Rising CO ₂ levels may induce nutritional deficits (protein, minerals, and vitamins) in the highest rice-consuming countries.

Plant responses to atmospheric carbon dioxide will be of great concern in the future, as carbon dioxide concentrations ([CO₂]) are predicted to continue to rise. Elevated [CO₂] causes increased photosynthesis in plants, which leads to greater production of carbohydrates and biomass. Which organ the extra carbohydrates are allocated to varies between species, but also within species. These carbohydrates are a major energy source for plant growth, but they also act as signaling molecules and have a range of uses beyond being a source of carbon and energy. Currently, there is a lack of information on how the sugar sensing and signaling pathways of plants are affected by the higher content of carbohydrates produced under elevated [CO₂]. Particularly, the sugar signaling pathways of roots are not well understood, along with how they are affected by elevated [CO₂]. At elevated [CO₂], some plants allocate greater amounts of sugars to roots where they are likely to act on gene regulation and therefore modify nutrient uptake and transport. Glucose and sucrose also promote root growth, an effect similar to what occurs under elevated [CO₂]. Sugars also crosstalk with hormones to regulate root growth, but also affect hormone biosynthesis. This review provides an update on the role of sugars as signaling molecules in plant roots and thus explores the currently known functions that may be affected by elevated [CO₂].

Myo-inositol hexaphosphate, also known as phytic acid (PA), is the most abundant storage form of phosphorus in seeds. PA acts as a strong chelator of metal cations to form phytate and is considered an anti-nutrient as it reduces the bioavailability of important micronutrients. Although the major nutrient source for more than one-half of the global population, rice is a poor source of essential micronutrients. Therefore, biofortification and reducing the PA content of rice have arisen as new strategies for increasing micronutrient bioavailability in rice. Furthermore, global climate change effects, particularly rising atmospheric carbon dioxide concentration, are expected to increase the PA content and reduce the concentrations of most of the essential micronutrients in rice grain. Several genes involved in PA biosynthesis have been identified and characterized in rice. Proper understanding of the genes related to PA accumulation during seed development and creating the means to suppress the expression of these genes should provide a foundation for manipulating the PA content in rice grain. Low-PA rice mutants have been developed that have a significantly lower grain PA content, but these mutants also had reduced yields and poor agronomic performance, traits that challenge their effective use in breeding programs. Nevertheless, transgenic technology has been effective in developing low-PA rice without hampering plant growth or seed development. Moreover, manipulating the micronutrient distribution in rice grain, enhancing micronutrient levels and reducing the PA content in endosperm are possible strategies for increasing mineral bioavailability. Therefore, a holistic breeding approach is essential for developing successful low-PA rice lines. In this review, we focus on the key determinants for PA concentration in rice grain and discuss the possible molecular methods and approaches for manipulating the PA content to increase micronutrient bioavailability.

Review Article Improving rice zinc biofortification success rates through genetic and crop management approaches in a changing environment Niluka Nakandalage1, Marc Nicolas2, Robert M Norton3, Naoki Hirotsu4, Paul J Milham5 and Saman Seneweera6* 1Faculty of Veterinary & Agricultural Sciences, University of Melbourne, Creswick, VIC 3363, Australia; Email: nnakandalage@student.unimelb.edu.au 2Faculty of Veterinary & Agricultural Sciences, University of Melbourne, Parkville, VIC3010, Australia; Email: marcen@unimelb.edu.au 3International Plant Nutrition Institute, 54 Florence St, Horsham, Victoria, Australia 4Faculty of Life Sciences, Toyo University, 1-1-1 Izumino, Itakura, Gunma 374-0193, Japan; Email: hirotsu@toyo.jp 5Hawkesbury Institute for the Environment, Western Sydney University, LB 1797 Penrith, NSW 2751, Australia; Email: p.milham@westernsydney.edu.au 6Centre for Crop Health, University of Southern Queensland, Toowoomba Qld 4350, Australia; Email: saman.seneweera@usq.edu.au * Correspondence: Saman Seneweera, Centre for Crop Health University of Southern Queensland Toowoomba Qld 4350 Australia Email: saman.seneweera@usq.edu.au Number of words: 5132 Number of figures: 02 Headings and sub headings of paper Abstract 1. Introduction 2. Zinc and human health on a global perspective 3. Rice as source of Zn 4. Key determinents of grain zinc concentration 4.1. Zn uptake and translocation in rice 4.2. Zn remobilization and loading into the grain 4.3. Phytosiderophores in zinc homeostasis 4.4.Plant available Zn in soil 4.5. Rising [CO2] and grain Zn 4.6. Fertilizers as a grain Zn determinant 4.7. Anti-nutrients and Zn bioavailability 5. Ways to overcome global dietary Zn deficiency 6. Conventional breeding as an effective tool for Zn biofortification 7. Conclusions 8. Future research focus Conflicts of Interest Authors and Contributors Acknowledgments References Though rice is the predominant source of energy and micronutrients for more than half of the world population, it does not provide enough zinc (Zn) to match human nutritional requirements. Moreover, climate change, particularly rising atmospheric carbon dioxide concentration, reduces the grain Zn concentration. Therefore, rice biofortification has been recognized as a key target to increase the grain Zn concentration to address global Zn malnutrition. Major bottlenecks for Zn biofortification in rice are identified as low Zn uptake, transport and loading into the grain; however, environmental and genetic contributions to grain Zn accumulation in rice have not been fully explored.