Terry Rose, Southern Cross Plant Science/Southern Cross GeoScience Enhancing internal phosphorus use efficiency in crops: concepts and approaches

Internal phosphorus use efficiency Three main ways we examine it: 1.Molecular and physiological responses from P starved model plants (e.g. arabidopsis) 2.Investigate mechanisms in highly P-efficient non-crop species 3.Investigate and attempt to exploit genetic variation within a crop species

Definitions of Internal Phosphorus Use Efficiency Grain yield per unit of P in above-ground tissue (g mg -1 ) - yield formation may be independent of P use - selects against genotypes with low grain yield potential Biomass yield per unit of P in above-ground tissue (g mg -1 ) - inverse of tissue P concentration Critical shoot P concentration for 90% maximum yield (mg g -1 ) - expensive when screening large numbers Shoot biomass/shoot P concentration (g 2 mg -1 )

Problem caused by differential P uptake 29 rice genotypes grown in low P soil for 50 d with four replicates

Can screening at equal P uptake help? low-P soil500 µg P hydroponic+P hydroponic Total PTotal DMTotal PTotal DMTotal PTotal DM ShtPUE-0.81 ** ** ** 0.51 ** ** SeedP ** 0.59 ** 0.10

Screening large numbers in hydroponics GWAS study needed +P controls for two reasons: 1.Can remove any genotypes that grew poorly in +P from any analyses – poor growth at low- P not related to low P but other artefacts 2.Can map GWAS peaks under +P conditions to find loci related to ‘general vigour’ and not specifically related to P

Genome-wide association study (GWAS) for PUE Main QTL on chromosomes 1 (indica) and 11 (aus)

Does internal PUE ranking change with shoot P content? There is no presumption that high internal PUE lines will grow well in the field because they may lack P uptake genes: The aim is to find loci/gene(s) that can be ‘pyramided’ into elite lines in local breeding programs.

What are the consequences of higher internal PUE ? These authors apply ‘The law of conservation of matter’ to nutrient use efficiency.

What are the consequences of higher internal PUE ? Source: Rose et al Frontiers in Plant Science

What are the consequences of higher internal PUE ? Obtained same biomass as the wild-type plants with a quarter of the P content in shoots, while seed yield was not reduced. No seed P concentrations shown

What are the consequences of lower seed P concentrations? Grain quality – milling traits in rice, dough quality in wheat? Human health 1. P deficiency in humans? Unlikely. 2. phytate - Some reports suggest it may have anti-cancer properties, recent review by Kumar et al. (2010) suggests that there is limited evidence for this - Strong evidence for its role in binding micronutrients, so reduction in phytate may be beneficial 3. phospholipids - play a role in grain quality and human health, but may be quite stable (Tong et al. 2014)

What are the consequences of lower seed P concentrations? Seedling germination and vigour Two lines of enquiry have led to the conclusion that reducing seed P is detrimental to seed germination and vigour. 1.Studies with low phytic acid (lpa) mutants - Low-phytic-acid mutants often have impaired germination and vigour BUT this is because whole genes are often knocked out - The only LPA mutant used in breeding programs (Barley lpa1-1; Bregitzer et al Crop Science) has no impact on seedling vigour BUT this mutant has a 10-14% reduction in seed total P due to mutation of a putative sulfate transporter.

Seed germination and seedling vigour 2. Studies with low-P seed from P-starved plants -Half a dozen studies with cereal crops where low-P seed have shown reduced germination and seedling vigour compared to high-P seed -These studies do NOT make fair comparisons because the low-P seed came from P-stressed parent plants -Most studies were not conducted with agricultural soils with a history of P fertilisation, so soils were typically highly P-deficient.

Seed germination and seedling vigour 2. Studies with low-P seed from P-starved plants -Seeds from P-starved plants performed poorly in P-deficient soil but no yield difference in agricultural soil supplied with P fertiliser -Subsequent studies with seed lower in P from environmental effects found no difference in seedling vigour between high- and low-P seed on any soil -Further studies have been conducted and will be the focus of the presentation by Elke Vandamme

High P input farming systems If seedling vigour can be maintained with lower seed P, then perhaps we could breed for lower seed P concentrations regardless of internal PUE at the vegetative stage. Go for high P uptake and low translocation to grains. May be a useful trait in high-input farming systems where the removal of P in grains is significant and drives the need for continual P fertiliser input World phosphate deposits: FAO data Australian deposits are < 1 % of world P resources. Over 70 % of resources are held by China and Morocco.

High P input farming systems

High P input farming systems

High P input farming systems

High P input farming systems

High P input farming systems In an average season in Australia, approximately 60, 000 tonnes of P (the equivalent of over half a million tonnes of super phosphate) is removed off-farm in wheat grain at harvest assuming grain contains 3 mg P/g. Most is exported overseas while some is consumed domestically and contributes to high-P landfill and the pollution of water bodies in Australia. A reduction in grain P concentrations to 2 mg P/g would save about $100 million being removed off farm each year at $5 per kg P.

Current research project: Global Rice Science Partnership (SCU, JIRCAS, IRRI, AfricaRice) project aims to reduce rice grain P by minimum 20% Approaches for breeding crops with low grain phosphorus Two approaches investigated in the project: 1.Exploiting genotypic variation 2.Molecular approach 3.Mutant approach – not investigated but may be an option later

GRiSP project: Exploiting genotypic variation Multi-location trials with 20+ rice genotypes over a number of years to look at G x E interactions for grain P concentration 1. Need to identify a genetic component that is independent of grain yield (yield-dilution effect) 2. Need to make sure low grain P isn’t associated with low plant P uptake! Elke Vandamme will be presenting data on this

GRiSP project: Molecular approach Identify P transporter(s) and use RNAi silencing to reduce gene expression in specific tissue at a specific time Understand regulatory pathway of genes involved in grain P loading and find targets for genetic manipulation

Summary 1.We have investigated internal PUE at the vegetative stage using a method which screens at equal P uptake and have mapped loci for high internal PUE 2.The consequences of high internal PUE will likely be a reduction in grain P concentration 3.Reducing grain P concentration may be a good option by itself, particularly in high-input systems 4.Further work is needed to ensure lower grain P concentration does not adversely affect grain quality or seedling vigour

Acknowledgements Southern Cross University Cecile Julia Kwanho Jeong Alicia Hidden Rachel Wood Japan International Research Centre for Agricultural Science Matthias Wissuwa Asako Mori Juan Pariasca-Tanaka Katsuhiko Kondo Africa Rice Elke Vandamme Kazuki Saito IRRI Tobias Kretschmar Funding Agencies Global Rice Science Partnership Japan Society for the Promotion of Science