| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
|
Plant Physiology 142:1-2 (2006) © 2006 American Society of Plant Biologists On the InsideThe Cause of Canola's Green Seed ProblemDeveloping canola (Brassica napus) seeds typically produce chloroplasts during early seed development and then catabolize the photosynthetic machinery during seed maturation, producing seeds that are essentially free of chlorophyll (Chl). However, an untimely frost during seed development can disrupt the Chl degradation process, causing green seed at harvest and devaluing the crop. Chung et al. (pp. 88–97) present evidence that freezing interferes with the induction of Pheophorbide a oxygenase (PaO), an enzyme that is a key regulator of Chl degradation. The authors report that the regulation of PaO activity was largely posttranslational, and it was at this level that freezing interfered with PaO activation in canola seeds. The mechanism of the posttranslational control is unknown, but the increase in PaO activity during seed maturation corresponds to a decrease in the phosphorylation of the PaO enzyme. In view of evidence that Ca2+ fluxes play a major role in plant responses to cold, it may be germane that canola has two PaO genes, each of which contains two possible Ca2+-dependent protein kinase phosphorylation sites. Geomagnetism and the Coconut Twist In plants with alternately arranged foliage, such as the coconut palm (Cocos nucifera), leaves are attached to the stem in either an ascending clockwise (left-handed [L]) or counterclockwise (right-handed [R]) spiral (Fig. 1 ). Foliar spiral direction (FSD) is a classic case of morphological antisymmetry, in which L and R forms are not inherited and are equally common within a species. Data collected from more than 70,000 coconut palms in more than 40 locations around the world revealed, however, that the FSD of coconut palms varies with latitude: R trees predominate in the northern hemisphere, and L trees predominate in the southern. A reanalysis of this data indicated that hemispheric asymmetries in FSD are significantly better correlated with magnetic (dip) latitude than with geographic or geomagnetic (centered dipole) latitude, suggesting that latitudinal asymmetries in FSD might be associated with the temporally varying component of Earth's magnetic field. The Induced Current Hypothesis proposes that asymmetries in FSD result from earth currents in trees that are induced by variations in the vertical Z component of the geomagnetic field, and that these earth currents consequently cause a tangential bias in the axial electrophoresis of phyllotaxy-determining morphogens (e.g. auxin transporters). Minorsky and Bronstein (pp. 40–44) report that asymmetries in FSD are also evident in populations of coconut palms on opposite sides of islands and that asymmetries between cohorts vary with an 11-year periodicity—two discoveries consistent with the hypothesis that geomagnetic variations underlie asymmetries in coconut palm FSD.
Phytosulfokines Affect Callus Formation and Longevity Even when sufficient amounts of growth regulators and nutrients are supplied, populations of living cells are often required to support callus growth in vitro. This population dependence is alleviated by the addition of conditioned medium in which cells have previously been grown, indicating the involvement of a chemical signal produced by growing cells in this phenomenon. Phytosulfokine (PSK) is a 5-amino-acid sulfated peptide that has been detected in conditioned medium of plant cell cultures. The addition of chemically synthesized PSK to culture medium, even at nanomolar concentrations, significantly increases the rate of callus growth, even when the initial cell population is below the critical density. These results suggest that PSK may be the critical component in conditioned media that gives such media their growth-promoting properties. PSK is known to act by binding to a membrane-localized PSK receptor, PSKR1. The carrot (Daucus carota) PSK receptor, DcPSKR1, exhibits a high percentage of amino acid identity with a Leu-rich repeat ribonuclease inhibitor (AtPSKR1) found in the Arabidopsis (Arabidopsis thaliana) genome. Matsubayashi et al. (pp. 45–53) have analyzed the function of this putative Arabidopsis PSK receptor gene by gain-of-function and loss-of-function strategies. Although AtPSKR1-deficient seedlings exhibited normal growth that was phenotypically indistinguishable from wild-type for the first 3 weeks after germination, they gradually lost their potential to form calluses as tissues matured. Moreover, the calluses derived from the immature tissues of AtPSKR1-deficient seedlings also exhibited premature senescence accompanied by browning within 3 weeks of culture. These results suggest that PSK signaling affects the growth potential and longevity of plant cells. Transcript Response to Elevated Carbon Dioxide Because soybean (Glycine max) is the most widely grown seed legume in the world, it is important to understand how it will respond to the 50% increase in atmospheric [CO2] that is projected to occur by the year 2050. Toward this end, a Free Air CO2 Enrichment (FACE) experiment was established in Illinois in 2001. From this long-term experiment and others, it has been established that elevated [CO2] increases carbon uptake, foliar carbohydrate content, plant growth, and yield, while decreasing stomatal conductance. The increased carbon assimilation and water-use efficiency under conditions of high atmospheric [CO2] also leads to increases in leaf area index (LAI). The combination of increased photosynthesis and increased LAI leads to significant increases in soybean seed yield. At the molecular level, the basis for changes in LAI at elevated [CO2] is largely unknown, although both cell production rates and cell expansion have been shown to be affected. Ainsworth et al. (pp. 135–147) have investigated the transcriptome responses of rapidly growing and fully expanded leaves to elevated [CO2] at the soybean FACE facility. Their research suggests that at the transcript level, elevated [CO2] stimulates the respiratory breakdown of carbohydrates, which likely provides increased fuel for leaf expansion and growth at elevated [CO2]. New Insights into Light-Regulated Transcription
The events leading to transcription of eukaryotic protein-coding genes culminate in the positioning of RNA polymerase II at the correct initiation site. The core promoter, which can extend approximately 35 bp upstream and/or downstream of this site, plays a central role in regulating initiation. Specific DNA elements within the core promoter bind the factors that nucleate the assembly of a functional pre-initiation complex and integrate stimulatory and repressive signals from factors bound at distal sites. The eukaryotic promoters of protein-coding genes have one or more of the three conserved sequences in the core promoter region, i.e. the TATA box, the initiator region, and the downstream promoter element. TATA boxes are T/A-rich DNA sequences (cis-regulatory elements) found in the promoter region of most genes about 25 to 30 bp upstream of the transcription site. TATA boxes have been highly conserved through evolution and have a core DNA sequence 5'-TATAAA-3', which is usually followed by two or more adenine bases. Although the minimal promoter is generally believed to determine the rate of transcription and the point of its initiation, there is some evidence suggesting that it also plays a role in promoter selectivity. For example, two different TATA boxes in transgenic mice expressing the human globin gene determine the regulation of KNOX Proteins Induce Cytokinin Biosynthesis in Rice KNOX proteins are transcriptional regulators that play critical roles in shoot apical meristem formation and maintenance. A complete understanding of KNOX protein function requires the identification of the genes targeted by them and knowledge of the transcriptional regulation of those genes. Previous studies in dicot species have revealed that KNOX proteins suppress the expression of genes that encode for GA 20-oxidase, the enzyme that catalyzes the rate-limiting step of bioactive GA synthesis. Another candidate for regulation by KNOX proteins is cytokinin (CK) biosynthesis because production of bioactive CKs such as trans-zeatin and isopentenyladenine is significantly increased in KNOX overproducers. To elucidate the functional interaction between KNOX proteins and CK biosynthesis in monocot plants, Sakamoto et al. (pp. 54–62) have identified eight rice (Oryza sativa) genes that code for adenosine phosphate isopentenyltransferase (IPT), the enzyme that catalyzes the rate-limiting step of CK biosynthesis. The overexpression of OsIPTs in transgenic rice inhibited root development and promoted axillary bud growth, indicating that OsIPTs are functional in vivo. The phenotypes of OsIPT overexpressors resembled those of KNOX-overproducing transgenic rice, although OsIPT overexpressors did not form roots or ectopic meristems, both of which are observed in KNOX overproducers. The authors propose that the ectopic expression of KNOX proteins induces specific IPT gene expression and de novo CK biosynthesis, and that this cascade is conserved in both monocots and dicots. Another important function of KNOX proteins—repression of GA biosynthesis by the suppression of GA 20-oxidase gene expression—is also apparently conserved between monocots and dicots. These results support the hypothesis that plant meristems need high-CK and low-GA conditions to maintain their activity, and that KNOX proteins act as central regulators to control these phytohormones at adequate levels in both monocots and dicots.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900202. Related articles in Plant Physiol.:
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| ASPB Publications | PLANT PHYSIOLOGY | THE PLANT CELL | |
|---|---|---|---|
-globin gene expression during embryogenesis and in the adult stage, respectively. Kiran et al. (pp. 
C or G, completely inactivated the expression of the minimal promoter in the light but not in the dark, and an A at the eighth position was specifically involved in the red-light response of the promoter.