Why do cabbage leaves smell? They’re storing sulphur.

Sulphur (S) is an essential macronutrient for seeds, so why do brassicas store it in leaves?

A team of botanists has found that leaves that become sinks for sulphur act as a store and later become a source as seeds become a sink at maturity. “Our evidence for S remobilization from leaves as primary S sink suggests that up- or downregulation of signalling molecules that mediate between secondary S sinks and sources may help modulate economically valuable S compounds in brassica seed.” said Priyakshee Borpatragohain and colleagues in their article in Annals of Botany.

Representation of sources and sinks of S in brassicas adapted from (Borpatragohain et al., 2016). Source and sink glyphs are adapted from the Systems Biology Graphical Notation (SBGN) project (http://sbgn.github.io/sbgn/). Red glyphs indicate primary S sources and sinks and blue glyphs indicate secondary S sources and sinks. Source Borpatragohain et al., 2019.

“Sulphur (S) is an essential macronutrient involved in numerous metabolic pathways required for plant growth, including the synthesis of amino acids, proteins, co-enzymes, vitamins and secondary metabolites such as glucosinolates (GSLs) and sulphoflavonoids,” said the authors. The team wanted to understand how plants were using sulphur and where it was going. “We consider soil as the primary source of S and roots and shoots both as primary and secondary S sources and sinks with overlapping functions. Leaves play a dual role, first as a primary sink and later in development as a secondary source. The model predicts that targeting S assimilation in crop breeding within the context of source–sink relationships could result in modified levels of seed GSL or storage proteins. At present, this approach is hindered by the lack of comprehensive data describing the uptake, distribution and fate of S and S-containing metabolites throughout crop development. Addressing this gap in Brassica crops would allow a better understanding of the distinct S sources and sinks, and thus guide optimized agronomic practice in terms of application of S fertilizer that may affect final seed composition.”

The authors found that the sulphur content of plants peaked at the time of floral initiation. At this stage most of the sulphur was in the leaves. When sulphur moved into the seeds it moved from the leaves. The authors concluded, “We have established that leaf S components that accumulated as primary S sinks at early developmental stages in condiment-type B. juncea become remobilized as a secondary S source to meet the demand for GSL as the dominant seed S sink at maturity… Future studies can now build on the current study to examine N × S interactions in B. juncea.”

Further reading

Borpatragohain, P., Rose, T. J., & King, G. J. (2016). Fire and Brimstone: Molecular Interactions between Sulfur and Glucosinolate Biosynthesis in Model and Crop Brassicaceae. Frontiers in Plant Science, 7. https://doi.org/10.3389/fpls.2016.01735

Borpatragohain, P., Rose, T. J., Liu, L., Barkla, B. J., Raymond, C. A., & King, G. J. (2019). Remobilization and fate of sulphur in mustard. Annals of Botany, 124(3), 471–480. https://doi.org/10.1093/aob/mcz101