CONSTANS delays Arabidopsis flowering under short days

Long days (LD) promote flowering of Arabidopsis thaliana compared with short days (SD) by activating the photoperiodic pathway. Here we show that growth under very‐SD (3 h) or darkness (on sucrose) also accelerates flowering on a biological scale, indicating that SD actively repress flowering compared with very‐SD. CONSTANS (CO) repressed flowering under SD, and the early flowering of co under SD required FLOWERING LOCUS T (FT). FT was expressed at a basal level in the leaves under SD, but these levels were not enhanced in co. This indicates that the action of CO in A. thaliana is not the mirror image of the action of its homologue in rice. In the apex, CO enhanced the expression of TERMINAL FLOWER 1 (TFL1) around the time when FT expression is important to promote flowering. Under SD, the tfl1 mutation was epistatic to co and in turn ft was epistatic to tfl1. These observations are consistent with the long‐standing but not demonstrated model where CO can inhibit FT induction of flowering by affecting TFL1 expression.     

Gibberellic acid signaling is required for ambient temperature‐mediated induction of flowering in Arabidopsis thaliana

Distinct molecular mechanisms integrate changes in ambient temperature into the genetic pathways that govern flowering time in Arabidopsis thaliana. Temperature‐dependent eviction of the histone variant H2A.Z from nucleosomes has been suggested to facilitate the expression of FT by PIF4 at elevated ambient temperatures. Here we show that, in addition to PIF4, PIF3 and PIF5, but not PIF1 and PIF6, can promote flowering when expressed specifically in phloem companion cells (PCC), where they can induce FT and its close paralog, TSF. However, despite their strong potential to promote flowering, genetic analyses suggest that the PIF genes seem to have only a minor role in adjusting flowering in response to photoperiod or high ambient temperature. In addition, loss of PIF function only partially suppressed the early flowering phenotype and FT expression of the arp6 mutant, which is defective in H2A.Z deposition. In contrast, the chemical inhibition of gibberellic acid (GA) biosynthesis resulted in a strong attenuation of early flowering and FT expression in arp6. Furthermore, GA was able to induce flowering at low temperature (15°C) independently of FT, TSF, and the PIF genes, probably directly at the shoot apical meristem. Together, our results suggest that the timing of the floral transition in response to ambient temperature is more complex than previously thought and that GA signaling might play a crucial role in this process.     

Natural variation and CRISPR/Cas9‐mediated mutation in GmPRR37 affect photoperiodic flowering and contribute to regional adaptation of soybean

Flowering time is a critical determinant of the geographic distribution and regional adaptability of soybean (Glycine max) and is strongly regulated by photoperiod and temperature. In this study, quantitative trait locus (QTL) mapping and subsequent candidate gene analysis revealed that GmPRR37, encoding a pseudo‐response regulator protein, is responsible for the major QTL qFT12‐2, which was identified from a population of 308 recombinant inbred lines (RILs) derived from a cross between a very late‐flowering soybean cultivar, ‘Zigongdongdou (ZGDD)’, and an extremely early‐flowering cultivar, ‘Heihe27 (HH27)’, in multiple environments. Comparative analysis of parental sequencing data confirmed that HH27 contains a non‐sense mutation that causes the loss of the CCT domain in the GmPRR37 protein. CRISPR/Cas9‐induced Gmprr37‐ZGDD mutants in soybean exhibited early flowering under natural long‐day (NLD) conditions. Overexpression of GmPRR37 significantly delayed the flowering of transgenic soybean plants compared with wild‐type under long photoperiod conditions. In addition, both the knockout and overexpression of GmPRR37 in soybean showed no significant phenotypic alterations in flowering time under short‐day (SD) conditions. Furthermore, GmPRR37 down‐regulated the expression of the flowering‐promoting FT homologues GmFT2a and GmFT5a, and up‐regulated flowering‐inhibiting FT homologue GmFT1a expression under long‐day (LD) conditions. We analysed haplotypes of GmPRR37 among 180 cultivars collected across China and found natural Gmprr37 mutants flower earlier and enable soybean to be cultivated at higher latitudes. This study demonstrates that GmPRR37 controls soybean photoperiodic flowering and provides opportunities to breed optimized cultivars with adaptation to specific regions and farming systems.     

Double loss-of-function mutation in EARLY FLOWERING 3 and CRYPTOCHROME 2 genes delays flowering under continuous light but accelerates it under long days and short days: an important role for Arabidopsis CRY2 to accelerate flowering time in continuous light

The photoperiodic response is one of the adaptation mechanisms to seasonal changes of lengths of day and night. The circadian clock plays pivotal roles in this process. In Arabidopsis, LHY, CCA1, ELF3, and other clock proteins play major roles in maintaining circadian rhythms. lhy;cca1 double mutants with severe defects in circadian rhythms showed accelerated flowering under short days (SDs), but delayed flowering under continuous light (LL). The protein level of the floral repressor SVP increased in lhy;cca1 mutants under LL, and the late-flowering phenotype of lhy;cca1 mutants was partially suppressed by svp, flc, or elf3. ELF3 interacted with both CCA1 and SVP, and elf3 suppressed the SVP accumulation in lhy;cca1 under LL. These results suggest that the unique mechanism of the inversion of the flowering response of lhy;cca1 under LL may involve both the ELF3-SVP/FLC-dependent and -independent pathways. In this work, elf3-1 seeds were mutagenized with heavy-ion beams and used to identify mutation(s) that delayed flowering under LL but not long days (LDs) or SDs even without ELF3. In this screening, seven candidate lines named suppressor of elf3 1 (self1), sel3, sel5, sel7, sel14, sel15, and sel20 were identified. Genetic analysis indicated that sel20 was a new deletion allele of a mutation in the blue light receptor, CRY2. A late-flowering phenotype and decrease of FT expression in the elf3;sel20 double mutant was obvious under LL but not under SDs or LDs. These results indicated that the late-flowering phenotype in the double mutant elf3;sel20 as well as in lhy;cca1 was affected by the presence of darkness. The results suggest that CRY2 may play more essential roles in the acceleration of flowering under LL than LDs or SDs.     

DAY NEUTRAL FLOWERING Represses CONSTANS to Prevent Arabidopsis Flowering Early in Short Days

The photoperiodic response in Arabidopsis thaliana requires the precise regulation of CONSTANS (CO) expression in relation to the light period during the day. In short days (SDs) levels of CO expression are normally low during the light period, and this results in delayed flowering compared with long days (LDs) when CO expression rises to high levels before the end of the light period. We identified a novel flowering time gene called DAY NEUTRAL FLOWERING (DNF) that acts in the same flowering pathway as CO. DNF is a membrane-bound E3 ligase that represses CO expression and plays an important role in maintaining low levels of CO expression in SDs. The effect of DNF on the rhythm of CO expression is essential for the photoperiodic response of Arabidopsis, enabling it to have a different flowering response in LDs and SDs.     

Effects of different photoperiodic treatments on flowering in two cultivars of American cotton

The photoperiodic behaviour of two cotton cultivars, H-14 andJ-34, was reinvestigated under more elaborate photoperiodictreatments, including combinations of photoperiodic regimesin different successions. When the plants were continually retainedunder a constant photoperiod, both cultivars seemed to behaveas quantitative LDP. More truly, however, they turned out tobe LSDP. SDs following 35 LDs brought about floral initiationearlier and at a lower node. The number of squares and flowerswas also larger with this treatment. Continuous SD or SDs precedingLDs depressed flowering. (Received November 16, 1971; )