How can a germinating plant seed determine how long its embryonic stem (hypocotyl) should grow before it unfurls its embryonic leaves? Because a seed under debris will receive much less light than a seed on bare ground, light signaling pathways are a major determinant of hypocotyl length. By comparing two natural strains of Arabidopsis thaliana, Loudet et al. mapped to chromosome 5 quantitative trait loci (QTLs) that affected light- and temperature-regulated growth of the hypocotyl. They named the QTL that accounted for the most variation LIGHT5. By generating lines that were nearly genetically identical (isogenic) except in the region of the QTL, LIGHT5 was mapped to a region containing three candidate genes. Loss- and gain-of-function strategies then pinpointed a conserved, single-copy gene, which they named tandem zip knuckle PLUS3 (TZP), as the causal gene of LIGHT5 phenotype. TZP encoded a nuclear protein that contains a domain that mediates protein-protein interactions (tandem zip knuckle), as well as a domain that may bind nucleotides (PLUS3). Plants that overexpressed TZP had an increased growth response to blue light. Noticing that the peak abundance of TZP expression occurred at dawn, the authors next looked at the relationship between TZP and the circadian clock, which controls hypocotyl elongation. The abundance of TZP was changed in plants with mutations that disrupted circadian rhythms, and microarray analysis in plants that overexpressed TZP revealed that the genes that were most disrupted by TZP overexpression displayed a peak transcript abundance at dawn. These included genes involved in cell wall expansion, auxin response (which controls growth in a time-of-day fashion), and a transcription factor required for certain types of far-red- and blue-light signaling. Thus, TZP may integrate cues from the circadian clock and light signaling pathways to control genes responsible for growth.
O. Loudet, T. P. Michael, B. T. Burger, C. Le Metté, T. C. Mockler, D. Weigel, J. Chory, A zinc knuckle protein that negatively controls morning-specific growth in Arabidopsis thaliana. Proc. Natl. Acad. Sci. U.S.A. 105, 17193-17198 (2008). [Abstract] [Full Text]