Nissan Motor shares climbed as much as 21% on Tuesday before closing up 13% – the highest jump in 15 years – following news that activist investor Suntera (Cayman) had taken a 2.5% stake in the company, according to a report by BNN Bloomberg.

Nissan’s short interest stood at 21% of its free float as of Friday, making it the third-most shorted stock on the Nikkei 225 Stock Average, according to data from S&P Global.

Suntera (Cayman), is a trustee of ECM Master Fund, which is reportedly managed by Effissimo Capital Management, a Singapore-based hedge fund known for buying into distressed companies.

Bloomberg cites an unnamed source familiar with the matter as saying that Effissimo controls the 2.5% stake in Nissan. However, neither Nissan nor the fund has confirmed this.

The rally in Nissan’s stock follows speculation that Suntera’s involvement in Nissan may lead to structural reforms aimed at fixing its ailing business, according to Ikuo Mitsui, a fund manager at Aizawa Securities. The stock plunged last week after Nissan announced plans to slash jobs and cut a fifth of its production capacity.

Effissimo has held shares in another company, Nissan Shatai – which runs Nissan’s manufacturing operations – since at least 2007. The fund is Nissan Shatai’s second-largest shareholder, according to a September filing.