Umami Vegan Broth?

Michael Natkin wanted to highlight umami (savoriness) in his vegetable broth:

Umami, of course, is that famed fifth flavor, sensed by receptors that look for glutamate and various ribonucleotides - basically indicators that a food contains protein. It isn't so surprising that we'd be wired to like protein, right?

I want to pack so much umami into this broth that you have one sip and feel your eyes roll back in your head involuntarily. I want to evoke a guttural groan, and in my small sample of testers, that is exactly what has happened.

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The vegetarian ingredients best known for high concentrations of glutamates are tomatoes, dried shiitake mushrooms, marmite, kombu seaweed, and parmesan cheese. I decided to include the first four, but omit the parmesan to keep this broth vegan.

Pretty informative. I don't think I'd ever considered what the "umami" ingredients for vegetarian cooking were. The recipe looks good too, but does require a pressure cooker.