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

/pdf/phosphorus-acquisition-and-use-critical-adaptations-by-56im8qzi8m.pdf

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

Summary
The diets of over two-thirds of the world's population lack one or more essential mineral elements. This can be remedied through dietary diversification, mineral supplementation, food fortification, or increasing the concentrations and/or bioavailability of mineral elements in produce (biofortification). This article reviews aspects of soil science, plant physiology and genetics underpinning crop biofortification strategies, as well as agronomic and genetic approaches currently taken to biofortify food crops with the mineral elements most commonly lacking in human diets: iron (Fe), zinc (Zn), copper (Cu), calcium (Ca), magnesium (Mg), iodine (I) and selenium (Se). Two complementary approaches have been successfully adopted to increase the concentrations of bioavailable mineral elements in food crops. First, agronomic approaches optimizing the application of mineral fertilizers and/or improving the solubilization and mobilization of mineral elements in the soil have been implemented. Secondly, crops have been developed with: increased abilities to acquire mineral elements and accumulate them in edible tissues; increased concentrations of ‘promoter’ substances, such as ascorbate, β-carotene and cysteine-rich polypeptides which stimulate the absorption of essential mineral elements by the gut; and reduced concentrations of ‘antinutrients’, such as oxalate, polyphenolics or phytate, which interfere with their absorption. These approaches are addressing mineral malnutrition in humans globally.

/pdf/biofortification-of-crops-with-seven-mineral-elements-often-3g2xc3trnk.pdf

Biofortification of crops with seven mineral elements often lacking in human diets--iron, zinc, copper, calcium, magnesium, selenium and iodine.

As the result of intensive research and breeding efforts over the last 20 years, the yield potential and yield quality of cereals have been greatly improved. Nowadays, yield safety has gained more importance because of the forecasted climatic changes. Drought and high temperature are especially considered as key stress factors with high potential impact on crop yield. Yield safety can only be improved if future breeding attempts will be based on the valuable new knowledge acquired on the processes determining plant development and its responses to stress. Plant stress responses are very complex. Interactions between plant structure, function and the environment need to be investigated at various phases of plant development at the organismal, cellular as well as molecular levels in order to obtain a full picture. The results achieved so far in this field indicate that various plant organs, in a definite hierarchy and in interaction with each other, are involved in determining crop yield under stress. Here we attempt to summarize the currently available information on cereal reproduction under drought and heat stress and to give an outlook towards potential strategies to improve yield safety in cereals.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1365-3040.2007.01727.x

The effect of drought and heat stress on reproductive processes in cereals.

Agricultural crops benefit from resistance to pathogens that endures over years and generations of both pest and crop. Durable disease resistance, which may be partial or complete, can be controlled by several genes. Some of the most devastating fungal pathogens in wheat are leaf rust, stripe rust, and powdery mildew. The wheat gene Lr34 has supported resistance to these pathogens for more than 50 years. Lr34 is now shared by wheat cultivars around the world. Here, we show that the LR34 protein resembles adenosine triphosphate-binding cassette transporters of the pleiotropic drug resistance subfamily. Alleles of Lr34 conferring resistance or susceptibility differ by three genetic polymorphisms. The Lr34 gene, which functions in the adult plant, stimulates senescence-like processes in the flag leaf tips and edges.

https://science.sciencemag.org/content/sci/323/5919/1360.full.pdf

A Putative ABC Transporter Confers Durable Resistance to Multiple Fungal Pathogens in Wheat

https://www.cell.com/trends/plant-science/pdf/S1360-1385(05)00250-5.pdf

Biofortifying crops with essential mineral elements

https://www.cell.com/trends/plant-science/pdf/S1360-1385(01)02222-1.pdf

Engineering crop plants: getting a handle on phosphate

The senescence process in wheat flag leaves was investigated over a time course from ear emergence until 50% yellowing of harvested leaf samples using an in-house fabricated cDNA microarray based on a 9K wheat unigene set. The top 1000 ranked differentially expressed probes were subjected to a cluster analysis and, from these, we selected 140 up-regulated genes with informative annotations. There was a considerable overlap between this list of genes and genes previously observed to be associated with senescence in other species, covering several functional categories involved in the degradation of macromolecules and nutrient remobilization, notably of nitrogen via the metabolism of carboxylic and amino acids. The up-regulation of a number of genes in this metabolism was confirmed by real-time polymerase chain reaction experiments. The data suggest a role for cytosolic/peroxisomal routes in the integration of the degradation of carbohydrates, fatty acids and proteins, leading to the remobilization of nitrogen. Illustrative examples of up-regulated genes comprise cytoplasmic aconitate hydratase and peroxisomal citrate synthase. The data support a protective role of the mitochondria towards oxidative cell damage via the up-regulation of the alternative oxidase, and possibly also involving the up-regulated succinate dehydrogenase. A number of up-regulated regulatory genes were also identified, notably NAC-domain and WRKY transcription factors. These factors have previously been identified as being associated with senescence in other species. The data support the notion that a generic senescence programme exists across monocot and dicot plant species. However, notable differences can also be recognized. We thus found transcriptional up-regulation of the biosynthetic pathway for benzoxazinoids, a group of graminaceous-specific secondary metabolites.

Transcriptome analysis of senescence in the flag leaf of wheat (Triticum aestivum L.)

The Aspergillus niger phytase-encoding gene (phyA) has been constitutively expressed in wheat. Transgenic wheat lines were generated by microprojectile bombardment of immature embryos, using the bar-Bialaphos selection system. The bar and the phyA gene expression were controlled by the maize ubiquitin-1 promoter. To ensure secretion and glycosylation of the microbial phytase, an expression cassette was designed (Ubi-SP-Phy) where an α-amylase signal peptide sequence was inserted between the promoter and the phytase coding region. A similar cassette was constructed without the signal peptide sequence (Ubi-Phy). Five lines of fertile wheat transformed with the Ubi-SP-Phy were generated and two lines with the Ubi-Phy construct. The inheritance of the phyA gene was monitored through three generations. Western blotting of leaf and seed derived protein revealed the presence of an immunoreacting polypeptide of the size expected for the Aspergillus phytase. Up to 25 days after pollination, the heterologous phytase was exclusively present in the pericarp-seed coat-aleurone fraction. Thereafter, it accumulated in the endosperm in amounts exceeding that found in the seed coat and aleurone. The phyA mRNA and derived protein could at no stage be detected in the embryo. The Ubi-SP-Phy transgenic seeds exhibited up to 4-fold increase of phytase activity while up to 56% increase was found in Ubi-Phy plants. It is concluded that a functional Aspergillus phytase can be produced in significant amounts in wheat grains. This may be of relevance for improving the phytate-phosphorus digestibility when wheat grains are used for non-ruminant animal feed.

Generation of transgenic wheat (Triticum aestivum L.) for constitutive accumulation of an Aspergillus phytase.

The objective of this study was to map quantitative trait loci (QTL) for the vernalization response in perennial ryegrass (Lolium perenne L.). The mapping population consisted of 184 F2 genotypes produced from a cross between one genotype of a synthetic perennial ryegrass variety “Veyo” and one genotype from the perennial ryegrass ecotype “Falster”. Veyo and Falster were chosen among four different populations because of their contrasting vernalization requirements. In total, five QTL for the vernalization response, measured as days to heading, were identified and mapped to linkage groups (LG) LG2, LG4, LG6 and LG7. Individually, these QTL explained between 5.4 and 28.0% of the total phenotypic variation. The overall contribution of these five QTL was 80% of the total phenotypic variation. A putative orthologue of Triticum monococcum VRN1 was amplified from genomic DNA from perennial ryegrass. PCR fragments covering the proximal part of the promoter and the 5′ end of the orthologue were subsequently PCR-amplified from both parents of the mapping population and shown to possess 95% DNA sequence identity to VRN1. Several polymorphisms were identified between Veyo and Falster in this fragment of the putative VRN1 orthologue. A CAPS marker, vrn-1, was developed and found to co-segregate with a major QTL on LG4 for the vernalization response. This indicates that the CAPS marker vrn-1 could be located in an orthologous gene of the wheat VRN1.

QTL mapping of vernalization response in perennial ryegrass (Lolium perenne L.) reveals co-location with an orthologue of wheat VRN1

A simple procedure is described for the mechanical isolation of protoplasts of unfertilized and fertilized barley egg cells from dissected ovules. Viable protoplasts were isolated from ~75% of the dissected ovules. Unfertilized protoplasts did not divide, whereas almost all fertilized protoplasts developed into microcalli. These degenerated when grown in medium only. When cocultivated with barley microspores undergoing microspore embryogenesis, the protoplasts of the fertilized egg cells developed into embryo-like structures that gave rise to fully fertile plants. On average, 75% of cocultivated protoplasts of fertilized egg cells developed into embryo-like structures. Fully fertile plants were regenerated from ~50% of the embryo-like structures. The isolation-regeneration techniques may be largely genotype independent, because similar frequencies were obtained in two different barley varieties with very different performance in anther and microspore culture. Protoplasts of unfertilized and fertilized eggs of wheat were isolated by the same procedure, and a fully fertile wheat plant was regenerated by cocultivation with barley microspores.

/pdf/regeneration-of-fertile-barley-plants-from-mechanically-3qccauqsmk.pdf

Preben Bach Holm

Papers

Engineering crop plants: getting a handle on phosphate

Transcriptome analysis of senescence in the flag leaf of wheat (Triticum aestivum L.)

Generation of transgenic wheat (Triticum aestivum L.) for constitutive accumulation of an Aspergillus phytase.

QTL mapping of vernalization response in perennial ryegrass (Lolium perenne L.) reveals co-location with an orthologue of wheat VRN1

Regeneration of Fertile Barley Plants from Mechanically Isolated Protoplasts of the Fertilized Egg Cell.