Age, as a threshold of floral competence acquisition, prevents precocious flowering when there is insufficient biomass, and ensures flowering independent of environmental conditions; however, the underlying regulatory mechanisms are largely unknown. In this study, silencing the expression of a nuclear factor gene, CmNF-YB8, from the short day plant chrysanthemum (Chrysanthemum morifolium), results in precocious transition from juvenile to adult, as well as early flowering, regardless of day length conditions. The expression of SQUAMOSA PROMOTER BINDING-LIKE (SPL) family members, SPL3, SPL5, and SPL9, is upregulated in CmNF-YB8-RNAi plants, while expression of the microRNA, cmo-MIR156, is downregulated. In addition, CmNF-YB8 is shown to bind to the promoter of the cmo-MIR156 gene. Ectopic expression of cmo-miR156, using a virus-based microRNA expression system, restores the early flowering phenotype caused by CmNF-YB8 silencing. These results show that CmNF-YB8 influences flowering time through directly regulating the expression of cmo-MIR156 in the aging pathway.

Background: Chrysanthemum is one of the most important ornamental crops in the world and drought stress seriously limits its production and distribution. In order to generate a functional genomics resource and obtain a deeper understanding of the molecular mechanisms regarding chrysanthemum responses to dehydration stress, we performed large-scale transcriptome sequencing of chrysanthemum plants under dehydration stress using the Illumina sequencing technology. Results: Two cDNA libraries constructed from mRNAs of control and dehydration-treated seedlings were sequenced by Illumina technology. A total of more than 100 million reads were generated and de novo assembled into 98,180 unique transcripts which were further extensively annotated by comparing their sequencing to different protein databases. Biochemical pathways were predicted from these transcript sequences. Furthermore, we performed gene expression profiling analysis upon dehydration treatment in chrysanthemum and identified 8,558 dehydration-responsive unique transcripts, including 307 transcription factors and 229 protein kinases and many well-known stress responsive genes. Gene ontology (GO) term enrichment and biochemical pathway analyses showed that dehydration stress caused changes in hormone response, secondary and amino acid metabolism, and light and photoperiod response. These findings suggest that drought tolerance of chrysanthemum plants may be related to the regulation of hormone biosynthesis and signaling, reduction of oxidative damage, stabilization of cell proteins and structures, and maintenance of energy and carbon supply. Conclusions: Our transcriptome sequences can provide a valuable resource for chrysanthemum breeding and research and novel insights into chrysanthemum responses to dehydration stress and offer candidate genes or markers that can be used to guide future studies attempting to breed drought tolerant chrysanthemum cultivars.

SUMMARY: Drought is an environmental factor that can severely influence plant development and distribution, and greatly affect the yield and economic value of crops. We characterized CmBBX19, a BBX family subgroup IV member gene, from the transcriptome database of Chrysanthemum morifolium in response to drought stress. Drought stress and ABA treatments downregulated the expression of CmBBX19. We generated CmBBX19‐overexpressing (CmBBX19‐OX) lines and CmBBX19‐suppressing lines (CmBBX19‐RNAi), and found that suppressed expression of CmBBX19 led to enhanced drought tolerance compared with the wild‐type (WT) controls, while CmBBX19‐OX lines exhibited reduced drought tolerance. Downstream gene analysis showed that CmBBX19 modulates drought tolerance mainly through inducing changes in the expression of ABA‐dependent pathway genes, including protective protein, redox balance and cell wall biogenesis genes, such as responsive to ABA 18, peroxidase 12, and cellulose synthase‐like protein G2. Moreover, CmBBX19 was shown to interact with CmABF3, a master ABA signaling component, to suppress expression of these downstream genes. We conclude that BBX19‐ABF3 module functions in the regulation of drought tolerance of chrysanthemum through an ABA‐dependent pathway.

Developmental transitions in plants require adequate carbon resources, and organ abscission often occurs due to competition for carbohydrates/assimilates. Physiological studies have indicated that organ abscission may be activated by Suc deprivation; however, an underlying regulatory mechanism that links Suc transport to organ shedding has yet to be identified. Here, we report that transport of Suc and the phytohormone auxin to petals through the phloem of the abscission zone (AZ) decreases during petal abscission in rose (Rosa hybrida), and that auxin regulates Suc transport into the petals. Expression of the Suc transporter RhSUC2 decreased in the AZ during rose petal abscission. Similarly, silencing of RhSUC2 reduced the Suc content in the petals and promotes petal abscission. We established that the auxin signaling protein RhARF7 binds to the promoter of RhSUC2, and that silencing of RhARF7 reduces petal Suc contents and promotes petal abscission. Overexpression of RhSUC2 in the petal AZ restored accelerated petal abscission caused by RhARF7 silencing. Moreover, treatment of rose petals with auxin and Suc delayed ethylene-induced abscission, whereas silencing of RhARF7 and RhSUC2 accelerated ethylene-induced petal abscission. Our results demonstrate that auxin modulates Suc transport during petal abscission, and that this process is regulated by a RhARF7-RhSUC2 module in the AZ.

Reactive oxygen species (ROS) are unstable reactive molecules that are toxic to cells. Regulation of ROS homeostasis is crucial to protect cells from dysfunction, senescence, and death. In plant leaves, ROS are mainly generated from chloroplasts and are tightly temporally restricted by the circadian clock. However, little is known about how ROS homeostasis is regulated in nonphotosynthetic organs, such as petals. Here, we showed that hydrogen peroxide (H₂O₂) levels exhibit typical circadian rhythmicity in rose (Rosa hybrida) petals, consistent with the measured respiratory rate. RNA-seq and functional screening identified a B-box gene, RhBBX28, whose expression was associated with H₂O₂ rhythms. Silencing RhBBX28 accelerated flower senescence and promoted H₂O₂ accumulation at night in petals, while overexpression of RhBBX28 had the opposite effects. RhBBX28 influenced the expression of various genes related to respiratory metabolism, including the TCA cycle and glycolysis, and directly repressed the expression of SUCCINATE DEHYDROGENASE 1, which plays a central role in mitochondrial ROS (mtROS) homeostasis. We also found that PHYTOCHROME-INTERACTING FACTOR8 (RhPIF8) could activate RhBBX28 expression to control H₂O₂ levels in petals and thus flower senescence. Our results indicate that the circadian-controlled RhPIF8–RhBBX28 module is a critical player that controls flower senescence by governing mtROS homeostasis in rose.

Drought is a major abiotic stress that limits plant growth and development. Adaptive mechanisms have evolved to mitigate drought stress, including the capacity to adjust water loss rate and to modify the morphology and structure of the epidermis. Here, we show that the expression of CmNF‐YB8, encoding a nuclear factor Y (NF‐Y) B‐type subunit, is lower under drought conditions in chrysanthemum (Chrysanthemum morifolium). Transgenic chrysanthemum lines in which transcript levels of CmNF‐YB8 were reduced by RNA interference (CmNF‐YB8‐RNAi) exhibited enhanced drought resistance relative to control lines, whereas lines overexpressing CmNF‐YB8 (CmNF‐YB8‐OX) were less tolerant to drought. Compared to wild type (WT), CmNF‐YB8‐RNAi plants showed reduced stomatal opening and a thicker epidermal cuticle that correlated with their water loss rate. We also identified genes involved in stomatal adjustment (CBL‐interacting protein kinase 6, CmCIPK6) and cuticle biosynthesis (CmSHN3) that are more highly expressed in CmNF‐YB8‐RNAi lines than in WT, CmCIPK6 being a direct downstream target of CmNF‐YB8. Virus‐induced gene silencing of CmCIPK6 or CmSHN3 in the CmNF‐YB8‐RNAi background abolished the effects of CmNF‐YB8‐RNAi on stomatal closure and cuticle deposition, respectively. CmNF‐YB8 thus regulates CmCIPK6 and CmSHN3 expression to alter stomatal movement and cuticle thickness in the leaf epidermis, thereby affecting drought resistance.