SUPPRESSOR OF LAZY QUADRUPLE 1 acts at ER–plasma membrane contact sites to control a gravitropism pathway in the <i>Arabidopsis</i> stem

LAZY proteins function early in the process of orienting growth of land plants with respect to the gravity vector (gravitropism). In <i>Arabidopsis</i>, an oppositely oriented and LAZY-independent form of gravitropism causes the inflorescence stems of a <i>lazy</i> quadruple mutant (<i>atlazy1</i>;<i>2</i>;<i>3</i>;<i>4</i>) 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 <i>suppressor of lazy quadruple 1</i> (<i>slq1</i>) 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 <i>atlazy1</i>;<i>2</i>;<i>3</i>;<i>4</i> 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 <i>slq1</i> mutation (S149F) prevented these bodies from forming and reinverted the auxin gradient in the <i>atlazy1</i>;<i>2</i>;<i>3</i>;<i>4</i> 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.