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

Terry Rose, Southern Cross Plant Science/Southern Cross GeoScience

Enhancing internal phosphorususe 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

www.scu.edu.au/scps/

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 (g2 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 soil 500 µg P hydroponic +P hydroponic

Total P Total DM Total P Total DM Total P Total DM

ShtPUE -0.81** -0.65** -0.43** 0.51** -0.30** -0.02

SeedP 0.10 0.07 0.40** 0.59** 0.10 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.



Source: Rose et al. 2013 Frontiers in Plant Science



• 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.

2007 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.











• 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 JuliaKwanho JeongAlicia HiddenRachel Wood

Japan International Research Centre for Agricultural Science

Matthias WissuwaAsako MoriJuan Pariasca-TanakaKatsuhiko Kondo

Africa Rice

Elke VandammeKazuki Saito

IRRI

Tobias Kretschmar

Funding Agencies

Global Rice Science Partnership

Japan Society for the Promotion of Science

Documents

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