Impaired phloem loading in zmsweet13a,b,c sucrose transporter triple knock‐out mutants in Zea mays

Summary Crop yield depends on efficient allocation of sucrose from leaves to seeds. In Arabidopsis, phloem loading is mediated by a combination of SWEET sucrose effluxers and subsequent uptake by SUT 1/ SUC 2 sucrose/H + symporters. Zm SUT 1 is essential for carbon allocation in maize, but the relative contribution to apoplasmic phloem loading and retrieval of sucrose leaking from the translocation path is not known. Here we analysed the contribution of SWEET s to phloem loading in maize. We identified three leaf‐expressed SWEET sucrose transporters as key components of apoplasmic phloem loading in Zea mays L. Zm SWEET 13 paralogues (a, b, c) are among the most highly expressed genes in the leaf vasculature. Genome‐edited triple knock‐out mutants were severely stunted. Photosynthesis of mutants was impaired and leaves accumulated high levels of soluble sugars and starch. RNA ‐seq revealed profound transcriptional deregulation of genes associated with photosynthesis and carbohydrate metabolism. Genome‐wide association study (GWAS) analyses may indicate that variability in Zm SWEET 13s correlates with agronomical traits, especifically flowering time and leaf angle. This work provides support for cooperation of three Zm SWEET 13s with Zm SUT 1 in phloem loading in Z. mays .