Abstract
The plant circadian clock coordinates environmental signals with internal processes including secondary metabolism, growth, flowering, and volatile emission. Plant tissues are specialized in different functions, and petals conceal the sexual organs while attracting pollinators. Here we analyzed the transcriptional structure of the petunia (Petunia x hybrida) circadian clock in leaves and petals. We recorded the expression of 13 clock genes in petunia under light:dark (LD) and constant darkness (DD). Under light:dark conditions, clock genes reached maximum expression during the light phase in leaves and the dark period in petals. Under free running conditions of constant darkness, maximum expression was delayed, especially in petals. Interestingly, the rhythmic expression pattern of PhLHY persisted in leaves and petals in LD and DD. Gene expression variability differed among leaves and petals, time of day and photoperiod. The transcriptional noise was higher especially in leaves under constant darkness. We found that PhPRR7, PhPRR5, and PhGI paralogs showed changes in gene structure including exon number and deletions of CCT domain of the PRR family. Our results revealed that petunia petals presented a specialized clock.
Highlights
Organisms, from bacteria to human beings, are subjected to periodic oscillations in the environment due the planet rotation around its axis
The genome of petunia has seven PSEUDO-RESPONSE REGULATOR gene family (PRR) genes as PRR7 and PRR5 are duplicated both in P. axillaris and P. integrifolia while Arabidopsis has the canonical set of five genes, PRR1 or TOC1, PRR3, PRR5, PRR7, and PRR9 involved in circadian regulation [11]
We reconstructed a phylogenetic tree of PRR genes, by using the conserved domain pseudo-receiver (PR), of Solanaceae and Arabidopsis (Table S1) in order to deduce the evolutionary relationships of the duplicated genes
Summary
From bacteria to human beings, are subjected to periodic oscillations in the environment due the planet rotation around its axis. A comprehensive analysis of the circadian clock genes found in the Petunia genomes shows that there is a set of genes that has remained as single copy These include the petunia orthologs for PRR9, PRR3, TOC1, and LHY. Other genes are present in two to four copies, PRR7, PRR5, GI, ELF3, or ELF4 [11] These data indicate a possible departure of the circadian clock network from the one known in Arabidopsis, and suggests the evolution of the clock at different levels including gene structure, expression pattern, and genetic functions. The initial transcriptional activation is followed at early stages by an autoregulatory positive regulation of the MADS-box genes controlling petal morphogenesis in Antirrhinum, Arabidopsis, and petunia [16,17,18,19,20,21]. Our results reflect the evolution of the plant circadian clock at the structural and expression level and suggests an organ specific transcriptional structure of the plant circadian clock
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