Research assistant University of Wisconsin-Madison
Gravitropism greatly shapes plant architecture. LAZY genes regulate gravitropism in both shoots and roots. Most LAZY genes share five short regions of conserved sequence, ranging from region I at the N-terminus of the predicted protein to the family-defining region V at the C-terminus. The biochemical function of LAZY proteins is not known or predictable from their sequence. This study concerns the LAZY1 gene of Arabidopsis. The LAZY1 protein resides in the nucleus and at the plasma membrane, where it controls the angle of inflorescence branches. We separately mutated each of the five conserved domains of LAZY1 and evaluated how well each variant rescued the branch angle phenotype of a null lazy1 mutant. We also examined subcellular localization of LAZY1-eGFP. Mutating Region I greatly reduced the amount of LAZY1 at the plasma membrane but not the nuclear pool. This variant could not rescue the branch angle phenotype. Mutating region V disrupted LAZY1 function but did not affect subcellular localization. Mutating regions III and IV affected neither subcellular localization nor the ability to rescue branch angle. However, a conservative change (I92A/L94A) to Region II caused a weeping, downward oriented inflorescence. The mechanical strength of the primary stem was normal, indicating that the downward bending was not due to sagging. Instead, reorientation assays showed downward, or reverse, gravitropism of the shoots. Gravitropism is a growth response to an auxin gradient. The oppositely-bending region II mutants respond normally to auxin. Their reverse gravitropism was explained by the development of a reversed gravity-induced auxin gradient, as shown by measurements of an auxin responsive mRNA (IAA5). We conclude that the I92A/L94A alteration to Arabidopsis LAZY1 switched the direction of stem gravitropism by reversing the auxin flow. These results indicate that modifying the LAZY1 pathway may be a way to tailor plant architecture.
Coauthors: Edgar Spalding – University of Wisconsin