Change in Auxin and Cytokinin Levels Coincides with Altered Expression of Branching Genes during Axillary Bud Outgrowth in Chrysanthemum

Abstract

In the production and breeding of Chrysanthemum sp., shoot branching is an important quality aspect as the outgrowth of axillary buds determines the final plant shape. Bud outgrowth is mainly controlled by apical dominance and the crosstalk between the plant hormones auxin, cytokinin and strigolactone. In this work the hormonal and genetic regulation of axillary bud outgrowth was studied in two differently branching cut flower Chrysanthemum morifolium (Ramat) genotypes. C17 is a split-type which forms an inflorescence meristem after a certain vegetative period, while C18 remains vegetative under long day conditions. Plant growth of both genotypes was monitored during 5 subsequent weeks starting one week before flower initiation occurred in C17. Axillary bud outgrowth was measured weekly and samples of shoot apex, stem and axillary buds were taken during the first two weeks. We combined auxin and cytokinin measurements by UPLC-MS/MS with RT-qPCR expression analysis of genes involved in shoot branching regulation pathways in chrysanthemum. These included bud development genes (CmBRC1, CmDRM1, CmSTM, CmLsL), auxin pathway genes (CmPIN1, CmTIR3, CmTIR1, CmAXR1, CmAXR6, CmAXR2, CmIAA16, CmIAA12), cytokinin pathway genes (CmIPT3, CmHK3, CmRR1) and strigolactone genes (CmMAX1 and CmMAX2). Genotype C17 showed a release from apical dominance after floral transition coinciding with decreased auxin and increased cytokinin levels in the subapical axillary buds. As opposed to C17, C18 maintained strong apical dominance with vegetative growth throughout the experiment. Here high auxin levels and decreasing cytokinin levels in axillary buds and stem were measured. A differential expression of several branching genes accompanied the different hormonal change and bud outgrowth in C17 and C18. This was clear for the strigolactone biosynthesis gene CmMAX1, the transcription factor CmBRC1 and the dormancy associated gene CmDRM1, that all showed a decreased expression in C17 at floral transition and an increased expression in C18 with continuous vegetative growth. These results offer a case study for Chrysanthemum, showing an altered cytokinin to auxin balance and differential gene expression between vegetative growth with apical dominance and transition to generative growth with loss of apical dominance and axillary bud outgrowth. This suggests a conservation of several aspects of the hormonal and genetical regulation of bud outgrowth in Chrysanthemum. Furthermore, 15 previously uncharacterised genes in chrysanthemum, were described in this study. Of those genes involved in axillary bud outgrowth we identified CmDRM1, CmBRC1 and CmMAX1 as having an altered expression preceding axillary bud outgrowth, which could be useful as markers for bud activity.

Sugar Transporter, CmSWEET17, Promotes Bud Outgrowth in Chrysanthemum Morifolium

“…This labor-intensive process limits the development of the ornamental chrysanthemum industry to a certain extent [1]. Currently, reports on the shoot branching of chrysanthemum focus mainly on the effects of hormones, including strigolactone [2], auxin [3], and cytokinin [4], as well as other factors such as temperature [5] and transcription factor BRC1 [6]. However, few studies have reported on the effect of sugar on the regulation of the shoot branching of chrysanthemum [7].…”

“…Since 1934, when Thimann and Skoog demonstrated that apical dominance was associated with auxin [17], this hormone has been regarded as the key signal controlling bud outgrowth and apical dominance [18]. The growth of axillary buds is accompanied by auxin export from the buds and an increase in the expression of the auxin transport gene PINs [4,19]. In recent years, studies have shown that the initial signaling molecule regulating plant apical dominance could be sugar rather than auxin [20,21]; and that auxin acts on subsequent continuous growth stages [22].…”

Plant Inflorescence Architecture: The Formation, Activity, and Fate of Axillary Meristems

“…Auxin produced in the shoot enters the rootward polar auxin transport (PAT) stream and prevents bud outgrowth in at least two ways: by modulating production of cytokinin and strigolactone, which act in the axillary buds (Dun et al 2012;Teichmann and Muhr 2015;Dierck et al 2016), and by regulating auxin flow out of buds (Sachs 1981;Prusinkiewicz et al 2009;Balla et al 2011). The latter response is less pronounced in pea (Chabikwa et al 2019).…”

Flowering behaviour inArabis alpinaensures the maintenance of a perennating dormant bud bank

“…Decapitation causes an elevation of sucrose levels followed by a depletion of the endogenous indole-3-acetic acid (IAA) in the polar auxin transport stream, which influences the auxin flux out of the axillary bud contributing to its sustainable growth 13–17 . Flowering transition triggers the activation of the upper most axillary buds in a basipetal sequence having a similar effect to decapitation 13,18 . In an annual plant, the relation between flowering and bud activation is easy to trace as its life cycle ends within one growing season.…”