Switching the Direction of Stem Gravitropism by Altering Two Amino Acids in AtLAZY1

From germination to flowering, gravity influences plant growth and development. A rice (Oryza sativa) mutant with a distinctly prostrate growth habit led to the discovery of a gene category that participates in the shaping of plant form by gravity. Each so-called LAZY gene includes five short regions of conserved sequence. The importance of each of these regions in the LAZY1 gene of Arabidopsis (Arabidopsis thaliana; AtLAZY1) was tested by mutating each region and measuring how well transgenic expression of the resulting protein variant rescued the large inflorescence branch angle of an atlazy1 mutant. The effect of each alteration on subcellular localization was also determined. Region I was required for AtLAZY1 to reside at the plasma membrane, which is necessary for its function. Mutating region V severely disrupted function without affecting subcellular localization. Regions III and IV could be mutated without large impact on function or localization. Altering region II with two conservative amino acid substitutions (L92A/I94A) had the profound effect of switching shoot gravity responses from negative (upward bending) to positive (downward bending), resulting in a "weeping" inflorescence phenotype. Mechanical weakness of the stem was ruled out as an explanation for the downward bending. Instead, experiments demonstrated that the L92A/I94A change to AtLAZY1 reversed the auxin gradient normally established across stems by the gravity-sensing mechanism. This discovery opens up new avenues for studying how auxin gradients form across organs and new approaches for engineering plant architecture for agronomic and other practical purposes.