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Here, plant HAM proteins are shown to physically interact with the transcription factor WUSCHEL and the related WOX proteins, with this interaction driving downstream transcriptional programs and determining the activities of stem cells. Regulation of plant stem cell function Postembryonic development of above-ground tissues and of roots of plants depends on stem cells in the shoot and root apical meristems, respectively. The transcription factor WUSCHEL (WUS) and the related WOX proteins are known to be involved in the specification and maintenance of stem cells within all meristems throughout the plant kingdom. The HAIRY MERISTEM (HAM) family of transcription regulators is known to contribute to shoot stem cell function in certain plant species. The present paper shows that WOX and HAM family proteins act together to control the production of all types of stem cells in diverse niches in Arabidopsis, and that WUS and HAM share some common targets in vivo. Differences in the combined expression patterns of WOX and HAM family members appear to determine the formation of diverse stem cell niche locations. These findings demonstrate how co-operative transcriptional regulators drive common regulatory pathways and point to mechanisms underlying the evolution of stem cell regulation in plants. Plant stem cells in the shoot apical meristem (SAM) and root apical meristem are necessary for postembryonic development of aboveground tissues and roots, respectively, while secondary vascular stem cells sustain vascular development.sup.1,2,3,4. WUSCHEL (WUS), a homeodomain transcription factor expressed in the rib meristem of the Arabidopsis SAM, is a key regulatory factor controlling SAM stem cell populations.sup.5,6, and is thought to establish the shoot stem cell niche through a feedback circuit involving the CLAVATA3 (CLV3) peptide signalling pathway.sup.7. WUSCHEL-RELATED HOMEOBOX 5 (WOX5), which is specifically expressed in the root quiescent centre, defines quiescent centre identity and functions interchangeably with WUS in the control of shoot and root stem cell niches.sup.8. WOX4, expressed in Arabidopsis procambial cells, defines the vascular stem cell niche.sup.9,10,11. WUS/WOX family proteins are evolutionarily and functionally conserved throughout the plant kingdom.sup.12 and emerge as key actors in the specification and maintenance of stem cells within all meristems.sup.13. However, the nature of the genetic regime in stem cell niches that centre on WOX gene function has been elusive, and molecular links underlying conserved WUS/WOX function in stem cell niches remain unknown. Here we demonstrate that the Arabidopsis HAIRY MERISTEM (HAM) family of transcription regulators act as conserved interacting cofactors with WUS/WOX proteins. HAM and WUS share common targets in vivo and their physical interaction is important in driving downstream transcriptional programs and in promoting shoot stem cell proliferation. Differences in the overlapping expression patterns of WOX and HAM family members underlie the formation of diverse stem cell niche locations, and the HAM family is essential for all of these stem cell niches. These findings establish a new framework for the control of stem cell production during plant development., Author(s): Yun Zhou [sup.1] , Xing Liu [sup.1] , Eric M. Engstrom [sup.2] [sup.6] , Zachary L. Nimchuk [sup.1] [sup.3] [sup.6] , Jose L. Pruneda-Paz [sup.4] , Paul T. Tarr [...]

The circadian clock is required for adaptive responses to daily and seasonal changes in environmental conditions (1-3). Light and the circadian clock interact to consolidate the phase of hypocotyl cell elongation to peak at dawn under diurnal cycles in Arabidopsis thaliana (4-7). Here we identify a protein complex (called the evening complex)--composed of the proteins encoded by EARLY FLOWERING 3 (ELF3), ELF4 and the transcription-factor-encoding gene LUX ARRHYTHMO (LUX; also known as PHYTOCLOCK 1)--that directly regulates plant growth (8-12). ELF3 is both necessary and sufficient to form a complex between ELF4 and LUX, and the complex is diurnally regulated, peaking at dusk. ELF3, ELF4 and LUX are required for the proper expression of the growth-promoting transcription factors encoded by PHYTOCHROME INTERACTING FACTOR 4 (PIF4)and PIF5 (also known as PHYTOCHROME INTERACTING FACTOR 3-LIKE 6) under diurnal conditions (4,6,13). LUX targets the complex to the promoters of PIF4 and PIF5 in vivo. Mutations in PIF4 and/or PIF5 are epistatic to the loss of the ELF4-ELF3-LUX complex, suggesting that regulation of PIF4 and PIF5 is a crucial function of the complex. Therefore, the evening complex underlies the molecular basis for circadian gating of hypocotyl growth in the early evening., The circadian clock is an endogenous molecular oscillator with a period of ~24 h that is almost ubiquitous1. In plants, multiple interlocking transcriptional feedback loops contribute to the robust architecture [...]

Author(s): Paloma Más [1]; Woe-Yeon Kim [2]; David E. Somers [2]; Steve A. Kay (corresponding author) [1] The underlying mechanism of circadian rhythmicity appears to be conserved among organisms, and [...]

Author(s): Takato Imaizumi [1]; Hien G. Tran [1]; Trevor E. Swartz [2]; Winslow R. Briggs [2]; Steve A. Kay (corresponding author) [1] Adaptation to seasonal change is a crucial component [...]

Author(s): Marcelo J. Yanovsky; Steve A. Kay (corresponding author) Several organisms have evolved the ability to measure daylength, or photoperiod, allowing them to adjust their development in anticipation of annual [...]

Author(s): Satchidananda Panda [1, 2]; John B. Hogenesch [2]; Steve A. Kay (corresponding author) [1, 2] The rotation of the Earth causes predictable changes in light and temperature in our [...]

Light is a crucial environmental signal that controls many photomorphogenic and circadian responses in plants (1). Perception and transduction of light is achieved by at least two principal groups of photoreceptors, phytochromes and cryptochromes (2,3). Phytochromes are red/far-red light-absorbing receptors encoded by a gene family of five members (phyA to phyE) (2,4) in Arabidopsis. Cryptochrome 1 (cry1), cryptochrome 2 (cry2) and phototropin are the blue/ultraviolet-A light receptors that have been characterized in Arabidopsis (5). Previous studies showed that modulation of many physiological responses in plants is achieved by genetic interactions between different photoreceptors (6); however, little is known about the nature of these interactions and their roles in the signal transduction pathway. Here we show the genetic interaction that occurs between the Arabidopsis photoreceptors phyB and cry2 in the control of flowering time, hypocotyl elongation and circadian period by the clock. PhyB interacts directly with cry2 as observed in co-immunoprecipitation experiments with transgenic Arabidopsis plants overexpressing cry2. Using fluorescent resonance energy transfer microscopy, we show that phyB and cry2 interact in nuclear speckles that are formed in a light-dependent fashion., [FIGURE 1 OMITTED] Light signals markedly affect the pace of the clock by activating different photoreceptors (7). In response to increasing light intensity, the period length of the circadian expressed [...]

Author(s): Motomu Endo (corresponding author) [1, 2]; Hanako Shimizu [1]; Maria A. Nohales [3]; Takashi Araki [1]; Steve A. Kay [3] Many organisms rely on a circadian clock system to [...]