Effect of auxin, cytokinin and nitrogen on anthocyanin biosynthesis in callus cultures of red-fleshed apple (Malus sieversii f.niedzwetzkyana)

We have induced callus tissues from one R6/R6 homozygous genotype red-fleshed apple individual which was the hybrid offspring of Malus sieversii f.niedzwetzkyana and 'Fuji', and investigated the effect of auxin alone and auxin combined with cytokinin or nitrogen deficiency on anthocyanin synthesis. In callus culture, auxin alone significantly inhibited anthocyanin biosynthesis with the increase of auxin concentration. The inhibitory effect of 2,4-dichlorophenoxyacetic acid (2,4-D) on anthocyanin accumulation was about tenfold stronger than naphthalene acetic acid. Anthocyanin regulatory genes (MdMYB10 and MdbHLH3) and structural genes were dramatically suppressed by 0.6 mg/L 2,4-D. The inhibitory effect of auxin on anthocyanin biosynthesis was influenced by cytokinins 6-benzylaminopurine (BAP) and thidiazuron (TDZ) as well as nitrogen deficiency. Auxin and cytokinin displayed the interaction in controlling anthocyanin biosynthesis. Co-treatment of auxin and cytokinin (BAP or TDZ) significantly enhanced the cytokinin-induced increase in anthocyanin levels but too high auxin concentration strongly inhibited anthocyanin synthesis even in the presence of cytokinin. Nitrogen deficiency could reverse the inhibition of anthocyanin accumulation by auxin.