Rice Genotype Differences in Tolerance of Zinc-Deficient Soils: Evidence for the Importance of Root-Induced Changes in the Rhizosphere

Zinc (Zn) is an essential component of thousands of proteins in plants, although it is toxic in excess. In this review, the dominant fluxes of Zn in the soil-root-shoot continuum are described, including Zn inputs to soils, the plant availability of soluble Zn(2+) at the root surface, and plant uptake and accumulation of Zn. Knowledge of these fluxes can inform agronomic and genetic strategies to address the widespread problem of Zn-limited crop growth. Substantial within-species genetic variation in Zn composition is being used to alleviate human dietary Zn deficiencies through biofortification. Intriguingly, a meta-analysis of data from an extensive literature survey indicates that a small proportion of the genetic variation in shoot Zn concentration can be attributed to evolutionary processes whose effects manifest above the family level. Remarkable insights into the evolutionary potential of plants to respond to elevated soil Zn have recently been made through detailed anatomical, physiological, chemical, genetic and molecular characterizations of the brassicaceous Zn hyperaccumulators Thlaspi caerulescens and Arabidopsis halleri.

The first cases of human Zn deficiency were described in the 1960s in the Middle East. Nevertheless, it was not until 2002 that Zn deficiency was included as a major risk factor in the global burden of disease, and only in 2004 did WHO/UNICEF include Zn supplements in the treatment of acute diarrhoea. Despite this recognition Zn is still not included in the UN micronutrient priority list, an omission that will continue to hinder efforts to reduce child and maternal mortality, combat HIV/AIDS, malaria and other diseases and achieve the UN Millennium Development Goals for improved nutrition in developing countries. Reasons for this omission include a lack of awareness of the importance of Zn in human nutrition, paucity of Zn and phytate food composition values and difficulties in identifying Zn deficiency. Major factors associated with the aetiology of Zn deficiency include dietary inadequacies, disease states inducing excessive losses or impairing utilization and physiological states increasing Zn requirements. To categorize countries according to likely risk of Zn deficiency the International Zinc Nutrition Consultative Group has developed indirect indicators based on the adequacy of Zn in the national food supplies and/or prevalence of childhood growth stunting. For countries identified as at risk confirmation is required through direct measurements of dietary Zn intake and/or serum Zn in a representative sample. Finally, in at risk countries either national or targeted Zn interventions such as supplementation, fortification, dietary diversification or modification, or biofortification should be implemented, where appropriate, by incorporating them into pre-existing micronutrient intervention programmes.

Zinc (Zn) deficiency is the most widespread micronutrient disorder in rice (Oryza sativa), but efforts to develop cultivars with improved tolerance have been hampered by insufficient understanding of genetic factors contributing to tolerance. The objective of this paper was to examine alternative evaluation methods and to identify the most informative traits that would provide realistic information for rice breeders and to map quantitative trait loci (QTLs) associated with tolerance. Screening experiments in low-Zn nutrient solution and in a Zn-deficient field did not produce similar tolerance rankings in a set of segregating lines, which suggested that rhizosphere effects were of greater importance for lowland rice than internal Zn efficiency. The most severe symptom in the field was high plant mortality. The occurrence of leaf bronzing, usually regarded as indicative of susceptibility, did not necessarily concur with high plant mortality, which implied that both were under independent genetic control. The QTL mapping experiment conducted in the field with a population derived from a cross of IR74 (intolerant) with Jalmagna (tolerant) largely confirmed this. Four QTLs associated with plant mortality were detected, and only one of those colocalized with one of the four QTLs detected for leaf bronzing. The two most influential QTLs for plant mortality were detected on chromosomes 2 and 12. They explained 16.6% and 24.2% of the variation, and alleles of the tolerant donor parent Jalmagna reduced mortality by 16.6% and 14.8%, respectively. QTLs for plant mortality acted in a purely additive manner, whereas digenic epistatic interactions were important for leaf bronzing.