Dissecting the physiological and molecular mechanisms of zinc loading into the wheat grain

Dissecting the physiological and molecular mechanisms of zinc loading into the wheat grain

Zinc (Zn) is an essential micronutrient for healthy growth, development, and for maintaining optimal health in plants, humans and animals. Inadequate dietary intake of Zn has adverse health impacts and is implicated in nearly 4% of global child mortality and morbidity. A convenient and economical means of overcoming Zn deficiency-related health problems in humans is to consume foods rich in Zn. Wheat (Triticum aestivum L.) is the most cultivated and highly consumed cereal crop globally. Therefore, wheat is one of the sources of daily dietary Zn requirements. However, wheat grain naturally contains a low level of Zn due to the limiting of Zn loading to the grain brought by the physiological and molecular mechanisms in the plant. Lack of understanding of the crucial regulatory mechanisms that govern the grain Zn level and its distribution in various grain compartments is a factor that hampers the progress in developing Zn-enriched wheat cultivars. Although shoot Zn uptake and detectable Zn remobilisation in wheat have been studied, it is unknown how these variables differ among genetically diverse cultivars or under conditions of different soil Zn levels to grain yield with varying Zn levels. Furthermore, despite the knowledge that the crease region of the grain is critically important in Zn loading, not much is known about the mechanism. Therefore, this project broadly aimed to dissect the key physiological and molecular mechanisms regulating wheat grain Zn and its distribution in various grain compartments. To achieve this, variation in the key processes of shoot Zn uptake and detectable Zn remobilisation that may account for the variation in levels of grain Zn were examined. Likewise, key molecular and physiological processes that govern the distribution of Zn along with Fe and Mn within the grain compartments were also dissected.

In the first, a study comparing 36 wheat cultivars showed that there was wide variation in their ability to accumulate grain Zn, grain Fe, grain Mn, grain yield, biomass harvest index (BHI), Zn harvest index (ZnHI), grain protein concentration (GPC), and shoot Zn concentration. BHI and ZnHI regulated the Zn partitioning into the grain, which depends on cultivars. Identification of cultivars with a high level of grain Zn together with high GPC, high grain Fe, high grain Mn, high ZnHI, high BHI and low concentration of Zn in shoots will be important in future research developing nutrient-rich cultivars. There was a positive correlation between Zn and Fe, and between Zn and GPC in the grain, suggesting that grain Zn can be increased while maintaining the other grain quality traits.

In the second study, variations in the key processes of shoot Zn uptake and detectable Zn remobilisation that regulate the levels of grain Zn were examined using genetically diverse wheat cultivars grown in soil with or without added Zn. The study revealed that the contribution of shoot Zn uptake and detectable Zn remobilisation to grain Zn is dependent on the cultivar and soil Zn status. Despite the importance of remobilisation and shoot uptake Zn for grain loading, a considerable amount of Zn remained in the shoot without translocating into the grain, indicating the existence of some mechanisms that limit the grain filling process.

In the third study, the distribution of Zn, its speciation and those of other inorganic nutrients within the grains of PBW343 and Goldmark grown in soil with or without added Zn were studied using synchrotron-based XFM and XANES. Uneven distribution of Zn was observed in the grain: Zn was concentrated in the embryo, aleurone and modified aleurone layers, while the endosperm contained a low concentration of Zn irrespective of the cultivar or soil Zn level. In the embryo, the concentration was higher in PBW343, while in the modified aleurone and aleurone layers, Goldmark had a higher concentration. Addition of Zn to the soil enhanced the concentration of Zn in the crease region of Goldmark. However, there was no proportionate increase in the level of Zn in the endosperm in both cultivars. Zn-phytic acid (Zn-PA) is the main Zn complex in the crease region regardless of cultivar or soil Zn level. The Zn-PA complexing regulates Zn loading into the endosperm because the binding of Zn limits its movement. Moreover, Zn, Fe and Mn in the aleurone and modified aleurone layers were co-localised with P and S, suggesting that complexation occurs with PA and S- containing compounds and govern the movement of micronutrients within the grain. In the embryo, the shoot and root primordium had a high accumulation of Zn, while Fe was concentrated in the scutellum.

The fourth study was conducted to dissect the key molecular and physiological processes that regulate the distribution of Zn together with Fe and Mn within wheat grain compartments of PBW343 and Goldmark. A higher concentration of P in the modified aleurone layer of Goldmark suggested more Zn-PA complexation in this tissue, which could restrict the movement of Zn into the endosperm resulting in a iv higher accumulation of Zn in the modified aleurone layer, compared to PBW343. In addition, a smaller crease region of Goldmark also restricts Zn movement, leading to higher accumulation in these tissues. The transporters TaYSL3, TaYSL5, TaYSL14, TaYSL1A, and TaZIP6 regulate the cellular trafficking of Zn through the crease tissues. The TaYSL6 can use to simultaneously increase Zn, Fe, and Mn in the grain. These findings help to fill some of the existing knowledge gaps relevant to the subject. As such, they would be useful in future research that targets the production of nutrient-rich wheat cultivars.

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