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Autophagy in plants is regulated by diverse signaling cascades in response to environmental changes. Fine-tuning of its activity is critical for the maintenance of cellular homeostasis under basal and stressed conditions. In this study, we compared the Arabidopsis autophagy-related (ATG) system transcriptionally under inorganic phosphate (Pi) deficiency versus nitrogen deficiency and showed that most ATG genes are only moderately upregulated by Pi starvation, with relatively stronger induction of AtATG8f and AtATG8h among the AtATG8 family. We found that Pi shortage increased the formation of GFP-ATG8f-labeled autophagic structures and the autophagic flux in the differential zone of the Arabidopsis root. However, the proteolytic cleavage of GFP-ATG8f and the vacuolar degradation of endogenous ATG8 proteins indicated that Pi limitation does not drastically alter the autophagic flux in the whole roots, implying a cell type-dependent regulation of autophagic activities. At the organismal level, the Arabidopsis atg mutants exhibited decreased shoot Pi concentrations and smaller meristem sizes under Pi sufficiency. Under Pi limitation, these mutants showed enhanced Pi uptake and impaired root cell division and expansion. Despite a reduced steady-state level of several PHOSPHATE TRANSPORTER 1s (PHT1s) in the atg root, cycloheximide treatment analysis suggested that the protein stability of PHT1;1/2/3 is comparable in the Pi-replete wild type and atg5-1. By contrast, the degradation of PHT1;1/2/3 is enhanced in the Pi-deplete atg5-1. Our findings reveal that both basal autophagy and Pi starvation-induced autophagy are required for the maintenance of Pi homeostasis and may modulate the expression of PHT1s through different mechanisms., (© The Author(s) 2023. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

Phosphorus (P) is the second most essential macronutrient in terms of limiting plant growth. The genes involved in P acquisition, transport, storage, utilization and respective regulation have been extensively studied. In addition, significant attention has been given to the crosstalk between P and other environmental stresses. In this review, we summarize recent discoveries pertaining to the emerging function of P in plant immunity. The roles of external soil P availability, internal cellular P in plants, P starvation signaling machinery and phosphate transporters in biotic interactions are discussed. We also highlight the impact of several phytohormones on the signaling convergence between cellular P and immune responses. This information may serve as a foundation for dissecting the molecular interaction between nutrient responses and plant immunity., (© The Author(s) 2021. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Homeostasis of phosphorus (P), an essential macronutrient, is vital for plant growth under diverse environmental conditions. Although plants acquire P from the soil as inorganic phosphate (Pi), its availability is generally limited. Therefore, plants employ mechanisms involving various Pi transporters that facilitate efficient Pi uptake against a steep concentration gradient across the plant-soil interface. Among the different types of Pi transporters in plants, some members of the PHOSPHATE TRANSPORTER 1 (PHT1) family, present in the plasma membrane of root epidermal cells and root hairs, are chiefly responsible for Pi uptake from the rhizosphere. Therefore, accurate regulation of PHT1 expression is crucial for the maintenance of P homeostasis. Previous investigations positioned the Pi-dependent posttranslational regulation of PHOSPHATE STARVATION RESPONSE 1 (PHR1) transcription factor activity at the center of the regulatory mechanism controlling PHT1 expression and P homeostasis; however, recent studies indicate that several other factors also regulate the expression of PHT1 to modulate P acquisition and sustain P homeostasis against environmental fluctuations. Together with PHR1, several transcription factors that mediate the availability of other nutrients (such as nitrogen and zinc), light, and stress signals form an intricate transcriptional network to maintain P homeostasis under highly diverse environments. In this review, we summarize this intricate transcriptional network for the maintenance of P homeostasis under different environmental conditions, with a main focus on the mechanisms identified in Arabidopsis., (© The Author(s) 2021. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Phosphorus (P) is an essential macronutrient for plant growth and development. Low inorganic phosphate (Pi) availability is a limiting factor for plant growth and yield. To cope with a complex and changing environment, plants have evolved elaborate mechanisms for regulating Pi uptake and use. Recently, the molecular mechanisms of plant Pi signaling have become clearer. Plants absorb Pi from the soil through their roots and transfer Pi to various organs or tissues through phosphate transporters, which are precisely controlled at the transcript and protein levels. Here, we summarize recent progress on the molecular regulatory mechanism of phosphate transporters in Arabidopsis and rice, including the characterization of functional transporters, regulation of transcript levels, protein localization and turnover of phosphate transporters. A more in-depth understanding of plant adaptation to a changing Pi environment will facilitate the genetic improvement of plant P efficiency., (© The Author(s) 2021. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

About 60-85% of total phosphorus (P) in cereal crops is finally allocated to seeds, where it is required for seed development, germination and early growth. However, little is known about the molecular mechanisms underlying P allocation to seeds. Here, we found that two members (OsPHO1;1 and OsPHO1;2) of the PHO1 gene family are involved in the distribution of P to seeds in rice. Both OsPHO1;1 and OsPHO1;2 were localized to the plasma membrane and showed influx transport activities for inorganic phosphate. At the reproductive stage, both OsPHO1;1 and OsPHO1;2 showed higher expression in node I, the uppermost node connecting to the panicle. OsPHO1;1 was mainly localized at the phloem region of diffuse vascular bundles (DVBs) of node I, while OsPHO1;2 was expressed in the xylem parenchyma cells of the enlarged vascular bundles (EVBs). In addition, they were also expressed in the ovular vascular trace, the outer layer of the inner integument (OsPHO1;1) and in the nucellar epidermis (OsPHO1;2) of caryopses. Knockout of OsPHO1;2, as well as OsPHO1;1 to a lesser extent, decreased the distribution of P to the seed, resulting in decreased seed size and delayed germination. Taken together, OsPHO1;2 expressed in node I is responsible for the unloading of P from the xylem of EVBs, while OsPHO1;1 is involved in reloading P into the phloem of DVBs for subsequent allocation of P to seeds. Furthermore, OsPHO1;1 and OsPHO1;2 expression in the caryopsis is important for delivering P from the maternal tissues to the filial tissues for seed development., (© The Author(s) 2020. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Phosphorus (P) is an essential macronutrient for plant life, although it is frequently not readily available to crops. Arbuscular mycorrhiza fungi (AMF) can improve plant P levels by inducing the expression of some phosphate (Pi) transporters. Symbiotic Pi uptake by Pi transporters is crucial for AMF colonization and arbuscule dynamics. However, the functions of mycorrhiza-inducible maize Pi transporters are largely unclear. We focused on the interaction between the Pi concentration and AMF colonization in maize, and detecting the induction of a Pi transporter. We investigated AMF colonization and arbuscular development in maize under high and low Pi environments. Low Pi increased AMF colonization and promoted arbuscular development. Further measurement of P concentration showed that AMF significantly improved the maize P status under low Pi conditions. Here, we identified the Pi transporter gene, ZmPt9, which was induced by mycorrhiza formation. In addition, ZmPt9-overexpressing roots were difficult to colonize by AMF. Pi response analysis showed that ZmPt9 complements a yeast mutant defective in Pi transporter activity and improves the P concentration in rice. Together, these data indicated that ZmPt9 is a mycorrhiza-inducible Pi transporter gene involved in Pi uptake.

Arsenate [As(V)] is toxic to nearly all organisms. Soil-borne As(V) enters plant cells mainly through the plasma membrane-localized phosphate (Pi) transporter PHT1 family proteins due to its chemical similarity to Pi. We report here that VPT1, a major vacuolar phosphate transporter which contributes to vacuolar Pi sequestration, is associated with As(V) tolerance in Arabidopsis. vpt1 mutants displayed enhanced tolerance to As(V) toxicity, whereas plants overexpressing VPT1 were more sensitive to As(V) as compared with the wild-type plants. Measurements of arsenic content indicated that vpt1 mutants accumulated less arsenic and, in contrast, up-regulating VPT1 expression contributed to higher levels of arsenic accumulation in plants. To examine further how VPT1 may modulate arsenic contents in plants, we surveyed the expression patterns of all the PHT1 family members that play roles in As(V) uptake, and found that many of the PHT1 genes were down-regulated in the vpt1 mutant as compared with the wild type under Pi-sufficient conditions, but not when Pi levels were low in the medium. Interestingly, As(V) sensitivity assays indicated that As(V) resistance in vpt1 mutants was prominent only under Pi-sufficient but not under Pi-deficient conditions. These results suggest that under Pi-sufficient conditions, loss of VPT1 leads to elevated levels of Pi in the cytosol, which in turn suppressed the expression of PHT1-type transporters and reduced accumulation of arsenic.

GmPT7 was originally identified as an arbuscular mycorrhiza-inducible gene of soybean that encodes a member of subfamily I in the PHOSPHATE TRANSPORTER 1 family. In the present study, we established conditions under which a number of dwarf soybean plants complete their life cycles in a growth chamber. Using this system, we grew transgenic soybean with a GmPT7 promoter-β-glucuronidase fusion gene and evaluated GmPT7 expression in detail. GmPT7 was highly expressed in mature, but not in collapsed, arbuscule-containing cortical cells, suggesting its importance in the absorption of fungus-derived phosphate and/or arbuscule development. GmPT7 was also expressed in the columella cells of root caps and in the lateral root primordia of non-mycorrhizal roots. The expression of GmPT7 occurred only in the late stage of phosphorus translocation from leaves to seeds, after water evaporation from the leaves ceased, and later than the expression of GmUPS1-2, GmNRT1.7a and GmNRT1.7b, which are possibly involved in nitrogen export. GmPT7 expression was localized in a pair of tracheid elements at the tips of vein endings of senescent leaves. Transmission electron microscopy revealed that the tip tracheid elements in yellow leaves were still viable and had intact plasma membranes. Thus, we think that GmPT7 on the plasma membranes transports phosphate from the apoplast into the tip elements. GmPT7 knockdown resulted in no significant effects, the function of GmPT7 remaining to be clarified. We propose a working model in which phosphate incorporated in vein endings moves to seeds via xylem to phloem transfer., (© The Author 2014. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

Phytic acid (PA) is the main phosphorus storage form in plant seeds. It is recognized as an anti-nutrient for humans and non-ruminant animals, as well as one of the major sources of phosphorus that contributes to eutrophication. Therefore, engineering plants with low PA content without affecting plant growth capacity has become a major focus in plant breeding. Nevertheless, lack of knowledge on the role of PA seed reserves in regulating plant growth and in maintaining ion homeostasis hinders such an agronomical application. In this context, we report here that the over-expression of the bacterial phytase PHY-US417 in Arabidopsis leads to a significant decrease in seed PA, without any effect on the seed germination potential. Interestingly, this over-expression also induced a higher remobilization of free iron during germination. Moreover, the PHY-over-expressor lines show an increase in inorganic phosphate and sulfate contents, and a higher biomass production after phosphate starvation. Finally, phosphate sensing was altered because of the changes in the expression of genes induced by phosphate starvation or involved in phosphate or sulfate transport. Together, these results show that the over-expression of PHY-US417 reduces PA concentration, and provide the first evidence for the involvement of PA in the regulation of sulfate and phosphate homeostasis and signaling., (© The Author 2014. Published by Oxford University Press on behalf of Japanese Society of Plant Physiologists. All rights reserved. For permissions, please email: journals.permissions@oup.com.)

In arbuscular mycorrhizal (AM) symbiosis, host plants supply photosynthates to AM fungi and, in return, they receive inorganic nutrients such as phosphate from finely branched fungal arbuscules. Plant cortical cells envelope arbuscules with periarbuscular membranes that are continuous with the plant plasma membranes. We prepared transgenic rice (Oryza sativa) plants that express a fusion of green fluorescent protein with rice AM-inducible phosphate transporter, OsPT11-GFP, and grew them with AM fungi. The fluorescence of the fusion transporter was observed in the arbuscule branch domain, where active nutrient exchange seems to occur. In contrast, a signal was not detected around intracellular hyphal coils on colonization by either Glomus mosseae or Gigaspora rosea, making the difference between Arum- and Paris-type mycorrhizae ambiguous. We also invented a simple device involving glass-bottomed Petri dishes for in planta observation of fluorescent proteins in living AM roots with an inverted fluorescence microscope. The plant bodies remain completely intact, avoiding any stressful procedure such as cutting, staining, etc. Since rice roots exhibit a very low level of autofluorescence, the device enabled clear time-lapse imaging to analyze the formation, function and degeneration of arbuscules. In cortical cells, arbuscules seemed to be functional for only 2-3 d. Suddenly, the arbuscular branches became fragile and they shrank. At this stage, however, the periarbuscular membranes appeared intact. Then, the fluorescence of the transporter disappeared within only 2.5-5.5 h. The collapse of arbuscules occurred in the subsequent several days. Thus, our device has a great advantage for investigation of dynamic features of AM symbiosis.

When growing under limiting phosphate (P) conditions, Arabidopsis thaliana plants show dramatic changes in root architecture, including a reduction in primary root length, increased formation of lateral roots and greater formation of root hairs. Here we report that primary root growth inhibition by low P is caused by a shift from an indeterminate to a determinate developmental program. In the primary root, the low P-induced determinate growth program initiates with a reduction of cell elongation followed by the progressive loss of meristematic cells. At later stages, cell proliferation ceases and cell differentiation takes place at the former cell elongation and meristematic regions of the primary root. In low P, not only the primary but also almost all mature lateral roots enter the determinate developmental program. Kinetic studies of expression of the cell cycle marker CycB1;1:uidA and the quiescent center (QC) identity marker QC46:GUS showed that in low P conditions, reduction in proliferation in the primary root was preceded by alterations in the QC. These results suggest that in Arabidopsis, P limitation can induce a determinate root developmental program that plays an important role in altering root system architecture and that the QC could act as a sensor of environmental signals.

To understand a physiological role of an abundant 34-kDa periplasmic protein in the denitrifying phototroph Rhodobacter sphaeroides f. sp. denitrificans grown in a medium containing malate as the carbon source, the gene for the protein was isolated. The deduced amino acid sequence of the protein had a sequence similarity of 66.2% to that of PstS from Sinorhizobium meliloti. The downstream sequence of the Rhodobacter pstS contained five genes similar to pstCAB and phoUB, and its upstream sequence contained a putative regulatory sequence that is analogous to the Pho box involved in phosphate-limitation-induced gene expression in Escherichia coli. Both the amount of the PstS and the pstS promoter-driven expression of lacZ activity increased about two-fold in response to phosphate limitation. This is the first isolation of pst genes encoding proteins of an ABC phosphate transporter system from phototrophic bacteria.

A full-length cDNA encoding mitochondrial phosphate transporter was isolated from symbiotic nodules of Lotus japonicus, a model legume. The gene appeared to be single-copied and its transcript was abundant in roots, symbiotic root nodules and shoots, in that order. The L. japonicus transporter has a conserved N-ethylmaleimide-reactive Cys residue. Site-directed mutagenesis and phosphate transport assaying in vitro showed that this Cys residue is preferable for the transport activity, although it can be replaced to some extent by Ser, but not by Ala.

It is well known that the P(i) uptake system via the high-affinity P(i) transporter and the organic acid exudation system via PEPC are enhanced in the roots of P(i)-starved plants. In this paper, we compared the expression of these two systems in Sesbania rostrata, a leguminous plant, on whose roots and stems it forms nodules. When S. rostrata plants were transferred to a 0 microM P(i) nutrient solution, the expression of both the high-affinity P(i) transporter and PEPC was enhanced within 2 d. The enhancement of the expression of the high-affinity P(i) transporter genes and the PEPC gene coordinated with the increases in the P(i) uptake rate and the PEPC activity, respectively. This suggests that the expression of the high-affinity P(i) transporters and PEPC is regulated in part at the transcript level. Furthermore, we examined which of the environmental or the endogenous P(i) level regulates the expression of these two systems. The P(i) content in the 6-day-old plants decreased to a lower level than that in the 15-day-old plants when grown in a 30 microM P(i) solution. At that time, the expression of the high-affinity P(i) transporters and PEPC was enhanced only in the 6-day-old plants. Moreover, the P(i) content in plants forming many nodules on their stems decreased. The expression of the high-affinity P(i) transporters and PEPC was then enhanced in the nodulated plants. These facts suggest that the expression of these two systems may be regulated by the P(i) content in the plants, not by the P(i) concentration in the soil.