Contents
I. Introduction  424
II. The phosphorus conundrum 424
III. Adaptations to low P 424
IV. Uptake of P  424
V. P deficiency alters root development and function 426
VI. P deficiency modifies carbon metabolism 431
VII. Acid phosphatase 436
VIII. Genetic regulation of P responsive genes 437
IX. Improving P acquisition 439
X. Synopsis  440


Summary
Phosphorus (P) is limiting for crop yield on > 30% of the world's arable land and, by some estimates, world resources of inexpensive P may be depleted by 2050. Improvement of P acquisition and use by plants is critical for economic, humanitarian and environmental reasons. Plants have evolved a diverse array of strategies to obtain adequate P under limiting conditions, including modifications to root architecture, carbon metabolism and membrane structure, exudation of low molecular weight organic acids, protons and enzymes, and enhanced expression of the numerous genes involved in low-P adaptation. These adaptations may be less pronounced in mycorrhizal-associated plants. The formation of cluster roots under P-stress by the nonmycorrhizal species white lupin (Lupinus albus), and the accompanying biochemical changes exemplify many of the plant adaptations that enhance P acquisition and use. Physiological, biochemical, and molecular studies of white lupin and other species response to P-deficiency have identified targets that may be useful for plant improvement. Genomic approaches involving identification of expressed sequence tags (ESTs) found under low-P stress may also yield target sites for plant improvement. Interdisciplinary studies uniting plant breeding, biochemistry, soil science, and genetics under the large umbrella of genomics are prerequisite for rapid progress in improving nutrient acquisition and use in plants.

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Phosphorus acquisition and use: critical adaptations by plants for securing a nonrenewable resource

The plant hormone auxin regulates diverse aspects of plant growth and development. Recent studies indicate that auxin acts by promoting the degradation of the Aux/IAA transcriptional repressors through the action of the ubiquitin protein ligase SCFTIR1. The nature of the signalling cascade that leads to this effect is not known. However, recent studies indicate that the auxin receptor and other signalling components involved in this response are soluble factors. Using an in vitro pull-down assay, we demonstrate that the interaction between transport inhibitor response 1 (TIR1) and Aux/IAA proteins does not require stable modification of either protein. Instead auxin promotes the Aux/IAA–SCFTIR1 interaction by binding directly to SCFTIR1. We further show that the loss of TIR1 and three related F-box proteins eliminates saturable auxin binding in plant extracts. Finally, TIR1 synthesized in insect cells binds Aux/IAA proteins in an auxin-dependent manner. Together, these results indicate that TIR1 is an auxin receptor that mediates Aux/IAA degradation and auxin-regulated transcription.

The F-box protein TIR1 is an auxin receptor

Acid soils significantly limit crop production worldwide because approximately 50% of the world's potentially arable soils are acidic. Because acid soils are such an important constraint to agriculture, understanding the mechanisms and genes conferring tolerance to acid soil stress has been a focus of intense research interest over the past decade. The primary limitations on acid soils are toxic levels of aluminum (Al) and manganese (Mn), as well as suboptimal levels of phosphorous (P). This review examines our current understanding of the physiological, genetic, and molecular basis for crop Al tolerance, as well as reviews the emerging area of P efficiency, which involves the genetically based ability of some crop genotypes to tolerate P deficiency stress on acid soils. These are interesting times for this field because researchers are on the verge of identifying some of the genes that confer Al tolerance in crop plants; these discoveries will open up new avenues of molecular/physiological inquiry that should greatly advance our understanding of these tolerance mechanisms. Additionally, these breakthroughs will provide new molecular resources for improving crop Al tolerance via both molecular-assisted breeding and biotechnology.

How do crop plants tolerate acid soils? Mechanisms of aluminum tolerance and phosphorous efficiency.

Aluminium tolerance in plants and the complexing role of organic acids

A molecular approach to investigate auxin signaling in plants has led to the identification of several classes of early/primary auxin response genes. Within the promoters of these genes, cis elements that confer auxin responsiveness (referred to as auxin-response elements or AuxREs) have been defined, and a family of trans-acting transcription factors (auxin-response factors or ARFs) that bind with specificity to AuxREs has been characterized. A family of auxin regulated proteins referred to as Aux/IAA proteins also play a key role in regulating these auxin-response genes. Auxin may regulate transcription on early response genes by influencing the types of interactions between ARFs and Aux/IAAs.

https://link.springer.com/content/pdf/10.1023%2FA%3A1015207114117.pdf

Auxin-responsive gene expression: genes, promoters and regulatory factors.

Auxin-induced growth of coleoptiles depends on the presence of potassium and is suppressed by K+ channel blockers. To evaluate the role of K+ channels in auxin-mediated growth, we isolated and functionally expressed ZMK1 and ZMK2 (Zea mays K+ channel 1 and 2), two potassium channels from maize coleoptiles. In growth experiments, the time course of auxin-induced expression of ZMK1 coincided with the kinetics of coleoptile elongation. Upon gravistimulation of maize seedlings, ZMK1 expression followed the gravitropic-induced auxin redistribution. K+ channel expression increased even before a bending of the coleoptile was observed. The transcript level of ZMK2, expressed in vascular tissue, was not affected by auxin. In patch-clamp studies on coleoptile protoplasts, auxin increased K+ channel density while leaving channel properties unaffected. Thus, we conclude that coleoptile growth depends on the transcriptional up-regulation of ZMK1, an inwardly rectifying K+ channel expressed in the nonvascular tissue of this organ.

https://www.pnas.org/content/pnas/96/21/12186.full.pdf

Auxin-induced K+ channel expression represents an essential step in coleoptile growth and gravitropism

In search for the cellular and molecular basis for differences in aluminum (Al) resistance between maize ( Zea mays ) cultivars we applied the patch-clamp technique to protoplasts isolated from the apical root cortex of two maize cultivars differing in Al resistance. Measurements were performed on protoplasts from two apical root zones: The 1- to 2-mm zone (DTZ), described as most Al-sensitive, and the main elongation zone (3–5 mm), the site of Al-induced inhibition of cell elongation. Al stimulated citrate and malate efflux from intact root apices, revealing cultivar differences. In the elongation zone, anion channels were not observed in the absence and presence of Al. Preincubation of intact roots with 90 μmAl for 1 h induced a citrate- and malate-permeable, large conductance anion channel in 80% of the DTZ protoplasts from the resistant cultivar, but only 30% from the sensitive cultivar. When Al was applied to the protoplasts in the whole-cell configuration, anion currents were elicited within 10 min in the resistant cultivar only. La 3+ was not able to replace or counteract with Al 3+ in the activation of this channel. In the presence of the anion-channel blockers, niflumic acid and 4, 4′-dinitrostilbene-2, 2′disulfonic acid, anion currents as well as exudation rates were strongly inhibited. Application of cycloheximide did not affect the Al response, suggesting that the channel is activated through post-translational modifications. We propose that the Al-activated large anion channel described here contributes to enhanced genotypical Al resistance by facilitating the exudation of organic acid anions from the DTZ of the maize root apex.

Aluminum Activates a Citrate-Permeable Anion Channel in the Aluminum-Sensitive Zone of the Maize Root Apex. A Comparison Between an Aluminum- Sensitive and an Aluminum-Resistant Cultivar

UNLABELLED Recently, two K(+) channel genes, ZMK1 and ZMK2, were isolated from maize coleoptiles. They are expressed in the cortex and vasculature, respectively. Expression in Xenopus oocytes characterized ZMK1 as an inwardly rectifying K(+) channel activated by external acidification, while ZMK2 mediates voltage-independent and proton-inhibited K(+) currents. In search of the related gene products in planta, we applied the patch-clamp technique to protoplasts isolated from the cortex and vasculature of Zea mays coleoptiles and mesocotyls. In the cortex, a 6-8 pS K(+) channel gave rise to inwardly rectifying K(+) currents. Like ZMK1, this channel was activated by apoplastic acidification. In contrast, protoplasts from vascular tissue expressing the sucrose transporter ZmSUT1 were dominated by largely voltage-independent K(+) currents with a single-channel conductance of 22 pS. The pronounced sensitivity to the extracellular protons Ca(2+), Cs(+) and Ba(2+) is reminiscent of ZMK2 properties in oocytes. Thus, the dominant K(+) channels in cortex and vasculature most likely represent the gene products of ZMK1 and ZMK2. Our studies on the ZMK2-like channels represent the first in planta analysis of a K+ channel that shares properties with the AKT3 K(+) channel family. KEYWORDS K(+) channel, voltage-independent, proton block, maize coleoptile.

Differential expression and regulation of K(+) channels in the maize coleoptile: molecular and biophysical analysis of cells isolated from cortex and vasculature.

The divalent cation Sr2+ induced repetitive transient spikes of the cytosolic Ca2+ activity [Ca2+]cy and parallel repetitive transient hyperpolarizations of the plasma membrane in the unicellular green alga Eremosphaera viridis. [Ca2+]cy measurements, membrane potential measurements, and cation analysis of the cells were used to elucidate the mechanism of Sr2+-induced [Ca2+]cy oscillations. Sr2+ was effectively and rapidly compartmentalized within the cell, probably into the vacuole. The [Ca2+]cy oscillations cause membrane potential oscillations, and not the reverse. The endoplasmic reticulum (ER) Ca2+-ATPase blockers 2,5-di-tert-butylhydroquinone and cyclopiazonic acid inhibited Sr2+-induced repetitive [Ca2+]cy spikes, whereas the compartmentalization of Sr2+ was not influenced. A repetitive Ca2+ release and Ca2+ re-uptake by the ER probably generated repetitive [Ca2+]cy spikes in E. viridis in the presence of Sr2+. The inhibitory effect of ruthenium red and ryanodine indicated that the Sr2+-induced Ca2+ release from the ER was mediated by a ryanodine/cyclic ADP-ribose type of Ca2+ channel. The blockage of Sr2+-induced repetitive [Ca2+]cy spikes by La3+ or Gd3+ indicated the necessity of a certain influx of divalent cations for sustained [Ca2+]cy oscillations. Based on these data we present a mathematical model that describes the baseline spiking [Ca2+]cy oscillations in E. viridis.

Strontium-Induced Repetitive Calcium Spikes in a Unicellular Green Alga

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/S0014-5793%2897%2900231-7

Claudia S. Bauer

Papers

Auxin-induced K+ channel expression represents an essential step in coleoptile growth and gravitropism

Aluminum Activates a Citrate-Permeable Anion Channel in the Aluminum-Sensitive Zone of the Maize Root Apex. A Comparison Between an Aluminum- Sensitive and an Aluminum-Resistant Cultivar

Differential expression and regulation of K(+) channels in the maize coleoptile: molecular and biophysical analysis of cells isolated from cortex and vasculature.

Strontium-Induced Repetitive Calcium Spikes in a Unicellular Green Alga

Repetitive Ca2+ spikes in a unicellular green alga