March 13 (Reuters) - Adobe's shares plunged 9% in premarket trading on Friday after the Photoshop maker said CEO Shantanu Narayen would step down after 18 years at the helm, unsettling investors already wary of AI-driven disruptions to the design software market.

The longtime CEO's exit comes at a critical juncture as Adobe works to reassure investors it can keep pace with sweeping changes brought by artificial intelligence in the software landscape.

It follows a broader slide in software stocks after fears that AI agents could supplant some traditional applications that led to a nearly $1 trillion rout in software stocks globally last month.

"The loss of an iconic leader at a time of peak uncertainty around the future of software more broadly, and the positioning of Adobe specifically in this new GenAI world is bound to further investor uncertainty and anxiety around the shares," said analysts at Morgan Stanley.

Adobe's shares are down about 23% so far this year, extending a slide that has stretched over the past two years.

The company, which makes Illustrator, Premiere Pro and other tools for creative professionals, is among a group of SaaS providers including Salesforce that have struggled to win new clients amid a wave of AI start-ups.

On Thursday, Adobe reported double-digit growth in total revenue and customer subscription segments in the first quarter, reflecting resilient spending on its product suite.

"After steering the Adobe ship through rough seas over the past several years, several data points from the most recent quarter suggest the captain (Narayen) may have brought this franchise into a safe harbor, from which it can continue to thrive," Morgan Stanley analysts said.