Crop yield depends on efficient allocation of sucrose from leaves to seeds.
In Arabidopsis, phloem loading is mediated by a combination of sucrose flow
regulated by SWEET genes and sucrose uptake by SUT1/SUC2 genes. ZmSUT1 is
essential for carbon allocation in maize, however the role of SWEET genes in
maize phloem loading is not known.

Margaret Bezrutczyk from Heinrich Heine University Düsseldorf in Germany
analyzed the role of SWEETs in maize. Bezrutczyk's team identified three
leaf-expressed SWEET sucrose transporters as key components of apoplasmic
phloem loading in maize (Zea mays). ZmSWEET13 paralogues a, b, and c were
the most highly expressed genes in the leaf vasculature. The team then used
CRISPR-Cas9 to knock-out these three genes.

The knock-out mutants exhibited severely stunted growth. The photosynthesis
of mutants was also impaired and their leaves accumulated high levels of
soluble sugars and starch. Analysis revealed that the knock-out also
downregulated genes associated with photosynthesis and carbohydrate
metabolism. Further analysis indicates that changes in expressions of
ZmSWEET13 genes correlate with other agronomical traits, specifically
flowering time and leaf angle.



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