Phosphorus (P) is an essential macronutrient for plant growth and development. To adapt to low inorganic-phosphate (Pi) environments, plants have evolved complex mechanisms and pathways that regulate ...the acquisition and remobilization of Pi and maintain P homeostasis. These mechanisms are regulated by complex gene regulatory networks through the functions of Pi transporters (PTs) and Pi starvation-induced (PSI) genes. This review summarizes recent progress in determining the molecular regulatory mechanisms of phosphate transporters and the Pi signaling network in the dicot Arabidopsis (Arabidopsis thaliana) and the monocot rice (Oryza sativa L.). Recent advances in this field provide a reference for understanding plant Pi signaling and specific mechanisms that mediate plant adaptation to environments with limited Pi availability. We propose potential biotechnological applications of known genes to develop plant cultivars with improved Pi uptake and use efficiency.
Abstract
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.
Arsenic (As) contamination in paddy soil can cause phytotoxicity and elevated As accumulation in rice grain. Rice varieties vary in As uptake and tolerance, but the underlying mechanisms remain ...unclear. In this study, the aus variety Kasalath was found to be more tolerant to arsenate As(V) than the japonica variety Nipponbare, but the two varieties showed similar arsenite As(III) tolerance. Nipponbare took up more phosphate (Pi) and As(V) than Kasalath. The expression of genes for Pi transporters or Pi homeostasis regulation was quantified. Nipponbare showed 2- to 3-fold higher expression of the Pi transporter genes OsPT2 and OsPT8 than Kasalath. Two ospt8 mutants were isolated from the Kasalath background and compared with an ospt8 mutant in the Nipponbare background. Mutation in OsPT8 in both backgrounds decreased As(V) uptake by 33–57%, increased As(V) tolerance assayed by root elongation by >100-fold, and abolished the varietal differences in As(V) uptake and tolerance. The results show that OsPT8 plays a key role in As(V) uptake and that As(V) uptake mediated by OsPT8 exerts a profound toxic effect on root elongation. The results also suggest that differential OsPT8 expression explains the varietal differences in As(V) uptake and tolerance between Kasalath and Nipponbare.
The major root system of cereals consists of crown roots (or adventitious roots), which are important for anchoring plants in the soil and for water and nutrient uptake. However, the molecular basis ...of crown root formation is largely unknown. Here, we isolated a rice (
) mutant with fewer crown roots, named
(
). Map-based cloning revealed that
is caused by a mutation of a putative transcription factor-coding gene,
(
). We demonstrate that the point mutation in
perturbs the
microRNA156 (OsmiR156)-directed cleavage of
transcripts, resulting in the mutant phenotype. Chromatin immunoprecipitation sequencing assays of OsSPL3 binding sites and RNA sequencing of differentially expressed transcripts in
further identified potential direct targets of OsSPL3 in basal nodes, including a MADS-box transcription factor, OsMADS50.
overexpressing plants produced fewer crown roots, phenocopying
, while knocking out
in the
background reversed this phenotype. We also show that
, another OsmiR156 target gene, regulates
and crown root development. Taken together, our findings suggest a novel regulatory pathway in which the OsmiR156-
module directly activates
in the node to regulate crown root development in rice.
Plant vacuoles serve as the primary intracellular compartments for phosphorus (P) storage. The Oryza sativa genome contains three genes that encode SPX (
SYG1/
PHO81/
XPR1)‐MFS (
Major
Facility
...Superfamily) proteins (OsSPX‐MFS1‐3). The physiological roles of the three transporters under varying P conditions in laboratory and field are not known. To address this knowledge gap, we generated single, double and triple mutants for three OsSPX‐MFS genes. All the mutants except Osspx‐mfs2 display lower vacuolar Pi concentrations and OsSPX‐MFSs overexpression plant display higher Pi accumulation, demonstrating that all OsSPX‐MFSs are vacuolar Pi influx transporters. OsSPX‐MFS3 plays the dominant role based on the phenotypes of single mutants in terms of growth, vacuolar and tissue Pi concentrations. OsSPX‐MFS2 is the weakest and only functions as vacuole Pi sequestration in an Osspx‐mfs1/3 background. The vacuolar Pi sequestration capacity was severely impaired in Osspx‐mfs1/3 and Osspx‐mfs1/2/3, which resulted in increased Pi allocation to aerial organs. High P in the panicle impaired panicle and fertility in Osspx‐mfs1/3 and Osspx‐mfs1/2/3. Osspx‐mfs2 resulted in a more stable yield compared to the wild type under low P in field grown plants. The results suggest that alteration of vacuolar Pi sequestration may be a novel effective strategy to improve rice tolerance to low phosphorus in cropping systems.
Roots are fundamentally important for growth and development, anchoring the plant to its growth substrate, facilitating water and nutrient uptake from the soil, and sensing and responding to ...environmental signals such as biotic and abiotic stresses. Understanding the molecular mechanisms controlling root architecture is essential for improving nutrient uptake efficiency and crop yields. In this review, we describe the progress being made in the identification of genes and regulatory pathways involved in the development of root systems in rice (
Oryza sativa
L.), including crown roots, lateral roots, root hairs, and root length. Genes involved in the adaptation of roots to the environmental nutrient status are reviewed, and strategies for further study and agricultural applications are discussed. The growth and development of rice roots are controlled by both genetic factors and environmental cues. Plant hormones, especially auxin and cytokinin, play important roles in root growth and development. Understanding the molecular mechanisms regulating root architecture and response to environmental signals can contribute to the genetic improvement of crop root systems, enhancing their adaptation to stressful environmental conditions.
Auxin is a phytohormone that plays an important role in plant growth and development by forming local concentration gradients. The regulation of auxin levels is determined by the activity of auxin ...efflux carrier protein PIN-formed (PIN). In Arabidopsis thaliana, PIN-formed1 (PIN1) functions in inflorescence and root development. In rice (Oryza sativa L.), there are four PIN1 homologs (OsPIN1a-1d), but their functions remain largely unexplored. Hence, in this study, we created mutant alleles of PIN1 gene-pin1a, pin1b, pin1c, pin1d, pin1a pin1b and pin1c pin1d- using CRISPR/Cas9 technology and used them to study the functions of the four OsPIN1 paralogs in rice. In wild-type rice, all four OsPIN1 genes were relatively highly expressed in the root than in other tissues. Compared with the wild type, the OsPIN1 single mutants had no dramatic phenotypes, but the pin1a pin1b double mutant had shorter shoots and primary roots, fewer crown roots, reduced root gravitropism, longer root hairs and larger panicle branch angle. Furthermore, the pin1c pin1d double mutant showed no observable phenotype at the seedling stage, but showed naked, pin-shape inflorescence at flowering. These data suggest that OsPIN1a and OsPIN1b are involved in root, shoot and inflorescence development in rice, whereas OsPIN1c and OsPIN1d mainly function in panicle formation. Our study provides basic knowledge that will facilitate the study of auxin transport and signaling in rice.
In plants, sensing the levels of external and internal nutrients is essential for reprogramming the transcriptome and adapting to the fluctuating environment. Phosphate (Pi) is a key plant nutrient, ...and a large proportion of Pi starvation-responsive genes are under the control of PHOSPHATE STARVATION RESPONSE REGULATOR 1 (PHR1) in Arabidopsis (AtPHR1) and its homologs, such as Oryza sativa (Os)PHR2 in rice. AtPHR1 and OsPHR2 expression is not very responsive to Pi starvation, raising the question as to how plants sense changes in cellular Pi levels to activate the central regulator. SPX named after SYG1 (suppressor of yeast gpa1 ), Pho81 (CDK inhibitor in yeast PHO pathway), and XPR1 (xenotropic and polytropic retrovirus receptor) proteins that harbor only the SPX domain are reported to be involved in the negative regulation of Pi starvation responses. Here, we show that the nuclear localized SPX proteins SPX1 and SPX2 are Pi-dependent inhibitors of the activity of OsPHR2 in rice. Indeed, SPX1 and SPX2 proteins interact with PHR2 through their SPX domain, inhibiting its binding to P1BS (the PHR1-binding sequence: GNATATNC). In vivo data, as well as results from in vitro experiments using purified SPX1, SPX2, and OsPHR2 proteins, showed that SPX1 and SPX2 inhibition of OsPHR2 activity is Pi-dependent. These data provide evidence to support the involvement of SPX1 and SPX2 in the Pi-sensing mechanism in plants.
Significance Phosphate (Pi) is a primary nutrient for plant growth. Because of the low availability of soil Pi, the Pi starvation signaling in plants is gaining great interest. Arabidopsis AtPHR1 and its rice homologue OsPHR2 are known to be central transcription factors in Pi homeostasis; however, the mechanism of how plants sense external Pi fluctuation to regulate the activity of AtPHR1/OsPHR2 has been elusive. Here, we identify rice SPX1 and SPX2 as Pi-dependent inhibitors of PHR2, implicating SPX1 and SPX2 in the Pi-sensing mechanism. We also show that the SPX domain of SPX1 and SPX2 is critical for repressing PHR2 binding to cis elements by protein interaction. The discovery of cellular nutrient concentration-dependent fine-tuning sheds light on a novel mechanism of plant adaption to environmental cues.
Flowering time is a critical agronomic trait that determines successful seed production and adaptation of crop plants. Photoperiodic control of this process in flowering plants is mediated by the ...long-distance mobile signal called florigen partly encoded by FLOWERING LOCUS T (FT) in Arabidopsis thaliana and its orthologs in other plant species. Despite the progress in understanding FT transport in the dicot model Arabidopsis, the mechanisms of florigen transport in monocots, which provide most of the biomass in agriculture, are unknown. Here, we show that rice FT-INTERACTING PROTEIN1 (OsFTIP1), a member of the family of multiple C2 domain and transmembrane region proteins (MCTPs) and the closest ortholog of Arabidopsis FTIP1, is required for export of RICE FLOWERING LOCUS T 1 (RFT1) from companion cells to sieve elements. This affects RFT1 movement to the shoot apical meristem and its regulation of rice flowering time under long days. We further reveal that a ubiquitin-like domain kinase γ4, OsUbDKγ4, interacts with OsFTIP1 and modulates its degradation in leaves through the 26S proteasome, which in turn affects RFT1 transport to the shoot apical meristem. Thus, dynamic modulation of OsFTIP1 abundance in leaves by a negative regulator OsUbDKγ4 is integral to the role of OsFTIP1 in mediating RFT1 transport in rice and provides key evidence for a conserved role of FTIP1-like MCTPs in mediating florigen transport in flowering plants.
Phosphorus (P) is an essential macronutrient for plant growth and development, but the molecular mechanism determining how plants sense external inorganic phosphate (Pi) levels and reprogram ...transcriptional and adaptive responses is incompletely understood.
In this study, we investigated the function of OsSPX6 (hereafter SPX6), an uncharacterized member of SPX domain (SYG1, Pho81 and XPR1)-containing proteins in rice, using reverse genetics and biochemical approaches.
Transgenic plants overexpressing SPX6 exhibited decreased Pi concentrations and suppression of phosphate starvation-induced (PSI) genes. By contrast, transgenic lines with decreased SPX6 transcript levels or spx6 mutant showed significant Pi accumulation in the leaf and upregulation of PSI genes. Overexpression of SPX6 genetically suppressed the overexpression of PHOSPHATE STARVATION RESPONSE REGULATOR 2 (PHR2) in terms of the accumulation of high Pi content. Moreover, direct interaction of SPX6 with PHR2 impeded PHR2 translocation into the nucleus, and inhibited PHR2 binding to the P1BS (PHR1 binding sequence) element. SPX6 protein was degraded in leaves under Pi-deficient conditions, whereas it accumulated in roots.
We conclude that rice SPX6 is another important negative regulator in Pi starvation signaling through the interaction with PHR2. SPX6 shows different responses to Pi starvation in shoot and root, which differ from those of other SPX proteins.
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