Nothing. But, Panspermia is not meant to address how life began, just the method that may cause its distribution in the Universe. - from the Panspermia Wikipedia article
[1] maybe after glancing off a planet or created by the collision of two large objects
There would be probably other consequences if a such large comet will fall in a planet. I can't see any happy ending for this organism to surviving its travel just by chance. If the comet is small or disintegrates in smaller pieces the plant is roasted and dead. Otherwise if fallen in one piece, all the life in the impact area of such large comet should be crushed by the pressure wave and vaporized also, including our plant