BEING ACYANIC: AN UNAVOIDABLE FATE FOR MANY WHITE FLOWERS?

For a range of white flowering ornamental species possessing favourable phenotypical and economical properties there is an ardent desire of breeders to get them coloured.
Unfortunately, classical approaches often fail in this respect which has led to an increased interest in understanding the underlying reasons for this failure.
Here attention is focussed on flavonoids as the main group of colour-giving compounds in flowers.
Flavonoid biosynthesis is a well-examined pathway.
In particular it has been possible to demonstrate and characterise the activity of flavonoid enzymes resulting in the formation of anthocyanins, which are responsible for colours ranging from orange and red to lilac and blue.
The lack of activity of one ore more enzymes of this pathway leads to acyanic flowers.
In white spatha of Zantedeschia aethiopica cv. ‘Nili’ and white petals of Osteospermum-Hybrid cv. ‘Zimba’ early enzymes of flavonoid biosynthesis – chalcone synthase (CHS), chalcone isomerase (CHI) and flavanone 3-hydroxylase (FHT) were proven to be active, resulting in the formation of dihydrokaempferol (DHK). This compound was shown to be 3’-hydroxylated to dihydroquercetin (DHQ) by flavonoid 3’-hydroxylase (F3’H) in Zantedeschia and to be 3’- and 5’-hydroxylated to DHQ and dihydromyrecetin (DHM) by flavonoid 3’5’-hydroxylase (F3’5’H) in Osteospermum.

Whereas reduction of dihydroflavonols by DFR to leucoanthocyanidins did not occure in ‘Zimba’, it was clearly detected in ‘Nili’. This suggests a lack of anthocyanidin synthase (ANS) activity in ‘Nili’. ANS converts leucoanthocyanidins to respective anthocyanidins.
Particle gun bombardment of ‘Nili’ spatha with plasmids containing Ans from M. incana led to red spots, thus supporting this hypothesis.
On the other hand, ‘Zimba’ petals did not show any spotting under bombardment both with plasmids containing Ans and plasmids containing Dfr from Gerbera-Hydrids suggesting the regulatory blocking of a complete set of late enzymes.