Effect Of 6-BA And IAA Spraying On Hormone And Related Gene Expression During Floral Induction And Initiation In ’fuji’ Apple(Malus Domestica Borkh)

To investigate the mechanism by which 6-BA and IAA affected flower bud formation, ‘Fuji’ apple trees were sprayed with 300 mg/l 6-BA and 300 mg/l IAA respectively before the onset of floral induction. The differential transcription responses of flowering genes(MdFT, AFL1 and MdTFL1) and endogenous hormones in terminal buds were studied during flower development. In addition, shoot growth and flowering rate were surveyed.The results showed as follows: after spraying with 6-BA, the flowering ratio of short shoots reach to 59.8%, which is 15% higher than the control. 6-BA treatment also had a significant effect on the shoot components and growth characteristics: The spur proportion increased to 49.9% while the cluster twig proportion and middle long shoot rate were reduced; 32 days after flowering, when most of shoots stopped growing, marked the onset of floral induction. At this time, the average length of shoot in 6-BA treatment was 15.0cm while the average length of shoot in control was 19.9cm. Meanwhile, the ZT concentration was elevated by 6-BA treatment in conjunction with change of ZT/IAA ratio, MdTFL1 expression decreased. The transcription of AFL1 unregulated followed(50 DAF). These results suggested that spraying with 300 mg/l 6-BA changed ZT accumulation in buds and break the ZT /IAA ratio which promoted MdTFL1 expression. Suppression of MdTFL1 expression resulted in greater AFL1 transcription at flower initiation, and then flower development was promoted. For MdIPT3 a and MdIPT5 a, higher cytokinin controlled their expression; rather the expression of MdIPT3 a and MdIPT5 a upregulated when cytokinin accumulation decreased. Nevertheless, the change of shoots component and growth characteristics caused by 6-BA treatment is beneficial for flower bud formation.IAA treatment decreased flowering rate, the spur flowering ratio was 21.8% and the flowering rate of cluster twigs was 4.9% in the treatment which were significantly lower than in the control. IAA treatment also had a significant effect on the shoot components and growth characteristics: The middle long shoot proportion increased to 32.58% while the spur proportion were reduced sharply; The growth of shoots was vigorously, although most shoots stopped growing at 32 days after flowering(32 DAF), the average growth length of IAA treatment was higher than control after the time of ceased growth. The levels of endogenous IAA, ZT, GAs and ABA in buds changed significantly. During flower induction, the content of IAA and GAs increased while ABA and ZT levels reduced; the transcript level of floral genes AFL1 was down-regulated and MdTFL1 expression was up-regulated. These results suggested that IAA treatment inhibited ‘Fuji’ apple flower development according to regulating endogenous hormone level and floral genes expression, promoting trees vegetative growth and reducing the proportion of short shoots, which were not good for flower bud formation.