A method by which many plants control organ shape is by regulated, differential cellular expansion. A gene involved in regulating the expansion of plants cells, such as Arabidopsis thaliana root cells is COBRA which encodes a protein with a putative GPI anchor. Plants comprising altered root morphologies may be produced by control of COBRA activity. For example, roots which lack COBRA activity comprise thicker, fatter roots (a CORE or cob phenotype) which are well suited for penetrating dense, compacted soil. Plants lacking COBRA activity may be useful in applications wherein plant growth in areas with dense soil would be beneficial. The present invention comprises the Arabidopsis thaliana COBRA gene and COBRA protein and homologues thereof as well as mutated alleles of COBRA. Also provided are anti-COBRA antibodies, transgenic plants which overexpress COBRA and methods for identifying COBRA modulating substances.