Belowground plant development measured with magnetic resonance imaging (MRI): exploiting the potential for non-invasive trait quantification using sugar beet as a proxy

Abstract

Both structural and functional properties of belowground plant organs are critical for the development and yield of plants but, compared to the shoot, much more difficult to observe due to soil opacity. Many processes concerning the belowground plant performance are not fully understood, in particular spatial and temporal dynamics and their interrelation with environmental factors. We used Magnetic Resonance Imaging (MRI) as a noninvasive method to evaluate which traits can be measured when a complex plant organ is monitored in-vivo while growing in the soil. We chose sugar beet (Beta vulgaris ssp. vulgaris) as a model system. The beet consists mainly of root tissues, is rather complex regarding tissue structure and responses to environmental factors, and thereby a good object to test the applicability of MRI for 3D phenotyping approaches. Over a time period of up to 3 months, traits such as beet morphology or anatomy were followed in the soil and the effect of differently sized pots on beet fresh weight calculated from MRI data was studied. There was a clear positive correlation between the pot size and the increase in fresh weight of a sugar beet over time. Since knowledge of the development of internal beet structures with several concentric cambia, vascular and parenchyma rings is still limited, we consecutively acquired 3D volumetric images on individual plants using the MRI contrast parameter T2 to map the development of rings at the tissue level. This demonstrates that MRI provides versatile protocols to non-invasively measure plant traits in the soil. It opens new avenues to investigate belowground plant performance under adverse environmental conditions such as drought, nutrient shortage, or soil compaction to seek for traits of belowground organs making plants more resilient to stress.

Metabolic architecture of the cereal grain and its relevance to maximize carbon use efficiency

“…In living plant tissue containing vacuolated cells, the majority of the water present is contained in the vacuoles, and as a result, T2 images are dominated by vacuolar water (Van As et al, 2009). Thus, increases over time in T2 probably reflect increases in vacuolar size (Metzner et al, 2014). Here, light-induced changes in T2 were shown to occur within the crease region (Fig.…”

Quantitative 3D Analysis of Plant Roots Growing in Soil Using Magnetic Resonance Imaging

“…With MRI it is possible to monitor growth over even a much longer time (e.g. 3 months) as already shown for the development of sugar beets (Metzner et al, 2014), which is a great strength of MRI. the fact that signal decay of water in our soil and at 4.7 T is much faster than that of the water in the root (relaxation time T 2,soil ,, T 2,root ).…”

“…The size and anatomy of below-ground storage organs as, for example, sugar beet ( Fig. 1) can also be addressed and the application of this was already described (Metzner et al, 2014;Schmittgen et al, 2015). However, relatively little is known about the root system of the sugar beet plant below the storage organ, and the demonstration that MRI is applicable even for large pots (compare with Fig.…”

“…This also applies to below ground storage organs such as sugar beets (Metzner et al, 2014). Figure 1E shows a magnetic resonance image of an 80-d-old sugar beet (Beta vulgaris) grown in a container with 117mm i.d.…”

“…The basic principles of MRI are described in detail in several textbooks (Callaghan, 1993;Haacke et al, 1999) or review articles (Köckenberger et al, 2004;Blümler et al, 2009;van As et al, 2009;Borisjuk et al, 2012). Research applications to plant roots range from phytopathology (Hillnhütter et al, 2012), across storage root internal structures (Metzner et al, 2014) and water uptake modeling (Stingaciu et al, 2013), to coregistration with positron emission tomography for investigating structure-function relations . Water mobility in roots and soil has also been shown to be detectable with MRI (MacFall and Johnson, 2012;Gruwel, 2014).…”