A single drop of sunlight fell from the heavens, and from this single drop of sun grew the golden flower. It had the ability to heal any sickness or injuries, even mortal wounds, as demonstrated by Rapunzel, delay aging, as demonstrated by Gothel, though that ability requires constant use of the power.
One day, Mother Gothel discovered the flower, and chose to hoard its healing power for herself. She used it to keep herself young for hundreds of years, and all she had to do was sing a special song. It appears that when the song is sung, the flower releases pollen which is breathed in. Even though Mother Gothel had hidden the flower, other people knew of its existence, suggesting that this flower was not the first of its kind.
As the centuries passed, across a lake from where the flower grew, a kingdom soon came into being. The kingdom was ruled by a benevolent King and Queen. The queen was about to have a baby, but fell seriously ill. Knowing that she and her unborn child would surely die if a cure was not found soon, the entire kingdom went searching for the flower.
When Gothel left the flower in plain sight by accident, one of the kingdom's soldiers found the flower, and it was uprooted and brought back to the castle, where it was made into an infusion for the queen. Soon afterwards the queen recovered, and gave birth to a healthy baby girl: Princess Rapunzel. Rapunzel was born with the rest of the flower's ability, which was the majority of it. A bit obviously went into the queen. Rapunzel gave a drop to Eugene to heal his stab wound that he received from Gothel, but still has most of the flower within. Perhaps as a thanks to the flower for saving the queen and Rapunzel, at the cost of its own existence, Corona adopted the sun as its symbol.