Molecular characterisation of an S-like RNase of Nicotiana alata that is induced by phosphate starvation

Intracellular phosphate recycling systems for survival during phosphate starvation in plants

Phosphorus (P) is an essential nutrient for plant growth and plants use inorganic phosphate (Pi) as their P source, but its bioavailable form, orthophosphate, is often limited in soils. Hence, plants have several mechanisms for adaptation to Pi starvation. One of the most common response strategies is "Pi recycling" in which catabolic enzymes degrade intracellular constituents, such as phosphoesters, nucleic acids and glycerophospholipids to salvage Pi. Recently, several other intracellular degradation systems have been discovered that salvage Pi from organelles. Also, one of sphingolipids has recently been identified as a degradation target for Pi recycling. So, in this mini-review we summarize the current state of knowledge, including research findings, about the targets and degradation processes for Pi recycling under Pi starvation, in order to further our knowledge of the whole mechanism of Pi recycling.

Evolutionary and structural aspects of Solanaceae RNases T2

Plant RNases T2 are involved in several physiological and developmental processes, including inorganic phosphate starvation, senescence, wounding, defense against pathogens, and the self-incompatibility system. Solanaceae RNases form three main clades, one composed exclusively of S-RNases and two that include S-like RNases. We identified several positively selected amino acids located in highly flexible regions of these molecules, mainly close to the B1 and B2 substrate-binding sites in S-like RNases and the hypervariable regions of S-RNases. These differences between S- and S-like RNases in the flexibility of amino acids in substrate-binding regions are essential to understand the RNA-binding process. For example, in the S-like RNase NT, two positively selected amino acid residues (Tyr156 and Asn134) are located at the most flexible sites on the molecular surface. RNase NT is induced in response to tobacco mosaic virus infection; these sites may thus be regions of interaction with pathogen proteins or viral RNA. Differential selective pressures acting on plant ribonucleases have increased amino acid variability and, consequently, structural differences within and among S-like RNases and S-RNases that seem to be essential for these proteins play different functions.

Transcriptome analysis of a near-isogenic line and its recurrent parent reveals the role of Pup1 QTL in phosphorus deficiency tolerance of rice at tillering stage

Phosphorus (P) is essential for cellular processes like respiration, photosynthesis, biosynthesis of membrane phospholipids, etc. To cope with P deficiency stress, plants adopt reprograming of the expression of genes involved in different metabolic/signaling pathways for survival, growth, and development. Plants use transcriptional, post-transcriptional, and/or post-translational machinery to achieve P homeostasis. Several transcription factors (TFs), miRNAs, and P transporters play important roles in P deficiency tolerance; however, the underlying mechanisms responsible for P deficiency tolerance remain poorly understood. Studies on P starvation/deficiency responses in plants at early (seedling) stage of growth have been reported but only a few of them focused on molecular responses of the plant at advanced (tillering or reproductive) stage of growth. To decipher the strategies adopted by rice at tillering stage under P deficiency stress, a pair of contrasting genotypes [Pusa-44 (a high-yielding, P deficiency sensitive cultivar) and its near-isogenic line (NIL-23, P deficiency tolerant) for Pup1 QTL] was used for morphophysiological, biochemical, and molecular analyses. Comparative analyses of shoot and root tissues from 45-day-old plants grown hydroponically under P sufficient (16 ppm) or P deficient (4 ppm) medium confirmed some of the known morphophysiological responses. Moreover, RNA-seq analysis revealed the important roles of phosphate transporters, TFs, auxin-responsive proteins, modulation in the cell wall, fatty acid metabolism, and chromatin architecture/epigenetic modifications in providing P deficiency tolerance to NIL-23, which were brought in due to the introgression of the Pup1 QTL in Pusa-44. This study provides insights into the molecular functions of Pup1 for P deficiency tolerance, which might be utilized to improve P-use efficiency of rice for better productivity in P deficient soils. KEY MESSAGE: Introgression of Pup1 QTL in high-yielding rice cultivar modulates mainly phosphate transporters, TFs, auxin-responsive proteins, cell wall structure, fatty acid metabolism, and chromatin architecture/epigenetic modifications at tillering stage of growth under phosphorus deficiency stress.

Fine-tuning the transcriptional regulatory model of adaptation response to phosphate stress in maize (Zea mays L.)

The post green revolution agriculture is based on generous application of fertilizers and high-yielding genotypes that are suited for such high input regimes. Cereals, like maize (Zea mays L.) are capable of utilizing less than 20% of the applied inorganic phosphate (Pi) - a non-renewable fertilizer resource. A greater understanding of the molecular mechanisms underlying the acquisition, transportation and utilization of Pi may lead to engineering genotypes with high phosphorus use efficiency. In this study, we carried out functional domain similarity analysis, promoter analysis and comparative transcriptional expression profiling of 12 selected Pi responsive genes in the Pi stress tolerant maize inbred line HKI-163 under sufficient and deficient Pi conditions. Pi starvation led to significant increase in root length; marked proliferation of root hairs and lesser number of crown roots. Eleven genes were significantly up or down regulated in Pi deficient condition. The putative acid phosphatase, ZmACP5 expression was up regulated by 162.81 and 74.40 fold in root and leaf tissues, respectively. The RNase, ZmRNS1 showed 115 fold up regulation in roots under Pi deprivation. Among the two putative high affinity Pi transporters ZmPht1;4 was found specific to root, whereas ZmPht2 was found to be up regulated in both root and leaf tissues. The genes involved in Pi homeostasis pathway (ZmSIZ1, SPX1 and Pho2) were up regulated in root and leaf. In light of the expression profiling of selected regulatory genes, an updated model of transcriptional regulation under Pi starvation in maize has been presented.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s12298-022-01155-x.

Genome-wide identification and expression pattern analysis of the ribonuclease T2 family in Eucommia ulmoides

The 2′,3′-cycling ribonuclease (RNase) genes are catalysts of RNA cleavage and include the RNase T2 gene family. RNase T2 genes perform important roles in plants and have been conserved in the genome of eukaryotic organisms. In this study we identified 21 EURNS genes in Eucommia ulmoides Oliver (E. ulmoides) and analyzed their structure, chromosomal location, phylogenetic tree, gene duplication, stress-related cis-elements, and expression patterns in different tissues. The length of 21 predicted EURNS proteins ranged from 143 to 374 amino acids (aa), their molecular weight (MW) ranged from 16.21 to 42.38 kDa, and their isoelectric point (PI) value ranged from 5.08 to 9.09. Two classifications (class I and class III) were obtained from the conserved domains analysis and phylogenetic tree. EURNS proteins contained a total of 15 motifs. Motif 1, motif 2, motif 3, and motif 7 were distributed in multiple sequences and were similar to the conserved domain of RNase T2. EURNS genes with similar structure and the predicted EURNS proteins with conserved motif compositions are in the same group in the phylogenetic tree. The results of RT-PCR and transcription data showed that EURNS genes have tissue-specific expression and exhibited obvious trends in different developmental stages. Gene duplication analysis results indicated that segment duplication may be the dominant duplication mode in this gene family. This study provides a theoretical basis for research on the RNase T2 gene family and lays a foundation for the further study of EURNS genes.

Tomato T2 ribonuclease LE is involved in the response to pathogens

T2 ribonucleases (RNases) are RNA-degrading enzymes that function in various cellular processes, mostly via RNA metabolism. T2 RNase-encoding genes have been identified in various organisms, from bacteria to mammals, and are most diverse in plants. The existence of T2 RNase genes in almost every organism suggests an important biological function that has been conserved through evolution. In plants, T2 RNases are suggested to be involved in phosphate scavenging and recycling, and are implicated in defence responses to pathogens. We investigated the function of the tomato T2 RNase LE, known to be induced by phosphate deficiency and wounding. The possible involvement of LE in pathogen responses was examined. Expression analysis showed LE induction during fungal infection and by stimuli known to be associated with pathogen inoculation, including oxalic acid and hydrogen peroxide. Analysis of LE-suppressed transgenic tomato lines revealed higher susceptibility to oxalic acid, a cell death-inducing factor, compared to the wild type. This elevated sensitivity of LE-suppressed lines was evidenced by visual signs of necrosis, and increased ion leakage and reactive oxygen species levels, indicating acceleration of cell death. Challenge of the LE-suppressed lines with the necrotrophic pathogen Botrytis cinerea resulted in accelerated development of disease symptoms compared to the wild type, associated with suppressed expression of pathogenesis-related marker genes. The results suggest a role for plant endogenous T2 RNases in antifungal activity.

NnSR1, a class III non-S-RNase specifically induced in Nicotiana alata under phosphate deficiency, is localized in endoplasmic reticulum compartments

A combined strategy of phosphate (Pi) remobilization from internal and external RNA sources seems to be conserved in plants exposed to Pi starvation. Thus far, the only ribonucleases (RNases) reported to be induced in Nicotiana alata undergoing Pi deprivation are extracellular S-like RNase NE and NnSR1. NnSR1 is a class III non S-RNase of unknown subcellular location. Here, we examine the hypothesis that NnSR1 is an intracellular RNase derived from the self-incompatibility system with specific expression in self-incompatible Nicotiana alata. NnSR1 was not induced in self-compatible Nicotiana species exposed to Pi deprivation. NnSR1 conjugated with a fluorescent protein and transiently expressed in Arabidopsis protoplasts and Nicotiana leaves showed that the fusion protein co-localized with an endoplasmic reticulum (ER) marker. Subcellular fractionation by ultracentrifugation of roots exposed to Pi deprivation revealed that the native NnSR1 migrated in parallel with the BiP protein, a typical ER marker. To our knowledge, NnSR1 is the first class III RNase reported to be localized in ER compartments. The induction of NnSR1 was detected earlier than the extracellular RNase NE, suggesting that intracellular RNA may be the first source of Pi used by the cell under Pi stress.

Structural and functional characteristics of S-like ribonucleases from carnivorous plants

Although the S-like ribonucleases (RNases) share sequence homology with the S-RNases involved in the self-incompatibility mechanism in plants, they are not associated with this mechanism. They usually function in stress responses in non-carnivorous plants and in carnivory in carnivorous plants. In this study, we clarified the structures of the S-like RNases of Aldrovanda vesiculosa, Nepenthes bicalcarata and Sarracenia leucophylla, and compared them with those of other plants. At ten positions, amino acid residues are conserved or almost conserved only for carnivorous plants (six in total). In contrast, two positions are specific to non-carnivorous plants. A phylogenetic analysis revealed that the S-like RNases of the carnivorous plants form a group beyond the phylogenetic relationships of the plants. We also prepared and characterized recombinant S-like RNases of Dionaea muscipula, Cephalotus follicularis, A. vesiculosa, N. bicalcarata and S. leucophylla, and RNS1 of Arabidopsis thaliana. The recombinant carnivorous plant enzymes showed optimum activities at about pH 4.0. Generally, poly(C) was digested less efficiently than poly(A), poly(I) and poly(U). The kinetic parameters of the recombinant D. muscipula enzyme (DM-I) and A. thaliana enzyme RNS1 were similar. The k cat/K m of recombinant RNS1 was the highest among the enzymes, followed closely by that of recombinant DM-I. On the other hand, the k cat/K m of the recombinant S. leucophylla enzyme was the lowest, and was ~1/30 of that for recombinant RNS1. The magnitudes of the k cat/K m values or k cat values for carnivorous plant S-like RNases seem to correlate negatively with the dependency on symbionts for prey digestion.

Overexpression of an S-like ribonuclease gene, OsRNS4, confers enhanced tolerance to high salinity and hyposensitivity to phytochrome-mediated light signals in rice

S-like ribonucleases (S-like RNases) are homologous to S-ribonucleases (S-RNases), but are not involved in self-incompatibility. In dicotyledonous plants, S-like RNases play an important role in phosphate recycling during senescence and are induced by inorganic phosphate-starvation and in response to defense and mechanical wounding. However, little information about the functions of the S-like RNase in monocots has been reported. Here, we investigated the expression patterns and roles of an S-like RNase gene, OsRNS4, in abscisic acid (ABA)-mediated responses and phytochrome-mediated light responses as well as salinity tolerance in rice. The OsRNS4 gene was expressed at relatively high levels in leaves although its transcripts were detected in various organs. OsRNS4 expression was regulated by salt, PEG and ABA. The seedlings overexpressing OsRNS4 had longer coleoptiles and first leaves than wild-type seedlings under red light (R) and far-red light (FR), suggesting negative regulation of OsRNS4 in photomorphogenesis in rice seedlings. Moreover, ABA-induced growth inhibition of rice seedlings was significantly increased in the OsRNS4-overexpression (OsRNS4-OX) lines compared with that in WT, suggesting that OsRNS4 probably acts as a positive regulator in ABA responses in rice seedlings. In addition, our results demonstrate that OsRNS4-OX lines have enhanced tolerance to high salinity compared to WT. Our findings supply new evidence on the functions of monocot S-like RNase in regulating photosensitivity and abiotic stress responses.

NnSR1, a class III non-S-RNase constitutively expressed in styles, is induced in roots and stems under phosphate deficiency in Nicotiana alata

BACKGROUND AND AIMS

Non-S-ribonucleases (non-S-RNases) are class III T2 RNases constitutively expressed in styles of species with S-RNase-based self-incompatibility. So far, no function has been attributed to these RNases. The aim of this work is to examine if NnSR1, a non-S-RNase from Nicotiana alata, is induced under conditions of phosphate (Pi) deprivation. The hypothesis is that under Pi-limited conditions, non-S-RNase functions may resemble the role of S-like RNases. To date, the only RNases reported to be induced by Pi deficiency are class I and class II S-like RNases, which are phylogenetically different from the class III clade of RNases.

METHODS

Gene and protein expression of NnSR1 were assayed in plants grown hydroponically with and without Pi, by combining RT-PCR, immunoblot and enzymatic activity approaches.

KEY RESULTS

NnSR1 transcripts were detected in roots 7 d after Pi deprivation and remained stable for several days. Transcript expression was correlated based on Pi availability in the culture medium. Antiserum against a peptide based on a hypervariable domain of NnSR1 recognized NnSR1 in roots and stems but not leaves exposed to Pi shortage. NnSR1 was not detected in culture medium and was pelleted with the microsomal fraction, suggesting that it was membrane-associated or included in large compartments. The anti-NnSR1 inhibited selectively the enzymatic activity of a 31-kDa RNase indicating that NnSR1 was induced in an enzymatically active form.

CONCLUSIONS

The induction of NnSR1 indicates that there is a general recruitment of all classes of T2 RNases in response to Pi shortage. NnSR1 appears to have regained ancestral functions of class III RNases related to strategies to cope with Pi limitation and also possibly with other environmental challenges. This constitutes the first report for a specific function of class III RNases other than S-RNases.

S-like ribonuclease gene expression in carnivorous plants

Functions of S-like ribonucleases (RNases) differ considerably from those of S-RNases that function in self-incompatibility. Expression of S-like RNases is usually induced by low nutrition, vermin damage or senescence. However, interestingly, an Australian carnivorous plant Drosera adelae (a sundew), which traps prey with a sticky digestive liquid, abundantly secretes an S-like RNase DA-I in the digestive liquid even in ordinary states. Here, using D. adelae, Dionaea muscipula (Venus flytrap) and Cephalotus follicularis (Australian pitcher plant), we show that carnivorous plants use S-like RNases for carnivory: the gene da-I encoding DA-I and its ortholog cf-I of C. follicularis are highly expressed and constitutively active in each trap/digestion organ, while the ortholog dm-I of D. muscipula becomes highly active after trapping insects. The da-I promoter is unmethylated only in its trap/digestion organ, glandular tentacles (which comprise a small percentage of the weight of the whole plant), but methylated in other organs, which explains the glandular tentacles-specific expression of the gene and indicates a very rare gene regulation system. In contrast, the promoters of dm-I, which shows induced expression, and cf-I, which has constitutive expression, were not methylated in any organs examined. Thus, it seems that the regulatory mechanisms of the da-I, dm-I and cf-I genes differ from each other and do not correlate with the phylogenetic relationship. The current study suggests that under environmental pressure in specific habitats carnivorous plants have managed to evolve their S-like RNase genes to function in carnivory.

Brassica napus PHR1 gene encoding a MYB-like protein functions in response to phosphate starvation

Phosphorus (P) is one of the essential nutrient elements for plant development. In this work, BnPHR1 encoding a MYB transcription activator was isolated from Brassica napus. The characterization of nuclear localization and transcription activation ability suggest BnPHR1 is a transcriptional activator. The tissue expression and histochemical assay showed that BnPHR1 was predominantly expressed in roots and modulated by exogenous Pi in transcriptional level in roots under Pi deficiency conditions. Furthermore, overexpression of BnPHR1 in both Arabidopsis and B. napus remarkably enhanced the expression of the Pi-starvation-induced genes including ATPT2 and BnPT2 encoding the high-affinity Pi transporter. Additionally, BnPHR1 can in vivo bind the promoter sequence of ATPT2 and BnPT2 in both Arabidopsis and B. napus. Possibly, due to the activation of ATPT2 and BnPT2, or even more high-affinity Pi transporters, the excessive Pi was accumulated in transgenic plants, resulting in the crucially Pi toxicity to cells and subsequently retarding plant growth. Given the data together, BnPHR1, as crucial regulator, is regulated by exogenous Pi and directly activates those genes, which promote the uptake and homeostasis of Pi for plant growth.

Modulation of kernel storage proteins in grain sorghum (Sorghum bicolor (L.) Moench)

Sorghum prolamins, termed kafirins, are categorized into subgroups α, β, and γ. The kafirins are co-translationally translocated to the endoplasmic reticulum (ER) where they are assembled into discrete protein bodies that tend to be poorly digestible with low functionality in food and feed applications. As a means to address the issues surrounding functionality and digestibility in sorghum, we employed a biotechnology approach that is designed to alter protein body structure, with the concomitant synthesis of a co-protein in the endosperm fraction of the grain. Wherein perturbation of protein body architecture may provide a route to impact digestibility by reducing disulphide bonds about the periphery of the body, while synthesis of a co-protein, with known functionality attributes, theoretically could impact structure of the protein body through direct association and/or augment end-use applications of sorghum flour by stabilizing ß-sheet formation of the kafirins in sorghum dough preparations. This in turn may improve viscoelasticity of sorghum dough. To this end, we report here on the molecular and phenotypic characterizations of transgenic sorghum events that are down-regulated in γ- and the 29-kDa α-kafirins and the expression of a wheat Dy10/Dx 5 hybrid high-molecular weight glutenin protein. The results demonstrate that down-regulation of γ-kafirin alone does not alter protein body formation or impacts protein digestibility of cooked flour samples. However, reduction in accumulation of a predicted 29-kDa α-kafirin alters the morphology of protein body and enhances protein digestibility in both raw and cooked samples.

Molecular and genetic characterization of novel S-RNases from a natural population of Nicotiana alata

Self-incompatibility in the Solanaceae is mediated by S-RNase alleles expressed in the style, which confer specificity for pollen recognition. Nicotiana alata has been successfully used as an experimental model to elucidate cellular and molecular aspects of S-RNase-based self-incompatibility in Solanaceae. However, S-RNase alleles of this species have not been surveyed from natural populations and consequently the S-haplotype diversity is poorly known. Here the molecular and functional characterization of seven S-RNase candidate sequences, identified from a natural population of N. alata, are reported. Six of these candidates, S ( 5 ), S ( 27 ), S ( 70 ), S ( 75 ), S ( 107 ), and S ( 210 ), showed plant-specific amplification in the natural population and style-specific expression, which increased gradually during bud maturation, consistent with the reported S-RNase expression. In contrast, the S ( 63 ) ribonuclease was present in all plants examined and was ubiquitously expressed in different organs and bud developmental stages. Genetic segregation analysis demonstrated that S ( 27 ), S ( 70 ), S ( 75 ), S ( 107 ), and S ( 210 ) alleles were fully functional novel S-RNases, while S ( 5 ) and S ( 63 ) resulted to be non-S-RNases, although with a clearly distinct pattern of expression. These results reveal the importance of performing functional analysis in studies of S-RNase allelic diversity. Comparative phylogenetic analysis of six species of Solanaceae showed that N. alata S-RNases were included in eight transgeneric S-lineages. Phylogenetic pattern obtained from the inclusion of the novel S-RNase alleles confirms that N. alata represents a broad sample of the allelic variation at the S-locus of the Solanaceae.

Petunia nectar proteins have ribonuclease activity

Plants requiring an insect pollinator often produce nectar as a reward for the pollinator's visitations. This rich secretion needs mechanisms to inhibit microbial growth. In Nicotiana spp. nectar, anti-microbial activity is due to the production of hydrogen peroxide. In a close relative, Petunia hybrida, limited production of hydrogen peroxide was found; yet petunia nectar still has anti-bacterial properties, suggesting that a different mechanism may exist for this inhibition. The nectar proteins of petunia plants were compared with those of ornamental tobacco and significant differences were found in protein profiles and function between these two closely related species. Among those proteins, RNase activities unique to petunia nectar were identified. The genes corresponding to four RNase T2 proteins from Petunia hybrida that show unique expression patterns in different plant tissues were cloned. Two of these enzymes, RNase Phy3 and RNase Phy4 are unique among the T2 family and contain characteristics similar to both S- and S-like RNases. Analysis of amino acid patterns suggest that these proteins are an intermediate between S- and S-like RNases, and support the hypothesis that S-RNases evolved from defence RNases expressed in floral parts. This is the first report of RNase activities in nectar.

CgSL2, an S-like RNase gene in 'Zigui shatian' pummelo (Citrus grandis Osbeck), is involved in ovary senescence

'Zigui shatian' pummelo (Citrus grandis Osbeck) is one nature mutant from 'Shatian' pummelo, which showed self-compatibility, because self-pollen tubes were not arrested in the style, moreover abnormal post-zygotic development in ovary caused seed abortion in the cultivar. Herein we constructed a cDNA library from flowers of 'Zigui shatian' pummelo and identified one RNase gene fragment. The full length of cDNA sequence of this gene, with an open reading frame of 834 bp, was isolated by 5'-RACE method. The gene, named as CgSL2, contained five conserved regions and two histidine residues essential for RNase activity. Phylogenetic analysis indicated that CgSL2 was mostly similar to AhSL28, an S-like RNase from Antirrhinum. Southern hybridization verified CgSL2 existed in the genome as multiple copies. qRT-PCR and RT-PCR analysis showed that the expression of CgSL2 was not tissue-specific. The expression of CgSL2 was down-regulated during senescence of stem, petal, style and stamen, whereas up-regulated during ovary senescence. Further in situ hybridization of CgSL2 in the ovary during the balloon stage to anthesis stage also showed that it dramatically increased in mature flower, consistent with qRT-PCR and RT-PCR results. These findings suggested that CgSL2 might play an important role during ovary senescence.

Gene expression profiles in rice roots under low phosphorus stress

Phosphorus (P), an important plant macronutrient, is a component of key molecules such as nucleic acids, phospholipids and ATP. P is often the limiting nutrient for crop yield potential because of the low concentration of soluble P that can be absorbed directly by plant. Plants have evolved a series of molecular and morphological adaptations to cope with P limitation. However, the molecular bases of these responses to P deficiency have not been thoroughly elucidated. In this report, the gene expression profiles of low-P-tolerant rice Zhongzao 18 (Oryza sativa ssp. Indica) and not-low-P-tolerant rice Lagrue (Oryza sativa ssp. Indica) roots at 6 h, 24 h and 72 h under low P stress were investigated and compared with a control (normal P conditions) profile, using a DNA chip of 60,000 oligos (70 mer) that represented all putative genes of the rice genome. A total of 1,518 and 2,358 genes exhibited alterations in expression in response to low P stress in at least one of the three time points in rice Zhongzao 18 and rice Lagrue, respectively. The differentially expressed genes included those involved in phosphate (Pi) transportation, transportations except for Pi transportation, phosphatase, enzymes other than phosphatase, primary metabolism, secondary metabolism and so on. Several genes involved in glycolysis and TCA cycle were up-regulated during the early stages of low P treatment in rice Zhongzao 18 roots, but not in rice Lagrue roots. The results may provide useful information to further studies of the molecular mechanism of plant adaptation to low P and thus facilitate research in improving P utilization in crop species.

The expression profile of genes in rice roots under low phosphorus stress

Phosphorus (P) is one of the most essential macronutrients required for plant growth. Although it is abundant in soil, P is often the limiting nutrient for crop yield potential because of the low concentration of soluble P that plants can absorb directly. The gene expression profile was investigated in rice roots at 6, 24 and 72 h under low P stress and compared with a control (normal P) profile, using a DNA chip of 60000 oligos (70 mer) that represented all putative genes of the rice genome. A total of 795 differentially expressed genes were identified in response to phosphate (Pi) starvation in at least one of the treatments. Based on the analysis, we found that: (i) The genes coding for the Pi transporter, acid phosphatase and RNase were up-regulated in rice roots; (ii) the genes involved in glycolysis were first up-regulated and then down-regulated; (iii) several genes involved in N metabolism and lipid metabolism changed their expression patterns; (iv) some genes involved in cell senescence and DNA or protein degradation were up-regulated; and (v) some transmembrane transporter genes were up-regulated. The results may provide useful information in the molecular process associated with Pi deficiency and thus facilitate research in improving Pi utilization in crop species.

Molecular mechanisms in response to phosphate starvation in rice

Phosphorus is one of the most important elements that significantly affect plant growth and metabolism. Among the macro-nutrients, phosphorus is the least available to the plants as major phosphorus content of the fertiliser is sorbed by soil particles. An increased knowledge of the regulatory mechanisms controlling plant's phosphorus status is vital for improving phosphorus uptake and P-use efficiency and for reducing excessive input of fertilisers, while maintaining an acceptable yield. Phosphorus use efficiency has been studied using forward and reverse genetic analyses of mutants, quantitative genomic approaches and whole plant physiology but all these studies need to be integrated for a clearer understanding. We provide a critical overview on the molecular mechanisms and the components involved in the plant during phosphorus starvation. Then we summarize the information available on the genes and QTLs involved in phosphorus signalling and also the methods to estimate total phosphate in plant tissue. Also, an effort is made to build a comprehensive picture of phosphorus uptake, homeostasis, assimilation, remobilization, its deposition in the grain and its interaction with other micro- and macro-nutrients as well as phytohormones.

An S-RNase-based gametophytic self-incompatibility system evolved only once in eudicots

It has been argued that the common ancestor of about 75% of all dicots possessed an S-RNase-based gametophytic self-incompatibility (GSI) system. S-RNase genes should thus be found in most plant families showing GSI. The S-RNase gene (or a duplicate) may also acquire a new function and thus genes belonging to the S-RNase lineage may also persist in plant families without GSI. Nevertheless, sequences that belong to the S-RNase lineage have been found in the Solanaceae, Scrophulariaceae, Rosaceae, Cucurbitaceae, and Fabaceae plant families only. Here we search for new sequences that may belong to the S-RNase lineage, using both a phylogenetic and a much faster and simpler amino acid pattern-based approach. We show that the two methods have an apparently similar false-negative rate of discovery (approximately 10%). The amino acid pattern-based approach produces about 15% false positives. Genes belonging to the S-RNase lineage are found in three new plant families, namely, the Rubiaceae, Euphorbiaceae, and Malvaceae. Acquisition of a new function by genes belonging to the S-RNase lineage is shown to be a frequent event. A putative S-RNase sequence is identified in Lotus, a plant genus for which molecular studies on GSI are lacking. The hypothesis of a single origin for S-RNase-based GSI (before the split of the Asteridae and Rosidae) is further supported by the finding of genes belonging to the S-RNase lineage in some of the oldest lineages of the Asteridae and Rosidae, and by Baysean constrained tree analyses.

Purification and characterization of a non-S-RNase and S-RNases from styles of Japanese pear (Pyrus pyrifolia)

In this study we biochemically characterized stylar ribonucleases (RNases) of Japanese pear (Pyrus pyrifolia), which exhibits S-RNase-based gametophytic self-incompatibility. We separated the RNase fractions NS-1, NS-2, and NS-3 from stylar extracts of the cultivar Nijisseiki (S(2)S(4)). The RNase in each fraction was purified to homogeneity through a series of chromatographic steps. Chemical analysis of the proteins revealed that the basic RNases in the NS-2 and NS-3 fractions were the S(4)- and S(2)-RNases, respectively. Five additional S-RNases were purified from other cultivars. An acidic RNase in the NS-1 fraction was also purified from other cultivars, and identified as a non-S-allele-associated RNase (non-S-RNase). The non-S-RNase is composed of 203 amino acids, is non-glycosylated and is a N-terminal-pyroglutamylated enzyme of the RNase T(2) family. The substrate specificities and optimum pH levels of the non-S-RNase and S-RNases were similar. Interestingly, the specific activity of the non-S-RNase was 7.5-221-fold higher than those of the S-RNases when tolura yeast RNA was used as the substrate. The specific activity of the S(2)-RNase was 8.8-28.6-fold lower than those of the other S-RNases. These differences in specific activities among the stylar RNases are discussed.

Suppression of LX ribonuclease in tomato results in a delay of leaf senescence and abscission

Although present in different organisms and conserved in their protein sequence, the biological functions of T2 ribonucleases (RNase) are generally unknown. Tomato (Lycopersicon esculentum) LX is a T2/S-like RNase and its expression is known to be associated with phosphate starvation, ethylene responses, and senescence and programmed cell death. In this study, LX function was investigated using antisense tomato plants in which the LX protein level was reduced. LX protein levels normally become elevated when leaves senesce and antisense inhibition of LX retarded the progression of senescence. Moreover, we observed a marked delay of leaf abscission in LX-deficient plants. This correlated with specific induction of LX protein in the tomato mature abscission zone tissue. LX RNase gene regulation and the consequences of antisense inhibition indicate that LX has an important functional role in both abscission and senescence.

Cloning and characterization of an RNase-related protein gene preferentially expressed in rice stems

RNase-related proteins (RRPs) are S- and S-like RNase homologs lacking the active site required for RNase activity. Here we describe the cloning and characterization of the rice (Oryza sativa) RRP gene (OsRRP). A single copy of OsRRP occurs in the rice genome. OsRRP contains three introns and an open reading frame encoding 252 amino acids, with the replacement of two histidines involved in the active site of RNase by lysine and tyrosine respectively. OsRRP is preferentially expressed in stems of wild-type rice and is significantly down-regulated in an increased tillering dwarf mutant ext37.

Characterization of RNASET2, the first human member of the Rh/T2/S family of glycoproteins

Ribonucleases are ubiquitous enzymes involved in RNA metabolism and are classified in several families on the basis of their structural, catalytic, and biological properties. Here, we describe characterization of the only human member of the Rh/T2/S family of acid hydrolases so far described, named RNASET2. This protein was previously reported to have an interesting biological function in the control of tumourigenesis and metastatization. We show that RNASET2 is present in multiple forms in human cell lines and mouse tissues, one of which represents the full length, glycosylated and secreted form, while the others are proteolytic products. RNASET2 is endowed with catalytic activity as demonstrated with purified recombinant protein expressed in the Baculovirus Expression Vector System and in a human cell line ectopically expressing various types of constructs. Furthermore, we document for this protein a lysosomal localization as described for other members of the Rh/T2/S family of ribonucleases. The results presented herein represent a further advancement toward the molecular understanding of the tumour suppressive properties of the human RNASET2 protein.

An S-like ribonuclease gene is used to generate a trap-leaf enzyme in the carnivorous plantDrosera adelae

Carnivorous plants usually grow in nutrient-deficient habitats, and thus they partly depend on insects for nitrogen and phosphate needed for amino acid and nucleotide synthesis. We report that a sticky digestive liquid from a sundew, Drosera adelae, contains an abundant amount of an S-like ribonuclease (RNase) that shows high amino acid-sequence similarity to S-like RNases induced by phosphate starvation or wounding in normal plants. By giving leaves an RNase "coat", D. adelae seems to achieve two requirements simultaneously to adapt itself to its specific surroundings: it obtains phosphates from insects, and defends itself against pathogen attack.

Rapid recent radiation of S-RNase lineages in Witheringia solanacea (Solanaceae)

Strong frequency-dependent selection as found in the self-incompatibility loci of flowering plants maintains allelic lineages for extremely long time scales, such that allelic genealogies can shed insight into long-term demographic patterns of species. Effective mutation rate, as well as demographic change such as population bottlenecks, can influence genealogical structure. In addition, loss of functionality at the self-incompatibility locus is likely to affect radiation rates. Partial sequences for 21 S-RNase alleles of the mid-elevation tropical species Witheringia solanacea were obtained in order to compare their substitution rates and genealogy with those of Witheringia maculata and two species in the closely related genus Physalis. Sequences for W. solanacea fell into the three clades within the Solanaceae already identified for the genus. Terminal branch lengths for W. solanacea, scaled to the total depth of its phylogeny, were intermediate between the unusually short terminal branches of W. maculata and those of the two Physalis species. In contrast to the Physalis species, where interspecific dN/dS for closely related alleles exceeded 1.0 to the same degree as did intraspecific dN/dS, in Witheringia only intraspecific comparisons showed an excess of nonsynonymous substitutions, suggesting postspeciation radiation of alleles. Alleles associated with lowered S-RNase production and self-compatibility showed extremely short terminal branches. In summary, it appears that rapid recent diversification of alleles characterizes the Witheringia lineages. In some cases, this rapid diversification can be attributed to relaxed constraints due to breakdown of self-incompatibility.

RNase activity requires formation of disulfide bonds and is regulated by the redox state

The activity of many RNases requires the formation of one or more disulfide bonds which can contribute to their stability. In this study, we show that RNase activity and, to a much lesser extent, nuclease activity, are redox regulated. Intracellular RNase activity was altered in vitro by changes in the glutathione redox state. Moreover, RNase activity was abolished following exposure to reducing agents such as beta-ME or DTT. Following reduction with glutathione (GSH), RNase activity could be fully reactivated with oxidized glutathione (GSSG). In contrast, RNase activity could not be reactivated when reduced with DTT. Decreasing the level of glutathione in vivo in wheat increased RNase activity. Tobacco engineered to have an increased glutathione redox state exhibited substantially lower RNase activity during dark-induced senescence. These results suggest that RNase activity requires the presence of one or more disulfide bonds that are regulated by glutathione and demonstrate for the first time that RNase activity can be altered with an alteration in cellular redox state.

Identification of a non-S RNase, a possible ancestral form of S-RNases, in Prunus

This study identifies and characterizes a basic non-S RNase in the styles with stigmas of sweet cherry (Prunus avium L.), a member of the Rosaceae subfamily Amygdaloideae, which has an RNase-based gametophytic self-incompatibility system. Internal sequences of putative non-S RNases (RNase PA1 and PA2) were determined, and a cDNA for PA1 was obtained. The deduced amino acid sequence of PA1 contained two conserved sequence motifs essential for T2/ S-type RNase activity. PA1 shows 20-30% sequence identity to S-RNases of Rosaceae, Solanaceae and Scrophulariaceae, and non-S RNases of higher plants. Transcription of the PA1 gene was specific to the styles with stigmas, and the gene was not expressed in other tissues. Although PA1 resembles RNase X2, a non-S RNase from Petunia inflata, the placement of PA1 and RNase X2 in the phylogenetic tree was quite different. Placement of PA1 was also distinct from that of rosaceous S-RNases, while RNase X2 was incorporated in the clade of S-RNases from the Solanaceae. The sole intron in the PA1 gene is located at a position equivalent to that of the second intron of amygdaloid S-RNase genes, and that of the only intron in most other S-RNase genes. Genomic Southern analysis revealed the presence of sequences homologous to PA1 in all of the other four Prunus species tested, suggesting that PA1 has an important physiological function. The significance of the discovery of PA1 is discussed in terms of the origin and evolution of S-RNases and self-incompatibility in Rosaceae.

Nylon filter arrays reveal differential gene expression in proteoid roots of white lupin in response to phosphorus deficiency

White lupin (Lupinus albus) adapts to phosphorus deficiency (-P) by the development of short, densely clustered lateral roots called proteoid (or cluster) roots. In an effort to better understand the molecular events mediating these adaptive responses, we have isolated and sequenced 2,102 expressed sequence tags (ESTs) from cDNA libraries prepared with RNA isolated at different stages of proteoid root development. Determination of overlapping regions revealed 322 contigs (redundant copy transcripts) and 1,126 singletons (single-copy transcripts) that compile to a total of 1,448 unique genes (unigenes). Nylon filter arrays with these 2,102 ESTs from proteoid roots were performed to evaluate global aspects of gene expression in response to -P stress. ESTs differentially expressed in P-deficient proteoid roots compared with +P and -P normal roots include genes involved in carbon metabolism, secondary metabolism, P scavenging and remobilization, plant hormone metabolism, and signal transduction.

AhSL28, a senescence- and phosphate starvation-induced S-like RNase gene in Antirrhinum

Several species of higher plants have been found to contain S-like ribonucleases (RNases), which are homologous to S-RNases controlling self-incompatibility. No S-like RNase genes have been isolated from self-incompatible Antirrhinum. To investigate the relationship between S- and S-like RNases, we cloned a gene named AhSL28 encoding an S-like RNase in Antirrhinum. Amino acid sequence, genomic structure and phylogenetic analyses indicated that AhSL28 is most similar to RNS2, an S-like RNase from Arabidopsis thaliana and formed a distinct subclass together with several other S-like RNases within the S-RNase superfamily. Unlike S-RNase genes in Antirrhinum, AhSL28 is not only expressed in pistils but also in leaves, petals, sepals and anthers, in particular, showing a strong expression in vascular tissues and transmitting track. Moreover, its RNA transcripts were induced during leaf senescence and phosphate (Pi) starvation but not by wounding, indicating that AhSL28 plays a role in remobilizing Pi and other nutrients, particularly when cells senesce and are under limited Pi conditions in Antirrhinum. Possible evolutionary relations of S- and S-like RNases as well as signal transduction pathways related to S-like RNase action are discussed.

A tobacco S-like RNase inhibits hyphal elongation of plant pathogens

Ribonuclease (RNase) NE gene expression is induced in tobacco leaves in response to Phytophthora parasitica. Using antibodies directed against RNase NE, we demonstrate that RNase NE is extracellular at the early steps of the interaction, while the fungal tip growth is initiated in the apoplastic compartment. After production in Pichia pastoris and biochemical purification, we show that the S-like RNase NE inhibits hyphal growth from P. parasitica zoospores and from Fusarium oxysporum conidia in vitro. Conversion into an enzymatically inactive form after mutagenesis of the active site-histidine 97 residue to phenylalanine leads to the suppression of this activity, suggesting that RNase NE inhibits the elongation of germ tubes by degradation of microbial RNAs. Exogenous application of RNase NE in the extracellular space of leaves inhibits the development of P. parasitica. Based on its induction by inoculation, its localization, and its activity against two plant pathogens, we propose that RNase NE participates in tobacco defense mechanisms by a direct action on hyphal development in the extracellular space. The RNase activity-dependent antimicrobial activity of the S-like RNase NE shares similarities with the only other biological activity demonstrated for plant RNases, the inhibition of elongation of pollen tubes by the S-RNase in gametophytic self-incompatibility, suggesting a functional link between self and nonself interactions in plants.

Molecular variation at the self-incompatibility locus in natural populations of the genera Antirrhinum and Misopates

The self-incompatibility system of flowering plants is a classic example of extreme allelic polymorphism maintained by frequency-dependent selection. We used primers designed from three published Antirrhinum hispanicum S-allele sequences in PCR reactions with genomic DNA of plants sampled from natural populations of Antirrhinum and Misopates species. Not surprisingly, given the polymorphism of S-alleles, only a minority of individuals yielded PCR products of the expected size. These yielded 35 genomic sequences, of nine different sequence types of which eight are highly similar to the A. hispanicum S-allele sequences, and one to a very similar unpublished Antirrhinum S-like RNase sequence. The sequence types are well separated from the S-RNase sequences from Solanaceae and Rosaceae, and also from most known "S-like" RNase sequences (which encode proteins not involved in self-incompatibility). An association with incompatibility types has so far been established for only one of the putative S-alleles, but we describe evidence that the other sequences are also S-alleles. Variability in these sequences follows the pattern of conserved and hypervariable regions seen in other S-RNases, but no regions have higher replacement than silent diversity, unlike the results in some other species.

Molecular cloning of cDNAs encoding ribonuclease-related proteins in Nicotiana glutinosa leaves, as induced in response to wounding or to TMV-infection

We earlier isolated a cDNA clone (NGR1) encoding a wound-inducible ribonuclease (RNase NW) from leaves of Nicotiana glutinosa [Kariu et al. Biosci. Biotechnol. Biochem., 62, 1144-1151 (1998)]. In this study, two distinct cDNA clones, NGR2 and NGR3, encoding proteins with a ribonuclease-related sequence in the N. glutinosa leaves, were amplified and sequenced. The nucleotide sequences of NGR2 and NGR3 consist of 1244 bp and 1069 bp, and have open reading frames encoding 277 (RNase NGR2) and 236 (RNase NGR3) amino acid residues, respectively. The deduced amino acid sequences of the putative RNases NGR2 and NGR3 showed 33% and 58% amino acid sequence identity, respectively, with that of RNase NW and 32% identity with each other. Sequence comparison showed that NGR2 is similar to RNase RNS2 (61%) from Arabidopsis thaliana, while NGR3 is related to RNase LX (84%) from tomato (Lycopersicon esculentum). RNA gel blot analysis showed that the RNase NGR2 gene is constitutively expressed to measurable levels; it is not increased by either wounding or TMV infection. In contrast, the expression of the NGR3 gene is induced after 48 h upon TMV infection.

Molecular control of acid phosphatase secretion into the rhizosphere of proteoid roots from phosphorus-stressed white lupin

White lupin (Lupinus albus) grown under P deficiency displays a suite of highly coordinated adaptive responses. Included among these is secretion of copious amounts of acid phosphatase (APase). Although numerous reports document that plants secrete APases in response to P deficiency, little is known of the biochemical and molecular events involved in this process. Here we characterize the secreted APase protein, cDNA, and gene from white lupin. The secreted APase enzyme is a glycoprotein with broad substrate specificity. It is synthesized as a preprotein with a deduced M(r) of 52,000 containing a 31-amino acid presequence. Analysis of the presequence predicts that the protein is targeted to outside the cell. The processed protein has a predicted M(r) of 49,000 but migrates as a protein with M(r) of 70,000 on sodium dodecyl sulfate gels. This is likely due to glycosylation. Enhanced expression is fairly specific to proteoid roots of P-stressed plants and involves enhanced synthesis of both enzyme protein and mRNA. Secreted APase appears to be encoded by a single gene containing seven exons interrupted by six introns. The 5'-upstream putative promoter of the white lupin-secreted APase contains a 50-base pair region having 72% identity to an Arabidopsis APase promoter that is responsive to P deficiency. The white lupin-secreted APase promoter and targeting sequence may be useful tools for genetically engineering important proteins from plant roots.

A senescence-associated S-like RNase in the multicellular green alga Volvox carteri

Asexual individuals of the green alga Volvox carteri consist of only two cell types: somatic and reproductive cells. The somatic cells are terminally differentiated, post-mitotic cells which undergo gradual senescence leading to cell death in every generation. To elucidate the self-degrading process of macromolecules associated with senescence, we attempted to clone an RNase whose mRNA accumulation is increased during senescence. The corresponding cDNA clone VRN1, encoding an S-like RNase of V. carteri, is the first T(2)/S-like RNase to be cloned from green algae. Semi-quantitative RT-PCR analysis revealed that a relative amount of VRN1 mRNA is more than three-fold higher in the senescent somatic cells than in young somatic cells when the mRNA of ribosomal protein S18 is used as an internal standard. VRN1 mRNA is not induced by phosphate starvation, indicating that its accumulation during senescence is not due to a self-induced defect in utilizing phosphates. Similar regulation has been reported for RNS3, which encodes the S-like RNase that is induced in senescent leaves of Arabidopsis thaliana. These observations imply that VRN1 may promote RNA degradation during senescence of somatic cells in V. carteri, and that its regulation has similarity with that of certain senescence-associated RNases in higher plants.

Amino acid sequence and characterization of a rice bran ribonuclease

A base-nonspecific and acid ribonuclease (RNase Os) belonging to the RNase T2 family was purified from rice bran to a homogeneous state by SDS-PAGE. The primary structure of RNase Os was determined by protein chemistry and molecular cloning. The RNase Os was a simple protein and consisted of 205 amino acid residues. Its molecular weight was 22578 and its amino acid sequence showed that it was most similar to barley RNase among the known RNase T2 family enzymes having 157 amino acid residues identical with barley RNase. However, its N-terminus was blocked by a gamma-pyroglutamyl residue. The optimal pH of RNase Os was around 5.5. The base preference at the B1 and B2 site of RNase Os was estimated from the rates of hydrolysis of 16 dinucleoside phosphates, to be guanine as the case of RNase LE from tomato. RNase Os was successfully expressed from yeast cells using the E. coli yeast expression vector pYE-RNAP.

Overexpression, biophysical characterization, and crystallization of ribonuclease I from Escherichia coli, a broad-specificity enzyme in the RNase T2 family

We have constructed a strain that overproduces ribonuclease I of Escherichia coli and we have purified large quantities of the enzyme. Data from fluorescence, CD, and DSC measurements showed that it was a very stable protein. The conformation energy determined from urea and guanidine hydrochloride denaturation experiments was 11.5 kcal mol(-1) at pH 7.5. Thermal denaturation studies indicated that it had a T(m) of 64 degrees C at pH 4.0. RNase I belongs to the RNase T2/S-RNase group of endoribonucleases, but near the amino terminus it has an unusually long hydrophilic segment. Part of this was removed in the deletion construct, RNase I Delta(26-38). We have obtained crystals of both RNase I and of an enzyme-G2'p5'G complex in the P2(1) space group and oligonucleotide complexes with both wild type and mutant enzymes. The current study lays the groundwork for extensive investigation into the structure, function, and physical properties of this widely distributed group of ribonucleases.

Partial purification and characterization of ribonucleases from roots, stem and leaves of cowpea

Partial purification and characterization of ribonucleases (RNase; EC 3.1.27.1) present in roots, stem and leaves of 5 day-old Pitiúba cowpea [Vigna unguiculata (L.) Walp.] seedlings are described. Crude extracts from the different tissues were precipitated with ammonium sulfate followed by ionic exchange chromatography (CM-Cellulose) resulting in purification factors of 48-fold for roots, 21 for stem and 42 for leaves. No deoxyribonuclease activity was practically observed. The molecular masses of the RNases did not significantly differ, averaging 16.3 kDa. Leaf RNase was stable up to 50ºC while the others were inactivated at this temperature. The maximal inactivation for both stem and roots RNases was reached at 70ºC while for leaf it occurred at 80ºC. The addition of KCl to the assay medium caused a shift of optimal pH from 6.0 toward the range of 5.2 - 5.6 for the enzymes extracted from the different tissues. RNase activities were strongly inhibited by Hg2+, Zn2+ and Cu2+, partially inhibited by Co2+ and Fe2+ and were not affected by EDTA, Ca2+ or Mg2+. In contrast to the leaf RNase, roots and stem enzymes were inactivated by urea and 2-mercaptoethanol (2-ME). Although there is a great similarity among the enzymes studied, leaf RNase appears to be more stable to heat and to chemical denaturation than root and stem RNases. The results also suggest that the enzymes extracted from different tissues of Pitiúba cowpea seedlings are ribonucleases and not nucleases.

Induction of an extracellular cyclic nucleotide phosphodiesterase as an accessory ribonucleolytic activity during phosphate starvation of cultured tomato cells

During growth under conditions of phosphate limitation, suspension-cultured cells of tomato (Lycopersicon esculentum Mill.) secrete phosphodiesterase activity in a similar fashion to phosphate starvation-inducible ribonuclease (RNase LE), a cyclizing endoribonuclease that generates 2':3'-cyclic nucleoside monophosphates (NMP) as its major monomeric products (T. Nürnberger, S. Abel, W. Jost, K. Glund [1990] Plant Physiol 92: 970-976). Tomato extracellular phosphodiesterase was purified to homogeneity from the spent culture medium of phosphate-starved cells and was characterized as a cyclic nucleotide phosphodiesterase. The purified enzyme has a molecular mass of 70 kD, a pH optimum of 6.2, and an isoelectric point of 8.1. The phosphodiesterase preparation is free of any detectable deoxyribonuclease, ribonuclease, and nucleotidase activity. Tomato extracellular phosphodiesterase is insensitive to EDTA and hydrolyzes with no apparent base specificity 2':3'-cyclic NMP to 3'-NMP and the 3':5'-cyclic isomers to a mixture of 3'-NMP and 5'-NMP. Specific activities of the enzyme are 2-fold higher for 2':3'-cyclic NMP than for 3':5'-cyclic isomers. Analysis of monomeric products of sequential RNA hydrolysis with purified RNase LE, purified extracellular phosphodiesterase, and cleared -Pi culture medium as a source of 3'-nucleotidase activity indicates that cyclic nucleotide phosphodiesterase functions as an accessory ribonucleolytic activity that effectively hydrolyzes primary products of RNase LE to substrates for phosphate-starvation-inducible phosphomonoesterases. Biosynthetical labeling of cyclic nucleotide phopshodiesterase upon phosphate starvation suggests de novo synthesis and secretion of a set of nucleolytic enzymes for scavenging phosphate from extracellular RNA substrates.

Isolation and characterisation of the cDNA encoding a glycosylated accessory protein of pea chloroplast DNA polymerase

The cDNA encoding p43, a DNA binding protein from pea chloroplasts (ct) that binds to cognate DNA polymerase and stimulates the polymerase activity, has been cloned and characterised. The characteristic sequence motifs of hydroxyproline-rich glyco-proteins (HRGP) are present in the cDNA corres-ponding to the N-terminal domain of the mature p43. The protein was found to be highly O-arabinosylated. Chemically deglycosylated p43 (i.e. p29) retains its binding to both DNA and pea ct-DNA polymerase but fails to stimulate the DNA polymerase activity. The mature p43 is synthesised as a pre-p43 protein containing a 59 amino acid long transit peptide which undergoes stromal cleavage as evidenced from the post-translational in vitro import of the precursor protein into the isolated intact pea chloroplasts. Surprisingly, p43 is found only in pea chloroplasts. The unique features present in the cloned cDNA indicate that p43 is a novel member of the HRGP family of proteins. Besides p43, no other DNA-polymerase accessory protein with O-glycosylation has been reported yet.

Chlamydomonas reinhardtii mutants abnormal in their responses to phosphorus deprivation

P-starved plants scavenge inorganic phosphate (Pi) by developing elevated rates of Pi uptake, synthesizing extracellular phosphatases, and secreting organic acids. To elucidate mechanisms controlling these acclimation responses in photosynthetic organisms, we characterized the responses of the green alga Chlamydomonas reinhardtii to P starvation and developed screens for isolating mutants (designated psr [phosphorus-stress response]) abnormal in their responses to environmental levels of Pi. The psr1-1 mutant was identified in a selection for cells that survived exposure to high concentrations of radioactive Pi. psr1-2 and psr2 were isolated as strains with aberrant levels of extracellular phosphatase activity during P-deficient or nutrient-replete growth. The psr1-1 and psr1-2 mutants were phenotypically similar, and the lesions in these strains were recessive and allelic. They exhibited no increase in extracellular phosphatase activity or Pi uptake upon starvation. Furthermore, when placed in medium devoid of P, the psr1 strains lost photosynthetic O2 evolution and stopped growing more rapidly than wild-type cells; they may not be as efficient as wild-type cells at scavenging/accessing P stores. In contrast, psr2 showed elevated extracellular phosphatase activity during growth in nutrient-replete medium, and the mutation was dominant. The mutant phenotypes and the roles of Psr1 and Psr2 in P-limitation responses are discussed.

PHOSPHATE ACQUISITION

Phosphorus is one of the major plant nutrients that is least available in the soil. Consequently, plants have developed numerous morphological, physiological, biochemical, and molecular adaptations to acquire phosphate (Pi). Enhanced ability to acquire Pi and altered gene expression are the hallmarks of plant adaptation to Pi deficiency. The intricate mechanisms involved in maintaining Pi homeostasis reflect the complexity of Pi acquisition and translocation in plants. Recent discoveries of multiple Pi transporters have opened up opportunities to study the molecular basis of Pi acquisition by plants. An increasing number of genes are now known to be activated under Pi starvation. Some of these genes may be involved in Pi acquisition, transfer, and signal transduction during Pi stress. This review provides an overview of plant adaptations leading to enhanced Pi acquisition, with special emphasis on recent developments in the molecular biology of Pi acquisition.

Cloning and characterization of a gibberellin-induced RNase expressed in barley aleurone cells

We cloned a cDNA for a gibberellin-induced ribonuclease (RNase) expressed in barley (Hordeum vulgare) aleurone and the gene for a second barley RNase expressed in leaf tissue. The protein encoded by the cDNA is unique among RNases described to date in that it contains a novel 23-amino acid insert between the C2 and C3 conserved sequences. Expression of the recombinant protein in tobacco (Nicotiana tabacum) suspension-cultured protoplasts gave an active RNase of the expected size, confirming the enzymatic activity of the protein. Analyses of hormone regulation of expression of mRNA for the aleurone RNase revealed that, like the pattern for alpha-amylase, mRNA levels increased in the presence of gibberellic acid, and its antagonist abscisic acid prevented this effect. Quantitative studies at early times demonstrated that cycloheximide treatment of aleurone layers increased mRNA levels 4-fold, whereas a combination of gibberellin plus cycloheximide treatment was required to increase alpha-amylase mRNA levels to the same extent. These results are consistent with loss of repression as an initial effect of gibberellic acid on transcription of those genes, although the regulatory pathways for the two genes may differ.

Structure-function relationships of acid ribonucleases: lysosomal, vacuolar, and periplasmic enzymes

It is surprising that only relatively recently has attention been directed to the characterization of the properties of acid ribonucleases (RNases), leading to some understanding of their biochemistry and their functional roles. The present review summarizes current progress in this field under the following general topics: (1) the wide distribution of acid RNases in organisms from viruses to animals; (2) recent findings concerning their primary and three-dimensional structure; (3) the structure-function relationship of acid RNases, with a fungal RNase from Rhizopus niveus as a model enzyme; (4) the unique localization of acid RNases in the periplasm of bacteria, vacuoles in plants, and lysosomes of animals and protozoa; and (5) the diversity of physiological roles, depending on the organism, such as self-incompatibility factors and defense proteins in some plants, the surface protein of an animal virus related to pathogenicity, and possible relationship to human cancer.

Regulation of S-like ribonuclease levels in Arabidopsis. Antisense inhibition of RNS1 or RNS2 elevates anthocyanin accumulation

The S-like ribonucleases (RNases) RNS1 and RNS2 of Arabidopsis are members of the widespread T2 ribonuclease family, whose members also include the S-RNases, involved in gametophytic self-incompatibility in plants. Both RNS1 and RNS2 mRNAs have been shown previously to be induced by inorganic phosphate (Pi) starvation. In our study we examined this regulation at the protein level and determined the effects of diminishing RNS1 and RNS2 expression using antisense techniques. The Pi-starvation control of RNS1 and RNS2 was confirmed using antibodies specific for each protein. These specific antibodies also demonstrated that RNS1 is secreted, whereas RNS2 is intracellular. By introducing antisense constructs, mRNA accumulation was inhibited by up to 90% for RNS1 and up to 65% for RNS2. These plants contained abnormally high levels of anthocyanins, the production of which is often associated with several forms of stress, including Pi starvation. This effect demonstrates that diminishing the amounts of either RNS1 or RNS2 leads to effects that cannot be compensated for by the actions of other RNases, even though Arabidopsis contains a large number of different RNase activities. These results, together with the differential localization of the proteins, imply that RNS1 and RNS2 have distinct functions in the plant.

Characterization of the Mt4 gene from Medicago truncatula

Previously we identified Mt4, a phosphate starvation inducible cDNA from Medicago truncatula which is down-regulated in roots in response to phosphate fertilization as well as colonization by arbuscular mycorrhizal (AM) fungi (AM). Here we present further studies of Mt4. Expression was highly sensitive to exogenous applications of phosphate fertilizer; transcripts were abundant in roots fertilized with nutrient solution lacking phosphate, reduced when fertilized with 0.02 or 0.1 mM phosphate and undetectable when fertilized with 1 or 5 mM phosphate. A time course experiment, to study the expression of Mt4 following colonization by AM fungi, revealed that Mt4 transcripts increased in uncolonized roots during the first three weeks of growth and then plateaued, while transcript levels in roots colonized with the AM fungus, Glomas versiforme, increased transiently and then decreased. Although the Mt4 gene is expressed exclusively in roots, transcripts were also detected in M. truncatula cell suspension cultures following phosphate starvation. A genomic clone containing the Mt4 gene and 1133 bp of the 5' flanking sequence was identified from a M. truncatula genomic library. The promoter region contains a conserved cis-element found in the promoters of phosphate starvation inducible genes of yeast and tomato. As Mt4 is the first cDNA reported to show independent regulation by both phosphate and mycorrhizal fungi, the genomic clone may provide a starting point from which to analyze the signal transduction pathways involved in these two processes.