Abstract
LAZY proteins function early in the process of orienting growth of land plants with respect to the gravity vector (gravitropism). In Arabidopsis, an oppositely oriented and LAZY-independent form of gravitropism causes the inflorescence stems of a lazy quadruple mutant (atlazy1;2;3;4) to grow down and along the surface of the soil. Here, we report on a suppressor mutation that restores an upright inflorescence to a plant lacking LAZY functions. The suppressor of lazy quadruple 1 (slq1) mutation alters one amino acid in a protein belonging to the NPH3/RPT2-like (NRL) family. The wild-type SLQ1 protein restores prostrate growth to the atlazy1;2;3;4 mutant when expressed specifically in the gravity-sensing endodermal cells of the stem. The SLQ1 protein interacts head-to-tail with itself and, regardless of direction, with a homologous protein called SETH6. Distinct subcellular bodies apparently containing concentrated head-to-tail SLQ1 oligomers coated with SETH6 occurred primarily at sites of contact between the ER and the plasma membrane (PM), including plasmodesmata. The suppressing slq1 mutation (S149F) prevented these bodies from forming and reinverted the auxin gradient in the atlazy1;2;3;4 inflorescence, switching its gravitropism from downward to upward. Thus, SLQ1 and at least one homolog (SETH6) function at ER-PM contact sites to counter the LAZY-dependent mechanism, not by inhibiting it but by regulating an apparently LAZY-independent gravity-directed process for creating an auxin gradient. Variation in the relative strengths of a LAZY-dependent and a SLQ1-mediated, LAZY-independent process of opposite effect may produce the great variety of stem organ postures observed in nature.