Visualizing plant metabolism

Abstract

The rationale of CEST is to transfer magnetization from low-concentration metabolites to highly concentrated water molecules using proton exchange, so that the saturation effects of metabolites can be effectively amplified and observed in water. The researchers used CEST to image metabolites in living plants, with a focus on saccharides and amino acids. They first applied CEST to visualize sugars and amino acids in the endosperm of pea seeds using chemical shift imaging (CSI) as a control. Unlike CSI, which measures the low-concentration metabolite signals, CEST measures the larger water signal and thus obtains larger metabolite signals. It also achieves higher image resolution in a much shorter time than CSI. During the mid-stage of pea seed development, the magnetic heterogeneity of the tissues prevented CSI from working, but CEST circumvented this issue and provided more reliable metabolic images. For the embryo cotyledon tissues, which showed homogenous magnetization, CEST and CSI both worked, but the former achieved fivefold higher resolution in a shorter time.

So, CEST outperforms CSI in terms of sensitivity and lower susceptibility to magnetic field heterogeneity. More interestingly, CEST showed a clear capacity to monitor the dynamics of sugars and amino acids in growing barley grains attached to spikes, and was used to produce time-lapse videos. In addition, CEST imaging revealed an early aberration in the grains of the HvSWEET11b-mutant barley line, seen as disturbed allocation of sugars, which causes defective endosperm development and grain abortion in the mutant.