Differential distribution of the plant hormone auxin within tissues mediates a variety of developmental processes. Cellular auxin levels are determined by metabolic processes including synthesis, degradation, and (de)conjugation, as well as by auxin transport across the plasma membrane. Whereas transport of free auxins such as naturally occurring indole-3-acetic acid (IAA) is well characterized, little is known about the transport of auxin precursors and metabolites. Here, we identify a mutation in the ABCG37 gene of Arabidopsis that causes the polar auxin transport inhibitor sensitive1 (pis1) phenotype manifested by hypersensitivity to auxinic compounds. ABCG37 encodes the pleiotropic drug resistance transporter that transports a range of synthetic auxinic compounds as well as the endogenous auxin precursor indole-3-butyric acid (IBA), but not free IAA. ABCG37 and its homolog ABCG36 act redundantly at outermost root plasma membranes and, unlike established IAA transporters from the PIN and ABCB families, transport IBA out of the cells. Our findings explore possible novel modes of regulating auxin homeostasis and plant development by means of directional transport of the auxin precursor IBA and presumably also other auxin metabolites.PDR9 | PDR8 | IBA transport | auxin synthesis P lants have evolved outstanding capacities to adapt their metabolism and development to respond to their environment. Changes in the availability and distribution of endogenous signaling molecules-plant hormones-play important roles in these responses (1). The phytohormone auxin, perceived by TIR1/AFB receptor proteins and interpreted by downstream nuclear signaling pathway, is an important signal that mediates transcriptional developmental reprogramming (reviewed in refs. 2 and 3). The differential distribution of auxin within tissues is essential for many adaptive responses including embryo and leaf patterning, root and stem elongation, lateral root initiation, and leaf expansion (4). Differential distribution of the major active auxin, IAA, depends on its intercellular transport and metabolic processes that involve biosynthesis by several pathways and release from storage forms including amide-or ester-linked conjugates with amino acids, peptides, and sugars (reviewed in ref. 5). The role of another endogenously occurring auxinic compound IBA is still unclear. It has been proposed that IBA acts independently of IAA (6), but a number of recent genetic findings suggest that IBA functions as an important precursor to IAA during conversion resembling peroxisomal fatty acid β-oxidation (5, 7). Besides metabolism, a crucial process controlling cellular auxin levels is the directional, intercellular auxin transport that depends on specialized influx and efflux carriers (reviewed in ref. 8). IAA transporters include amino acid permeases-like AUXIN RESISTANT1 (AUX1) mediating auxin influx (9-11), the PIN-FORMED (PIN) efflux carriers (12)(13)(14), and the MULTIDRUG RESISTANCE/P-GLYCOPROTEIN (PGP) class of ATP-Binding Cassette (ABC) auxin trans...

Synechocystis: sp. PCC 6803 is a unicellular motile cyanobacterium, which shows positive or negative phototaxis on agar plates under lateral illumination. By gene disruption in a substrain showing of positive phototaxis, it was demonstrated that mutants defective in sll0038, sll0039, sll0041, sll0042 or sll0043 lost positive phototaxis but showed negative phototaxis away from the light source. Mutants of sll0040, which is located within the cluster of these genes, retained the capacity of positive phototaxis but to a lesser extent than the parent cells. These genes are homologous to che genes, which are involved in flagellar switching for bacterial chemotaxis. Interestingly, sll0041 (designated pisJ1) is predicted to have a chromophore-binding motif of phytochrome-like proteins and a signaling motif of chemoreceptors for bacterial chemotaxis. It is strongly suggested that the positive phototactic response was mediated by a phytochrome-like photoreceptor and CheA/CheY-type signal transduction system.

Stomata, valves on the plant epidermis, are critical for plant growth and survival, and the presence of stomata impacts the global water and carbon cycle. Although transcription factors and cell-cell signaling components regulating stomatal development have been identified, it remains unclear as to how their regulatory interactions are translated into two-dimensional patterns of stomatal initial cells. Using molecular genetics, imaging, and mathematical simulation, we report a regulatory circuit that initiates the stomatal cell-lineage. The circuit includes a positive feedback loop constituting self-activation of SCREAMs that requires SPEECHLESS. This transcription factor module directly binds to the promoters and activates a secreted signal, EPIDERMAL PATTERNING FACTOR2, and the receptor modifier TOO MANY MOUTHS, while the receptor ERECTA lies outside of this module. This in turn inhibits SPCH, and hence SCRMs, thus constituting a negative feedback loop. Our mathematical model accurately predicts all known stomatal phenotypes with the inclusion of two additional components to the circuit: an EPF2-independent negative-feedback loop and a signal that lies outside of the SPCH•SCRM module. Our work reveals the intricate molecular framework governing self-organizing two-dimensional patterning in the plant epidermis.

The developmental program of nodulation is regulated systemically in leguminous host species. A mutant astray (Ljsym77) in Lotus japonicus has lost some sort of its ability to regulate this symtem, and shows enhanced and early nodulation. In the absence of rhizobia, this mutant exhibits characteristics associated with defects in light and gravity responses. These nonsymbiotic phenotypes of astray are very similar to those observed in photomorphogenic Arabidopsis mutant hy5. Based on this evidence, we predicted that astray might contain a mutation in the HY5 homologue of L. japonicus. The homologue, named LjBzf, encodes a basic leucine zipper protein in the C-terminal half that shows the highest level of identity with HY5 of all Arabidopsis proteins. It also encodes legume-characteristic combination of motifs, including a RING-finger motif and an acidic region in the N-terminal half. The astray phenotypes were cosegregated with LjBzf, and the failure to splice the intron was detected. Nonsymbiotic and symbiotic phenotypes of astray were complemented by introduction of CaMV35S::LjBzf. It is noteworthy that although Arabidopsis hy5 showed an enhancement of lateral root initiation, Lotus astray showed an enhancement of nodule initiation but not of lateral root initiation. Legume-characteristic combination of motifs of ASTRAY may play specific roles in the regulation of nodule development.

A fundamental question in developmental biology is how spatial patterns are self-organized from homogeneous structures. In 1952, Turing proposed the reaction-diffusion model in order to explain this issue. Experimental evidence of reaction-diffusion patterns in living organisms was first provided by the pigmentation pattern on the skin of fishes in 1995. However, whether or not this mechanism plays an essential role in developmental events of living organisms remains elusive. Here we show that a reaction-diffusion model can successfully explain the shoot apical meristem (SAM) development of plants. SAM of plants resides in the top of each shoot and consists of a central zone (CZ) and a surrounding peripheral zone (PZ). SAM contains stem cells and continuously produces new organs throughout the lifespan. Molecular genetic studies using Arabidopsis thaliana revealed that the formation and maintenance of the SAM are essentially regulated by the feedback interaction between WUSHCEL (WUS) and CLAVATA (CLV). We developed a mathematical model of the SAM based on a reaction-diffusion dynamics of the WUS-CLV interaction, incorporating cell division and the spatial restriction of the dynamics. Our model explains the various SAM patterns observed in plants, for example, homeostatic control of SAM size in the wild type, enlarged or fasciated SAM in clv mutants, and initiation of ectopic secondary meristems from an initial flattened SAM in wus mutant. In addition, the model is supported by comparing its prediction with the expression pattern of WUS in the wus mutant. Furthermore, the model can account for many experimental results including reorganization processes caused by the CZ ablation and by incision through the meristem center. We thus conclude that the reaction-diffusion dynamics is probably indispensable for the SAM development of plants.

Arabidopsis thaliana plants were grown at 23°C and changes in carbohydrate metabolism, photosynthesis and photosynthetic gene expression were studied after the plants were shifted to 5°C. The responses of leaves shifted to 5°C after development at 23°C are compared to leaves that developed at 5°C. Shifting warm developed leaves to 5°C lead to a severe suppression of photosynthesis that correlated with a rapid and sustained accumulation of hexose phosphates and soluble sugars. Associated with the suppression of photosynthesis and the accumulation of soluble sugars was a reduction in the amount of transcript for genes encoding photosynthetic proteins (cab and rbcS). In contrast, leaves that developed at 5°C showed an increase in photosynthesis and control levels of photosynthetic gene expression. This recovery occurred even though leaves that developed at 5°C maintained large pools of soluble sugars. Leaves that developed at 5°C also showed a strong upregulation of the cytosolic pathway for soluble sugar synthesis but not of the chloroplastic pathway for starch synthesis. This was shown at the level of both enzyme activity and the amount of transcript. Thus, development of Arabidopsis leaves at 5°C resulted in metabolic changes that enabled them to produce and accumulate large soluble sugar pools without any associated suppression of photosynthesis or photosynthetic gene expression. These changes were also associated with enhanced freezing tolerance. We suggest that this reprogramming of carbohydrate metabolism associated with development at low temperature is essential to the development of full freezing tolerance and for winter survival of over‐wintering herbaceous annuals.

Polar auxin transport inhibitors, including N-1-naphthylphthalamic acid (NPA) and 2,3,5-triiodobenzoic acid (TIBA), have various effects on physiological and developmental events, such as the elongation and tropism of roots and stems, in higher plants. We isolated NPA-resistant mutants of Arabidopsis thaliana, with mutations designated pir1 and pir2, that were also resistant to TIBA. The mutations specifically affected the root-elongation process, and they were shown ultimately to be allelic to aux1 and ein2, respectively, which are known as mutations that affect responses to phytohormones. The mechanism of action of auxin transport inhibitors was investigated with these mutants, in relation to the effects of ethylene, auxin, and the polar transport of auxin. With respect to the inhibition of root elongation in A. thaliana, we demonstrated that (1) the background level of ethylene intensifies the effects of auxin transport inhibitors, (2) auxin transport inhibitors might act also via an inhibitory pathway that does not involve ethylene, auxin, or the polar transport of auxin, (3) the hypothesis that the inhibitory effect of NPA on root elongation is due to high-level accumulation of auxin as a result of blockage of auxin transport is not applicable to A. thaliana, and (4) in contrast to NPA, TIBA itself has a weak auxin-like inhibitory effect.

Complex morphology is an evolutionary outcome of phenotypic diversification. In some carnivorous plants, the ancestral planar leaf has been modified to form a pitcher shape. However, how leaf development was altered during evolution remains unknown. Here we show that the pitcher leaves of Sarracenia purpurea develop through cell division patterns of adaxial tissues that are distinct from those in bifacial and peltate leaves, subsequent to standard expression of adaxial and abaxial marker genes. Differences in the orientation of cell divisions in the adaxial domain cause bifacial growth in the distal region and adaxial ridge protrusion in the middle region. These different growth patterns establish pitcher morphology. A computer simulation suggests that the cell division plane is critical for the pitcher morphogenesis. Our results imply that tissue-specific changes in the orientation of cell division underlie the development of a morphologically complex leaf.