett-White)