In this paper from the July 2012 issue of MPMI (Molecular Plant Microbe Interactions), French scientists isolated compounds from the roots and rhizosphere of Brassica napus (aka rapeseed or canola), which stimulate the germination of the B. napus pathogen, Phelipanche ramosa, a parasitic weed. They first used gas chromatography-mass spec to isolate 8 glucosinolate break-down products (glucosinolates are metabolites found often and almost exclusively in the Brassicas, the Order that includes broccoli). Then they found they found that two of these products, both found in the rhizosphere and both isothiocyanates, strongly and specifically triggered P. ramosa germination.
This group also spent some time looking at strigolactones, which are known to induce the growth of mycorrhizal (plant-associated) fungi as well as the germination of plant parasites like Striga (from which the metabolite got its name – according to Wikipedia). But they found (a) no strigolactones in the roots/rhizosphere of B. napus, (b) organisms that respond to strigolactones didn’t respond to isothiocyanates, and (c) P. ramosa did not germinate in response to strigolactones.
Here’s a link to the abstract, with links to download the pdf.