Prolonged Exposure to Elevated Temperature Induces Floral Transition via Up-Regulation of Cytosolic Ascorbate Peroxidase 1 and Subsequent Reduction of the Ascorbate Redox Ratio in Oncidium Hybrid Orchid

Abstract

The bolting time of the Oncidium hybrid orchid is not season dependent and so it is a useful year-round model system to study thermal-induced flowering mechanisms in planta. Previously, we reported that a low ascorbate (AsA) content is essential for floral transition in Oncidium; however, the environmental factors governing initiation of the flowering process remained to be elucidated. The current study revealed that a prolonged elevated temperature treatment (30°C over a 14 d period) induces floral transition. This floral induction in response to thermal stress was associated with a significantly increased reactive oxygen species (ROS) level and a lowered AsA redox ratio, as well as prominently up-regulated expression of cytosolic ascorbate peroxidase (cytAPX1). Transcriptome analysis confirmed that increased temperature affected the differential expression of genes involved in antioxidant metabolism. Likewise, transgenic Arabidopsis ectopically overexpressing Oncidium cytAPX1 displayed an early-flowering phenotype and low AsA redox ratio under thermal stress, while cytAPX1 mutants, apx1-1 and apx1-2, exhibited a delayed-flowering phenotype and a high AsA redox ratio. Our present data illustrate that the floral transition response to thermal stress is mediated by the AsA redox ratio, and that CytAPX plays a pivotal role in modulating the AsA redox ratio in Oncidium hybrid orchid. Taken together, the results from this investigation of the thermal-induced flowering mechanism indicated that the AsA redox ratio is a master switch to mediate phase transition from the vegetative to reproductive stage.

Nitric oxide participates in plant flowering repression by ascorbate

“…It is well-known that the reduced form of ascorbate (AsA) and reactive oxygen species are essential for phase transition in Oncidium 6 22 23 . To further elucidate the relationship of AsA and NO during phase transition of Oncidium, Oncidium tissue culture of a protocorm-like body (PLB) was used to monitor the effect of AsA on NO production.…”

“…Although AsA is known to be involved in cellular redox signaling but does not act specifically in any of the known flowering pathways, it plays a general role in responding to environmental signals 5 . The reduced form of AsA is essential for Oncidium phase transition and flowering process under high ambient temperature 6 22 23 . A plausible explanation may be plants would rather produce NO than H 2 O 2 to balance redox homeostasis for its physiological functions, that are tampered by AsA ( Fig.…”

“…The Arabidopsis vtc1 mutant line did not respond to hydrogen peroxide (H 2 O 2 ) and calcium chloride (CaCl 2 ) compared to wild type (Wt), suggesting the role of cyt APX1 maintained a low AsA ratio to scavenge H 2 O 2 ( Fig. 6 ) 5 6 22 23 . Molecular studies of the Arabidopsis noa1 mutant further showed that AsA could not rescue flowering time ( Fig.…”

“…The inflorescent bud (I) and axillary bud (AX) are formed concomitantly at each side of the pseudobulb (P) base at the vegetative stage. Either bolting or developing to next generation is regulated by a complicated genetic network (modified based upon previously published 6 22 ). L. denotes the leaf numbering from the top to the base.…”

“…Previously, we have demonstrated that endogenous AsA is essential for phase transition and the flowering process 6 . Similarly, the redox homeostasis of Oncidium is reliant on ambient temperature as well as phase transition signaled by the decrease in AsA levels in ‘pseudobulb with inflorescent bud’ (PIB) tissues 22 23 . However, the specific mechanism of AsA and NO signaling in repressing flowering is poorly understood.…”