We propose that AtLAZY proteins control plant architecture by coupling gravity sensing to the formation of auxin gradients that override a LAZY-independent mechanism that creates an opposing gravity-induced auxin gradient. A reversed auxin gradient across the root visualized by a fluorescent signaling reporter explained the reversed, upward bending response. In light-grown seedlings, the root of the atlazy2, 3, 4 mutant was also agravitropic but when adapted to dim red light it displayed a reversed gravitropic response. Etiolated hypocotyls of the quadruple atlazy1, 2, 3, 4 mutant were essentially agravitropic, but their phototropic response was robust. Lateral roots of the atlazy2, 4 double mutant emerged slightly upward, approximately 10° greater than perpendicular to the primary root axis, and they were agravitropic. AtLAZY2 and AtLAZY4 determined lateral root branch angle. The weeping inflorescence phenotype of atlazy1, 2, 4 mutants may be due at least in part to a reversal in the gravitropism mechanism. AtLAZY1 is the principal determinant of inflorescence branch angle.
The results identify the major contributors to root and shoot branch angles and gravitropic behavior of seedling hypocotyls and primary roots. Branch angle phenotypes previously associated with single LAZY genes were here studied in roots and shoots of single and higher-order atlazy mutants. Arabidopsis ( Arabidopsis thaliana) possesses six LAZY genes having spatially distinct expression patterns. A rice ( Oryza sativa) mutant led to the discovery of a plant-specific LAZY1 protein that controls the orientation of shoots.
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