In the year 2000, a team of scientists at the Plant Gene Expression Center at the University of California, Berkeley, sequenced a gene that would later be named LUX, yet they could not have known that this tiny segment of DNA held the key to how plants measure time. This gene, located on the third chromosome of Arabidopsis thaliana, does not merely exist; it actively participates in a molecular dance that dictates when a plant sleeps and when it wakes. The LUX gene codes for a protein called LUX ARRHYTHMO, which serves as a critical component of the Evening Complex, a trio of proteins that includes Early Flowering 3 and Early Flowering 4. Together, these proteins form a repressilator model, a biological mechanism that ensures the plant's internal clock ticks with precision. Without this gene, the plant loses its ability to maintain circadian rhythms, leading to a state of arrhythmia where biological processes drift without order. The discovery of LUX was not an isolated event but part of a larger collaborative effort involving the Salk Institute for Biological Studies and the Center for Gene Research at Nagoya University, which eventually proved that LUX is essential for the survival and proper functioning of Arabidopsis thaliana.
Molecular Architecture
The physical structure of the LUX gene reveals a sophisticated design that allows it to interact with the genetic code of the plant. Located on the third chromosome, the gene contains three exons and is preceded by a promoter region housing a specific cis-regulatory element known as the evening element. This element carries the sequence AAAATATCT and is overrepresented in genes that are expressed during the evening hours. The protein produced by LUX, LUX ARRHYTHMO, is composed of 323 amino acids and features a Myb-like GARP family transcription factor DNA-binding domain. This domain allows the protein to attach to specific DNA sequences, although the exact sequence it binds to to repress PRR9 remains a mystery to scientists. The Evening Complex, formed by LUX, ELF3, and ELF4, is assembled during the evening to repress the transcription of the PRR9 gene, which is part of the Midday Complex. This repression creates a cycle where PRR9 subsequently represses CCA1 and LHY, genes that express components of the Morning Complex. The system is self-regulating, as LUX also represses its own transcription, creating a feedback loop that maintains the rhythm of the plant's internal clock.The Growth Paradox
When the LUX gene fails to function, the consequences for the plant are immediate and visually striking, manifesting as an elongated hypocotyl phenotype due to excess growth during the night. The Evening Complex normally binds to the promoters of Phytochrome Interacting Factor 4 and Phytochrome Interacting Factor 5, repressing their expression and thereby inhibiting plant growth in the evening. These factors, PIF4 and PIF5, are basic helix-loop-helix domain transcription factors that are implicated in the induction of Flowering Locus T, which produces a florigen that promotes flowering. In mutants lacking functional LUX, the repression of PIF4 and PIF5 is lost, leading to their premature accumulation and subsequent early growth. This phenomenon highlights the critical role of LUX in ensuring that growth occurs at the appropriate time of day. The inability to repress these factors results in a plant that grows unchecked during the night, disrupting the natural balance required for healthy development. This growth paradox underscores the importance of the Evening Complex in regulating the timing of plant development and flowering.