A point mutation in Phytochromobilin Synthase alters the circadian clock and photoperiodic flowering of Medicago truncatula

Abstract

Plants use seasonal cues to initiate flowering at an appropriate time of year to ensure optimal reproductive success. The circadian clock integrates these daily and seasonal cues with internal cues to initiate flowering. The molecular pathways that control the sensitivity of flowering to photoperiod (daylength) are well described in the model plant Arabidopsis. However, much less is known in crop species, such as the legume family species. Here we performed a flowering time screen of a TILLING population of Medicago truncatula and found a line with late-flowering and altered light-sensing phenotypes. Using RNA-sequencing, we identified a nonsense mutation in the Phytochromobilin Synthase (MtPΦBS) gene, which encodes an enzyme that carries out the final step in the biosynthesis of the chromophore required for phytochrome (PHY) activity. The analysis of the circadian clock in the MtpΦbs mutant revealed a shorter circadian period, which was shared with the phyA mutant. The MtpΦbs and MtphyA mutants showed downregulation of FT floral regulators MtFTa1, MtFTb1/b2 and a shift in phase for morning and night core clock genes. Our findings show that PHYA is necessary to synchronize the circadian clock and integration of light signaling to promote expression of the MtFT genes to precisely time flowering.

Genomic evidence supports the genetic convergence of a supergene controlling the distylous floral syndrome

“…In addition, we found that the candidate anther‐position gene contained a unique mutation, which caused a change in coding in the amino acid sequence. Mutations can affect gene expression and/or functions (Feng et al ., 2006; Li et al ., 2020, 2022; Perez‐Santangelo et al ., 2022) altering floral phenotype. Given the similar expression patterns of GeGA3OX in the S‐ and H‐morphs, it is possible that this mutation caused the failure of anther elevation through functional loss or neofunctionalization in the H‐morph.…”

Transcriptomic Analysis of Medicago truncatula under Long-Day Conditions

“…LHY and CCA1 are also part of A. thaliana's core circadian oscillator, and both of their expression levels peak in the morning. The response regulator (PSEUDO-RESPONSE REGULATOR) family of genes peaks between 11 a.m. and 18 p.m. [5], which is also part of the LHY negative feedback loop. Finally, GI, EARLY 3, and EARLY 4 reach their maximum at night, and they exhibit repressive effects on genes expressed earlier in the day [6].…”