Overexpression of CONSTANS homologs CO1 and CO2 fails to alter normal reproductive onset and fall bud set in woody perennial poplar.

Chuan-Yu Hsu,Richard Meilan,Kyoungok No,John E Carlson,Grier P Page,Abdelali Barakat,Haiying Liang,Cetin Yuceer,Olga Pechanova,Joshua P Adams,Miguel A Blazquez

doi:10.1371/journal.pone.0045448

Chuan-Yu Hsu, Richard Meilan + Show 9 more

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https://doi.org/10.1371/journal.pone.0045448

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Abstract

CONSTANS (CO) is an important flowering-time gene in the photoperiodic flowering pathway of annual Arabidopsis thaliana in which overexpression of CO induces early flowering, whereas mutations in CO cause delayed flowering. The closest homologs of CO in woody perennial poplar (Populus spp.) are CO1 and CO2. A previous report [1] showed that the CO2/FLOWERING LOCUS T1 (FT1) regulon controls the onset of reproduction in poplar, similar to what is seen with the CO/FLOWERING LOCUS T (FT) regulon in Arabidopsis. The CO2/FT1 regulon was also reported to control fall bud set. Our long-term field observations show that overexpression of CO1 and CO2 individually or together did not alter normal reproductive onset, spring bud break, or fall dormancy in poplar, but did result in smaller trees when compared with controls. Transcripts of CO1 and CO2 were normally most abundant in the growing season and rhythmic within a day, peaking at dawn. Our manipulative experiments did not provide evidence for transcriptional regulation being affected by photoperiod, light intensity, temperature, or water stress when transcripts of CO1 and CO2 were consistently measured in the morning. A genetic network analysis using overexpressing trees, microarrays, and computation demonstrated that a majority of functionally known genes downstream of CO1 and CO2 are associated with metabolic processes, which could explain their effect on tree size. In conclusion, the function of CO1 and CO2 in poplar does not appear to overlap with that of CO from Arabidopsis, nor do our data support the involvement of CO1 and CO2 in spring bud break or fall bud set.

Highlights

The CONSTANS (CO) gene encodes a zinc finger transcription factor that plays a major role in the photoperiodic flowering pathway of the annual and facultative long-day plant Arabidopsis thaliana [2]
Mutations in CO cause delayed flowering under long days in Arabidopsis, but do not affect flowering time relative to wildtype plants grown under short days, suggesting that CO promotes flowering under long days
CO protein is degraded in darkness by a CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1, an E3 ubiquitin ligase)-dependent mechanism when plants are grown under short days [8], but light stabilizes CO under long days through cryptochrome 2 and phytochrome A [9]

Summary

Introduction

The CONSTANS (CO) gene encodes a zinc finger transcription factor that plays a major role in the photoperiodic flowering pathway of the annual and facultative long-day plant Arabidopsis thaliana [2]. Constitutive expression of COL9 resulted in plants with delayed flowering, whereas mutants with reduced COL9 transcription flowered early under long days, suggesting that COL9, like COL3, is a floral repressor [26]. These studies show that COL genes are functionally unrelated to CO, with respect to photoperiodic flowering regulation, suggesting that they may have other roles in controlling growth and development. If CO2 and/ or CO1 are functionally conserved in poplar relative to CO in Arabidopsis [3,4], their constitutive overexpression should induce early reproductive onset and delay fall bud set To test this hypothesis, we conducted physiological and genetic experiments of CO1 and CO2, including expression analysis, in poplar. Our longterm field experiments showed no evidence for involvement of these genes, singly or in combination, in reproductive onset, spring bud break, or fall bud set, suggesting that CO1 and CO2 in poplar are not functional orthologs of Arabidopsis CO

Results

Discussion

Experimental Procedures