P is an important plant macronutrient, making up about 0.2% of a plant's dry weight. It is a component of key molecules such as nucleic acids, phospholipids, and ATP, and, consequently, plants cannot grow without a reliable supply of this nutrient. Pi is also involved in controlling key enzyme

Phosphorus Uptake by Plants: From Soil to Cell

Phosphate (Pi) is considered to be one of the least available plant nutrients in the soil. High-affinity Pi transporters are generally accepted as entry points for Pi in the roots. The physiological, genetic, molecular and biochemical analysis of phosphate starvation response mechanisms highlight the ability of plants to adapt and thrive under phosphate limiting conditions. These responses help them enhance the availability of Pi, increase its uptake and improve the use-efficiency of Pi within a plant. Enhanced ability to acquire Pi appears to be regulated at the level of transcription of high-affinity phosphate transporters. These transporters are encoded by a family of small number of genes having characteristic tissue and organ associated expression patterns. Many of them are strongly induced during phosphate deficiency thus providing plants with enhanced ability to acquire and transfer phosphate. In addition, plants also activate biochemical mechanisms that could lead to increased acquisition of phosphate from both inorganic and organic phosphorus sources in the soil. Furthermore, altered root morphology and mycorrhizal symbiosis further enhance the ability of plants to acquire Pi. Interestingly most of these responses appear to be coordinated by changes in cellular phosphate levels. It is becoming apparent that phosphate acquisition and utilization are associated with activation or inactivation of a host of genes in plants. In this article we describe molecular, biochemical and physiological factors associated with phosphate acquisition by plants.

Phosphate Acquisition

INTRODUCTION .. ... ..... . .. . ..... . .... .. . .. ...... .. . 397 G EN ERAL MECHANIS MS OF R EG ULATION IN VOLVING A S ECOND MESSENGER.. .. . .. . ..... .... . . .. ....... . . .. . . . . ..... .... . . .. . . . ......... . . .. ........ . . . . . 398 CELLULAR METABOLI S M OF C a2+ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... . . . . . . . . . . 401 C a2+ CHEMIS TR y .. . . . . . . . . . . . . . . . ........ 401 Ca2+ -B IND ING PROTEINS . . . ... . . . ... . . . ... . . . . . . . . .. . . .... . . . . . . . . ... . . . . . . . . . . .. . . . .. . . . . . . . . . . 403 THE FITNESS OF Ca2+ . .. . . .. . . . .. . .. 404 Ca2+ -MEDIATED PROCESS ES . . . ... . . . ... . . . .. . . . . . . . . . .... . . . . . . . .. . . . . . .. . . . . . . . . . . .. .. . . . . . . . 404 Polar ized Growth ... . . . . . . . . . . . . . . . . . . . . . . ... .... . . . . . . . . .... ... ... . . . . . . . . ..... . . . . . . . .. . . . . . . . . . 405 M ito sis an d Cyto kin esis.. .. . . . .. . . . . . . . . . .. . . . . .. . . . . . . . . .. . . ..... .... . . . ... . . .... . . . .... . . . . . . . . 407 Cytoplasm ic Stream in g. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . .. . 409 STI MULUS-RESPONS E CO UPLING . . . . . . .. . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . 412 Light . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . .. . . . . . . . . . . . . . . . . . . . . . . . . 412 Gravity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 416 Cyto kin in . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 419 Gibberellin .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ... . . . . . . . . .. . . 421 Auxin . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 423 AN EVALUATION OF Ca2+ IN PLANT D EVELOPMENT . . . . .. '" ......... 425

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Calcium and plant development

Bacteria belonging to the genera Rhizobium, Mesorhizobium, Sinorhizobium, Bradyrhizobium, and Azorhizobium (collectively referred to as rhizobia) grow in the soil as free-living organisms but can also live as nitrogen-fixing symbionts inside root nodule cells of legume plants. The interactions between several rhizobial species and their host plants have become models for this type of nitrogen-fixing symbiosis. Temperate legumes such as alfalfa, pea, and vetch form indeterminate nodules that arise from root inner and middle cortical cells and grow out from the root via a persistent meristem. During the formation of functional indeterminate nodules, symbiotic bacteria must gain access to the interior of the host root. To get from the outside to the inside, rhizobia grow and divide in tubules called infection threads, which are composite structures derived from the two symbiotic partners. This review focuses on symbiotic infection and invasion during the formation of indeterminate nodules. It summarizes root hair growth, how root hair growth is influenced by rhizobial signaling molecules, infection of root hairs, infection thread extension down root hairs, infection thread growth into root tissue, and the plant and bacterial contributions necessary for infection thread formation and growth. The review also summarizes recent advances concerning the growth dynamics of rhizobial populations in infection threads.

Infection and Invasion of Roots by Symbiotic, Nitrogen-Fixing Rhizobia during Nodulation of Temperate Legumes

The most well-characterized organelle contact sites are those between the endoplasmic reticulum (ER) and mitochondria. Increased understanding is being gained of how ER–mitochondria contact sites are organized and which factors converge at this interface, some of which may provide a tethering function. The role of the ER–mitochondria junction in coordinating the functions of these two organelles is also becoming clearer, and it has been shown to be involved in the regulation of lipid synthesis, Ca2+ signalling and the control of mitochondrial biogenesis and intracellular trafficking.

Endoplasmic reticulum-mitochondria contacts: function of the junction

Plants exhibit an ultimate case of the intracellular motility involving rapid organelle trafficking and continuous streaming of the endoplasmic reticulum (ER). Although it was long assumed that the ER dynamics is actomyosin-driven, the responsible myosins were not identified, and the ER streaming was not characterized quantitatively. Here we developed software to generate a detailed velocity-distribution map for the GFP-labeled ER. This map revealed that the ER in the most peripheral plane was relatively static, whereas the ER in the inner plane was rapidly streaming with the velocities of up to ∼3.5 μm/sec. Similar patterns were observed when the cytosolic GFP was used to evaluate the cytoplasmic streaming. Using gene knockouts, we demonstrate that the ER dynamics is driven primarily by the ER-associated myosin XI-K, a member of a plant-specific myosin class XI. Furthermore, we show that the myosin XI deficiency affects organization of the ER network and orientation of the actin filament bundles. Collectively, our findings suggest a model whereby dynamic three-way interactions between ER, F-actin, and myosins determine the architecture and movement patterns of the ER strands, and cause cytosol hauling traditionally defined as cytoplasmic streaming.

https://www.pnas.org/content/pnas/107/15/6894.full.pdf

Myosin-dependent endoplasmic reticulum motility and F-actin organization in plant cells

Cytoplasmic streaming in plants.

High velocity cytoplasmic streaming is found in various plant cells from algae to angiosperms. We characterized mechanical and enzymatic properties of a higher plant myosin purified from tobacco bright yellow-2 cells, responsible for cytoplasmic streaming, having a 175 kDa heavy chain and calmodulin light chains. Sequence analysis shows it to be a class XI myosin and a dimer with six IQ motifs in the light chain-binding domains of each heavy chain. Electron microscopy confirmed these predictions. We measured its ATPase characteristics, in vitro motility and, using optical trap nanometry, forces and movement developed by individual myosin XI molecules. Single myosin XI molecules move processively along actin with 35 nm steps at 7 μm/s, the fastest known processive motion. Processivity was confirmed by actin landing rate assays. Mean maximal force was ∼0.5 pN, smaller than for myosin IIs. Dwell time analysis of beads carrying single myosin XI molecules fitted the ATPase kinetics, with ADP release being rate limiting. These results indicate that myosin XI is highly specialized for generation of fast processive movement with concomitantly low forces.

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Higher plant myosin XI moves processively on actin with 35 nm steps at high velocity

Fifty years ago, an important paper appeared in Botanical Magazine Tokyo. Kamiya and Kuroda proposed a sliding theory for the mechanism of cytoplasmic streaming. This pioneering study laid the basis for elucidation of the molecular mechanism of cytoplasmic streaming--the motive force is generated by the sliding of myosin XI associated with organelles along actin filaments, using the hydrolysis energy of ATP. The role of the actin-myosin system in various plant cell functions is becoming evident. The present article reviews progress in studies on cytoplasmic streaming over the past 50 years.

The sliding theory of cytoplasmic streaming: fifty years of progress.

The mechanism of the cessation of cytoplasmic streaming upon membrane excitation inCharaceae internodal cells was investigated. Cell fragments containing only cytoplasm were prepared by collecting the endoplasm at one cell end by centrifugation. In such cell fragments lacking the tonoplast, an action potential induced streaming cessation, indicating that an action potential at the plasmalemma alone is enough to stop the streaming. The active rotation of chloroplasts passively flowing together with the endoplasm also stopped simultaneously with the streaming cessation upon excitation. The time lag or interval between the rotation cessation and the electrical stimulation for inducing the action potential increased with the distance of the chloroplasts from the cortex. The time lag was about 1 second/15 μm, suggesting that an agent causing the rotation cessation is diffused throughout the endoplasm. Using internodes whose tonoplast was removed by replacing the cell sap with EGTA-containing solution (tonoplast-free cells,Tazawa
et al. 1976), we investigated the streaming rate with respect to the internal Ca2+ concentration. The rate was roughly identical to that of normal cells at a Ca2+ concentration of less than 10−7 M. It decreased with an increase in the internal Ca2+ concentration and was zero at 1 mM Ca2+. The above results, together with the two facts that Ca2+ reversibly inhibits chloroplast rotation (Hayama andTazawa, unpublished) and the streaming in tonoplast-free cells does not stop upon excitation (Tazawa
et al. 1976), lead us to conclude that a transient increase in the Ca2+ concentration in the cytoplasm directly stops the cytoplasmic streaming. Both Ca influxes across the resting and active membranes were roughly proportional to the external Ca2+ concentration, which did not affect the rate of streaming recovery. Based on these results, several possibilities for the increase in Ca2+ concentration in the cytoplasm causing streaming cessation were discussed.

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Teruo Shimmen

Papers

Myosin-dependent endoplasmic reticulum motility and F-actin organization in plant cells

Cytoplasmic streaming in plants.

Higher plant myosin XI moves processively on actin with 35 nm steps at high velocity

The sliding theory of cytoplasmic streaming: fifty years of progress.

Participation of Ca＾ in cessation of cytoplasmic streaming induced by membrane excitation in Characeae internodal cells