Syndrome “basses richesses” disease induced structural deformations and sectorial distribution of photoassimilates in sugar beet taproot revealed by combined MRI-PET imaging

The disease syndrome "basses richesses" (SBR) leads to a significant reduction in sugar beet biomass and sugar content, negatively affecting the sugar economy. The mechanistic understanding regarding growth and photoassimilates distribution within the sugar beet taproot diseased with SBR is currently incomplete. We combined two tomographic methods, magnetic resonance imaging (MRI) and positron emission tomography (PET) using ¹¹C as tracer, to non-invasively determine SBR effects on structural growth and photoassimilates distribution within the developing taproot over six weeks. MRI analysis revealed a deformed cross-sectional anatomical structure from an early stage, as well as a reduction in taproot volume and width of inner cambium ring structures of up to 26 and 24 ​%, respectively. These SBR disease effects were also confirmed by post-harvest analysis of the taproot. PET analysis revealed a heterogeneous distribution of labeled photoassimilates for diseased plants: sectors of the taproot with characteristic SBR symptoms showed little to very low ¹¹C tracer signal. The heterogeneity of SBR disease effects is most likely due to a partial inoculation of leaves leading to an uneven distribution of the SBR pathogen in the taproot through the strong vascular interconnection between shoot and root. Also, the pathogen needs to spread non-uniformly within the taproot to explain the observed marked increase of the SBR disease effects over time. Our results indicate that SBR affects photoassimilates sink capacity at an early stage of taproot development. Co-registration of MRI and PET may support an early judging of susceptibility and selection of promising genotype candidates for future breeding programs.