Many monocot seedlings rely on elongation of the ‘mesocotyl’ tissue to help the shoot emerge from the soil. The length of this mesocotyl tissue varies between germplasm and is also affected by various environmental factors. We have exploited this variation of mesocotyl length in rice (Oryza sativa) by screening over 600 germplasm from the 3KG population in order and performed a Genome Wide Association Study to understand the genetic basis underlying this trait. Several candidate genes were identified, that may confer the ‘long mesocotyl’ trait in rice seedlings. One of the most interesting candidates located on chromosome 7 is cellulose synthase 3 (CesA3), LOC_Os07g24190, whose expression level also correlates with mesocotyl length. Rice has 11 CesA genes and CesA3 is most similar to Arabidopsis thaliana CesA6. In Arabidopsis, CesA6 encodes a subunit of the cellulose synthase complex which synthesizes a cellulose microfibril in the primary cell wall. Analysis of mesocotyl elongation in multiple independent homozygous overexpressor (OE) and knock-out (KO) CesA3 lines suggests that overexpressing CesA3 increases mesocotyl length while disrupting CesA3 reduces mesocotyl length. We also demonstrate that CesA3 overexpression leads to increased cell elongation while editing CesA3 led to a reduction in the cell length in the mesocotyl. Due to the role of CesA genes in cell wall synthesis, we are currently analysing the monosaccharide composition of the primary and secondary cell wall in the OE and KO CesA3 lines. Furthermore, we are testing whether rice CesA3 can complement the Arabidopsis cesa6 mutant phenotype.