Abstract
In spite of short-day (SD) nature, rice (Oryza sativa) shares a conserved photoperiodic network for flowering control with long-day plants like Arabidopsis thaliana. Flowering or heading is an important agronomic trait in rice. NAC transcription factors (TFs) are well-conserved and one of the largest families of plant TFs. However, their function in flowering or heading time is not well-known yet. A preferential expression of a membrane-bound NAC-like TF OsNTL5 in developing leaves and panicles of rice indicated to us its putative role in flowering. To examine its function, three independent constructs was generated, one with a deletion in the C terminus membrane-spanning domain (OsNTL5∆C), OsNTL5∆C fused with the SRDX transcriptional repressor motif and OsNTL5∆C used with the VP16 activation domain under the Ubiquitin promoter to produce the overexpressing lines OsNTL5∆C, OsNTL5∆C-SRDX, and OsNTL5∆C-VP, respectively in rice. The OsNTL5∆C-VP line showed an early-flowering phenotype. In contrast to this, the plants with OsNTL5∆C and OsNTL5∆C-SRDX showed a very strong late-flowering phenotype, suggesting that OsNTL5 suppresses flowering as a transcriptional repressor. The protein subcellular localization assay suggested that N-terminal part of the OsNTL5 is localized to the nucleus after the protein is cleaved from its membrane-spanning domain at the C-terminal end and functions as a TF. Expression of flowering genes responsible for day length signals such as Early Heading Date 1 (Ehd1), Heading Date 3a (Hd3a), and Rice Flowering Locus T1 (RFT1) was significantly changed in the overexpression lines of OsNTL5∆C-VP, OsNTL5∆C, and OsNTL5∆C-SRDX as analyzed by Quantitative Real-time PCR. ChIP-qPCR and rice protoplasts assays indicate that OsNTL5 directly binds to the promoter of Ehd1 and negatively regulates the expression of Ehd1, which shows antagonistic photoperiodic expression patterns of OsNTL5 in a 24-h SD cycle. Hence in conclusion, the NAC-like TF OsNTL5 functions as a transcriptional repressor to suppress flowering in rice as an upstream factor of Ehd1.
Highlights
Flowering or “heading” is an essential process from vegetative to reproductive growth shift in plants governed by environmental and endogenous signals
Our results suggest that OsNTL5 is translocated from ER to nucleus and acts as a repressor of rice flowering by directly regulating the expression of Early Heading Date 1 (Ehd1)
The phylogeny result demonstrated that OsNTL5 (Os08g44820) is a rice orthologous to Arabidopsis ANTHER INDEHISCENCE FACTOR (AIF) with 60% identity and there is another homolog of OsNTL6 (Os02g57650), which may share at least a partial redundancy with OsNTL5
Summary
Flowering or “heading” is an essential process from vegetative to reproductive growth shift in plants governed by environmental and endogenous signals. Plants integrate the environmental signals (photoperiod, light quality, and temperature) as well as endogenous cues (plant developmental status) to control flowering (Simpson and Dean, 2002; Cho et al, 2016). In LD plants like Arabidopsis, flowering is promoted by the integration of circadian clock and light signals by the GI-CO-FT genetic pathway (Turck et al, 2008; Tsuji et al, 2011). Previous studies have showed that the ambient temperature is an important factor regulating flowering in Arabidopsis. The main studies on ambient temperature focus on the genetic pathways of SHORT VEGETATIVE PHASE (SVP) and FLOWERING LOCUS M (FLM) (Lee et al, 2007, 2013; Pose et al, 2013)
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