Improving phosphate use efficiency in the aquatic crop watercress (Nasturtium officinale)

Watercress is a nutrient-dense leafy green crop, traditionally grown in aquatic outdoor systems and increasingly seen as well-suited for indoor hydroponic systems. However, there is concern that this crop has a detrimental impact on the environment through direct phosphate additions causing environmental pollution. Phosphate-based fertilisers are supplied to enhance crop yield, but their use may contribute to eutrophication of waterways downstream of traditional watercress farms. One option is to develop a more phosphate use efficient (PUE) crop. This review identifies the key traits for this aquatic crop (the ideotype), for future selection, marker development and breeding. Traits identified as important for PUE are (i) increased root surface area through prolific root branching and adventitious root formation, (ii) aerenchyma formation and root hair growth. Functional genomic traits for improved PUE are (iii) efficacious phosphate remobilisation and scavenging strategies and (iv) the use of alternative metabolic pathways. Key genomic targets for this aquatic crop are identified as: PHT phosphate transporter genes, global transcriptional regulators such as those of the SPX family and genes involved in galactolipid and sulfolipid biosynthesis such as MGD2/3, PECP1, PSR2, PLDζ1/2 and SQD2. Breeding for enhanced PUE in watercress will be accelerated by improved molecular genetic resources such as a full reference genome sequence that is currently in development.