Chapter 3 - Ionomics-based imaging, localization and quantification of zinc and other micronutrients in rice grains for biofortification research

Abstract

Plant ionomics involves analyzing the total elemental composition of plants to solve biological problems such as Zinc biofortification. For high-throughput ionomic studies, the elemental analysis is performed by bulk analysis techniques such as atomic absorption spectroscopy, inductively coupled plasma optical emission spectroscopy, or inductively coupled plasma mass spectrometry. Recent advanced mapping techniques such as synchrotron-based X-ray fluorescent microscopy (XFM), particle-induced X-ray emission (PIXE) spectroscopy, energy dispersive X-ray fluorescence (XRF), and X-ray imaging, offers the possibility of gaining spatial information about inorganic nutrients alongside their concentration. Though versatile, some difficulties reported in these advanced techniques are the expense of XRF instruments, the poor detection limit for lighter elements, and the need for separate analyses for micronutrients and heavier metal species. PIXE spectroscopy is similar to XRF in its fundamental concepts and is widely preferred over other methods. In PIXE, the superior peak background sensitivity reduces the need for large sample size and has enhanced sensitivity to a broader range of 0.5–5 parts per million which is not seen with XRF. Moreover, PIXE has the advantage over XFM because it can detect lighter elements and enables simultaneous analysis of micronutrients and heavy metals. When used together, these techniques can be used to identify any alterations in the relative repartitioning of mineral elements such as Zn during seed germination and grain development. It is also applied to measure the outcome of biofortification programmes that enrich rice seeds with micronutrients, especially Zn and Iron. In a scenario where each ionomics technique has its advantages and drawbacks, most of the element profiling is conducted with complementary combinations where one method is used for assessment and another method is used for validation. This chapter explains the different ionomics-based elemental mapping techniques alongside new approaches to map micronutrients in rice seeds using Zn-biofortified rice as the primary focus.