Positional Signaling and Expression of ENHANCER OF TRY AND CPC1 Are Tuned to Increase Root Hair Density in Response to Phosphate Deficiency in Arabidopsis thaliana

Phosphate (Pi) deficiency induces a multitude of responses aimed at improving the acquisition of Pi, including an increased density of root hairs. To understand the mechanisms involved in Pi deficiency-induced alterations of the root hair phenotype in Arabidopsis (Arabidopsis thaliana), we analyzed the patterning and length of root epidermal cells under control and Pi-deficient conditions in wild-type plants and in four mutants defective in the expression of master regulators of cell fate, CAPRICE (CPC), ENHANCER OF TRY AND CPC 1 (ETC1), WEREWOLF (WER) and SCRAMBLED (SCM). From this analysis we deduced that the longitudinal cell length of root epidermal cells is dependent on the correct perception of a positional signal (‘cortical bias’) in both control and Pi-deficient plants; mutants defective in the receptor of the signal, SCM, produced short cells characteristic of root hair-forming cells (trichoblasts). Simulating the effect of cortical bias on the time-evolving probability of cell fate supports a scenario in which a compromised positional signal delays the time point at which non-hair cells opt out the default trichoblast pathway, resulting in short, trichoblast-like non-hair cells. Collectively, our data show that Pi-deficient plants increase root hair density by the formation of shorter cells, resulting in a higher frequency of hairs per unit root length, and additional trichoblast cell fate assignment via increased expression of ETC1.     

SIZ1 regulation of phosphate starvation-induced root architecture remodeling involves the control of auxin accumulation

Phosphate (Pi) limitation causes plants to modulate the architecture of their root systems to facilitate the acquisition of Pi. Previously, we reported that the Arabidopsis (Arabidopsis thaliana) SUMO E3 ligase SIZ1 regulates root architecture remodeling in response to Pi limitation; namely, the siz1 mutations cause the inhibition of primary root (PR) elongation and the promotion of lateral root (LR) formation. Here, we present evidence that SIZ1 is involved in the negative regulation of auxin patterning to modulate root system architecture in response to Pi starvation. The siz1 mutations caused greater PR growth inhibition and LR development of seedlings in response to Pi limitation. Similar root phenotypes occurred if Pi-deficient wild-type seedlings were supplemented with auxin. N-1-Naphthylphthalamic acid, an inhibitor of auxin efflux activity, reduced the Pi starvation-induced LR root formation of siz1 seedlings to a level equivalent to that seen in the wild type. Monitoring of the auxin-responsive reporter DR5::uidA indicated that auxin accumulates in PR tips at early stages of the Pi starvation response. Subsequently, DR5::uidA expression was observed in the LR primordia, which was associated with LR elongation. The time-sequential patterning of DR5::uidA expression occurred earlier in the roots of siz1 as compared with the wild type. In addition, microarray analysis revealed that several other auxin-responsive genes, including genes involved in cell wall loosening and biosynthesis, were up-regulated in siz1 relative to wild-type seedlings in response to Pi starvation. Together, these results suggest that SIZ1 negatively regulates Pi starvation-induced root architecture remodeling through the control of auxin patterning.     

Differential effects of sucrose and auxin on localized phosphate deficiency-induced modulation of different traits of root system architecture in Arabidopsis

Phosphorus, one of the essential elements for plants, is often a limiting nutrient in soils. Low phosphate (Pi) availability induces sugar-dependent systemic expression of genes and modulates the root system architecture (RSA). Here, we present the differential effects of sucrose (Suc) and auxin on the Pi deficiency responses of the primary and lateral roots of Arabidopsis (Arabidopsis thaliana). Inhibition of primary root growth and loss of meristematic activity were evident in seedlings grown under Pi deficiency with or without Suc. Although auxin supplementation also inhibited primary root growth, loss of meristematic activity was observed specifically under Pi deficiency with or without Suc. The results suggested that Suc and auxin do not influence the mechanism involved in localized Pi sensing that regulates growth of the primary root and therefore delineates it from sugar-dependent systemic Pi starvation responses. However, the interaction between Pi and Suc was evident on the development of the lateral roots and root hairs in the seedlings grown under varying levels of Pi and Suc. Although the Pi+ Suc- condition suppressed lateral root development, induction of few laterals under the Pi- Suc- condition point to increased sensitivity of the roots to auxin during Pi deprivation. This was supported by expression analyses of DR5uidA, root basipetal transport assay of auxin, and RSA of the pgp19 mutant exhibiting reduced auxin transport. A significant increase in the number of lateral roots under the Pi- Suc- condition in the chalcone synthase mutant (tt4-2) indicated a potential role for flavonoids in auxin-mediated Pi deficiency-induced modulation of RSA. The study thus demonstrated differential roles of Suc and auxin in the developmental responses of ontogenetically distinct root traits during Pi deprivation. In addition, lack of cross talk between local and systemic Pi sensing as revealed by the seedlings grown under either the Pi- Suc- condition or in the heterogeneous Pi environment highlighted the coexistence of Suc-independent and Suc-dependent regulatory mechanisms that constitute Pi starvation responses.     

拟南芥SPL1基因参与调节低磷条件下的根际酸化反应

根际酸化是植物适应低磷胁迫的重要策略, 但植物是如何感知和转导低磷信号, 进而促进根际酸化的分子机制至今还不十分清楚。利用pH指示剂(溴甲酚紫)显色法从拟南芥(Arabidopsis thaliana) T-DNA插入突变体库中分离得到了1株低磷诱导根际酸化缺失突变体spl1。在含溴甲酚紫的低磷培养基上培养8小时, 野生型拟南芥根际培养基的颜色变为黄色, 而突变体spl1根际培养基的颜色没有明显变化, 表明spl1的低磷根际酸化反应能力降低。当低磷胁迫处理延长20天, spl1叶片的花青素积累明显高于野生型。同时也出现, 即使在磷营养正常条件下, spl1突变体也表现出根毛数量与长度增加的特征。进一步的研究表明, 在低磷条件下, spl1突变体根部的磷含量略高于野生型, 与磷转运相关基因的表达量明显高于野生型。分子遗传学分析结果表明, SPL1基因受低磷胁迫诱导, 主要在拟南芥的叶片和花等组织中表达, 其编码的蛋白广泛分布在细胞的各个部位。以上结果表明, SPL1参与介导低磷诱导的拟南芥根际酸化反应, 调节多种低磷胁迫反应及低磷条件下磷饥饿诱导基因的表达。     

HPS4/SABRE regulates plant responses to phosphate starvation through antagonistic interaction with ethylene signalling

The phytohormone ethylene plays important roles in regulating plant responses to phosphate (Pi) starvation. To date, however, no molecular components have been identified that interact with ethylene signalling in regulating such responses. In this work, an Arabidopsis mutant, hps4, was characterized that exhibits enhanced responses to Pi starvation, including increased inhibition of primary root growth, enhanced expression of Pi starvation-induced genes, and overproduction of root-associated acid phosphatases. Molecular cloning indicated that hps4 is a new allele of SABRE, which was previously identified as an important regulator of cell expansion in Arabidopsis. HPS4/SABRE antagonistically interacts with ethylene signalling to regulate plant responses to Pi starvation. Furthermore, it is shown that Pi-starved hps4 mutants accumulate more auxin in their root tips than the wild type, which may explain the increased inhibition of their primary root growth when grown under Pi deficiency.     

A soybean β-expansin gene GmEXPB2 intrinsically involved in root system architecture responses to abiotic stresses

Root system architecture responds plastically to some abiotic stresses, including phosphorus (P), iron (Fe) and water deficiency, but its response mechanism is still unclear. We cloned and characterized a vegetative β-expansin gene, GmEXPB2, from a Pi starvation-induced soybean cDNA library. Transient expression of 35S::GmEXPB2-GFP in onion epidermal cells verified that GmEXPB2 is a secretory protein located on the cell wall. GmEXPB2 was found to be primarily expressed in roots, and was highly induced by Pi starvation, and the induction pattern was confirmed by GUS staining in transgenic soybean hairy roots. Results from intact soybean composite plants either over-expressing GmEXPB2 or containing knockdown constructs, showed that GmEXPB2 is involved in hairy root elongation, and subsequently affects plant growth and P uptake, especially at low P levels. The results from a heterogeneous transformation system indicated that over-expressing GmEXPB2 in Arabidopsis increased root cell division and elongation, and enhanced plant growth and P uptake at both low and high P levels. Furthermore, we found that, in addition to Pi starvation, GmEXPB2 was also induced by Fe and mild water deficiencies. Taken together, our results suggest that GmEXPB2 is a critical root β-expansin gene that is intrinsically involved in root system architecture responses to some abiotic stresses, including P, Fe and water deficiency. In the case of Pi starvation responses, GmEXPB2 may enhance both P efficiency and P responsiveness by regulating adaptive changes of the root system architecture. This finding has great agricultural potential for improving crop P uptake on both low-P and P-fertilized soils.     

Ethylene's role in phosphate starvation signaling: more than just a root growth regulator

Phosphate (Pi) is a common limiter of plant growth due to its low availability in most soils. Plants have evolved elaborate mechanisms for sensing Pi deficiency and for initiating adaptive responses to low Pi conditions. Pi signaling pathways are modulated by both local and long-distance, or systemic, sensing mechanisms. Local sensing of low Pi initiates major root developmental changes aimed at enhancing Pi acquisition, whereas systemic sensing governs pathways that modulate expression of numerous genes encoding factors involved in Pi transport and distribution. The gaseous phytohormone ethylene has been shown to play an integral role in regulating local, root developmental responses to Pi deficiency. Comparatively, a role for ethylene in systemic Pi signaling has been more circumstantial. However, recent studies have revealed that ethylene acts to modulate a number of systemically controlled Pi starvation responses. Herein we highlight the findings from these studies and offer a model for how ethylene biosynthesis and responsiveness are integrated into both local and systemic Pi signaling pathways.     

The GLABRA2 homeodomain protein directly regulates CESA5 and XTH17 gene expression in Arabidopsis roots

Arabidopsis root hair formation is determined by the patterning genes CAPRICE ( CPC ), GLABRA3 ( GL3 ), WEREWOLF ( WER ) and GLABRA2 ( GL2 ), but little is known about the later changes in cell wall material during root hair formation. A combined Fourier-transform infrared microspectroscopy–principal components analysis (FTIR-PCA) method was used to detect subtle differences in the cell wall material between wild-type and root hair mutants in Arabidopsis. Among several root hair mutants, only the gl2 mutation affected root cell wall polysaccharides. Five of the 10 genes encoding cellulose synthase ( CESA1 – 10 ) and 4 of 33 xyloglucan endotransglucosylase ( XTH1 – 33 ) genes in Arabidopsis are expressed in the root, but only CESA5 and XTH17 were affected by the gl2 mutation. The L1-box sequence located in the promoter region of these genes was recognized by the GL2 protein. These results indicate that GL2 directly regulates cell wall-related gene expression during root development.     

Nitrate transporter NPF7.3/NRT1.5 plays an essential role in regulating phosphate deficiency responses in Arabidopsis

AtNPF7.3/AtNRT1.5, which is a nitrate transporter that drives root-to-shoot transport of NO₃^?, is also involved in modulating the response to K⁺ deprivation in Arabidopsis by affecting root development and K⁺ transport. However, whether NPF7.3/NRT1.5 functions in regulating plant responses to deficiencies of other nutrients remains unknown. In this study, we found that the expression of AtNPF7.3/AtNRT1.5 was predominant in the roots and was substantially induced by phosphate (Pi) starvation. The atnrt1.5 mutants displayed conspicuously longer primary roots along with a significantly reduced lateral root density under Pi-deficient conditions than did the wild-type plants, and these morphological differences in the roots were eliminated to a certain extent by the ethylene synthesis antagonist Co²⁺. Further analyses revealed that the expression of important Pi starvation-induced genes, which are directly involved in Pi transport, mobilization and distribution, were significantly higher in the atnrt1.5 mutants than that in the wild-type plants under Pi-starvation conditions; therefore, the atnrt1.5 mutants retained higher tissue Pi concentrations. Taken together, our results suggest that NPF7.3/NRT1.5 is an important component in the regulation of phosphate deficiency responses in Arabidopsis.     

Cell specification in the Arabidopsis root epidermis requires the activity of ECTOPIC ROOT HAIR 3--a katanin-p60 protein.

The Arabidopsis root is composed of radial cell layers, each with distinct identities. The epidermal layer is composed of rows of hair cells flanked on either side by rows of non-hair epidermal cells. The development of hair and non-hair cells is dependent on domains of positional information with strict boundaries. The pattern of cell differentiation and the expression of molecular markers of cell fate is altered in the ectopic root hair 3 (erh3) mutant epidermis indicating that ERH3 is required for the specification of cell fates from early in development (in the meristem) through differentiation. Furthermore the expression of molecular markers indicates that the specification of cell identities is defective within other radial cell layers. ERH3 encodes a p60 katanin protein that is expressed throughout the plant. Katanin proteins are known to sever microtubules, and have a role in the organisation of the plant cell wall since mutants with decreased katanin activity have been shown to have defective walls. We suggest that microtubules are involved in the specification of cell identities in cells of the Arabidopsis root. Microtubules may be required for the localization of positional cues in the wall that have previously been shown to operate in the development of the root epidermis. Alternatively microtubules may be involved in another as yet undefined process required for the specification of cell identity in plants.     

Environmentally induced plasticity of root hair development in Arabidopsis

Postembryonic development of plants is dependent on both intrinsic genetic programs and environmental factors. The plasticity of root hair patterning in response to environmental signals was investigated in the Columbia-0 wild type and 19 Arabidopsis mutants carrying lesions in various parts of the root hair developmental pathway by withholding phosphate or iron (Fe) from the nutrient medium. In the aging primary root and in laterals of the wild type, the number of root hairs increased in response to phosphate and Fe deficiency in a manner typical of each growth type. Although an increase in root hair density in -phosphorus plants was mainly achieved by the formation of extra hairs over both tangential and radial wall of underlying cortical cells, roots of -Fe plants were characterized by a high percentage of extra hairs with two tips. Root hair patterning and hair length was differentially affected by the presence or absence of phosphate and Fe among the genotypes under investigation, pointing to separate cascades of gene activation under all three growth conditions. Divergence in root hair patterning was most pronounced among mutants with defects in genes that affect the first stages of differentiation, suggesting that nutritional signals are perceived at an early stage of epidermal cell development. During elongation of the root hairs, no differences in the requirement of gene products between the growth types were obvious. The role of genes involved in root hair development in the aging primary root of Arabidopsis under the various growth conditions is discussed.