Role of Cytokinin, Strigolactone, and Auxin Export on Outgrowth of Axillary Buds in Apple

Abstract

Shoot branching is regulated by phytohormones, including cytokinin (CK), strigolactone (SL), and auxin in axillary buds. The correlative importance of these phytohormones in the outgrowth of apple axillary buds remains unclear. In this study, the outgrowth dynamics of axillary buds of a more-branching mutant (MB) and its wild-type (WT) of Malus spectabilis were assessed using exogenous chemical treatments, transcriptome analysis, paraffin section, and reverse transcription-quantitative PCR analysis (RT-qPCR). High contents of CK and abscisic acid coincided in MB axillary buds. Exogenous CK promoted axillary bud outgrowth in the WT but not in MB, whereas exogenous gibberellic had no significant effect on bud outgrowth in the WT. Functional analysis of transcriptome data and RT-qPCR analysis of gene transcripts revealed that MB branching were associated with CK signaling, auxin transport, and SL signaling. Transcription of the SL-related genes MsMAX1, MsD14 , and MsMAX2 in the axillary buds of MB was generally upregulated during bud outgrowth, whereas MsBRC1 / 2 were generally downregulated both in WT and MB. Exogenous SL inhibited outgrowth of axillary buds in the WT and the apple varieties T337, M26, and Nagafu 2, whereas axillary buds of the MB were insensitive to SL treatment. Treatment with N -1-naphthylphalamic acid (NPA; an auxin transport inhibitor) inhibited bud outgrowth in plants of the WT and MB. The transcript abundance of MsPIN1 was generally decreased in response to NPA and SL treatments, and increased in CK and decapitation treatments, whereas no consistent pattern was observed for MsD14 and MsMAX2 . Collectively, the present results suggest that in apple auxin transport from the axillary bud to the stem may be essential for the outgrowth of axillary buds, and at least, is involved in the process of bud outgrowth.

Overexpression of SHORT-ROOT2 transcription factor enhanced the outgrowth of mature axillary buds in poplar trees

“…4E). Type-A ARRs are prime cytokinin response genes with inducibility by cytokinin and MsARR3/5 have been shown to be required for CK-mediated bud outgrowth in apple trees (Tan et al ., 2019). Type-B ARRs are transcription factors acting as mediators of cytokinin responses and Arabidopsis ARR2 has been previously shown to directly bind and activate the expression of WUS to define the stem cell niche during axillary meristem formation (Meng et al ., 2017; Wang et al ., 2017).…”

The Biological Function and Roles in Phytohormone Signaling of the F-Box Protein in Plants

“…Strigolactones are newly identified plant hormones that are derived from β-carotene. SLs can not only regulate axillary bud growth [115,116], leaf senescence [117], and root development [21], but can also respond to multiple stresses [118] and interactions with plant symbiotic bacteria [119]. In plants, SL is perceived by the α/β hydrolase DWARF14 (D14).…”

Effect of Cytokinin on Fruit Crops

“…Apple Tan et al, (2019) observed on role of cytokinin on outgrowth of auxiliary buds in apple and he opined that high content cytokinin present in more branching mutant type and in wild type exogenous application of ck promotes axillary bud out growth. Magyaretal.…”