THREONINE SYNTHASE1 is Indispensable for Maintenance of the Root Stem Cell Niche and the Apical Meristem in Arabidopsis Thaliana

Blanca Jazmín Reyes-Hernández ¹ Svetlana Shishkova ¹ Rachel Amir ^2,3 Aranza Xhaly Quintana-Armas ¹ Selene Napsucialy-Mendivil ¹ Rocio Guadalupe Cervantes-Gamez ^1,4 Hector Hugo Torres-Martínez ¹ Jesús Montiel ^1,5 Yamel Ugartechea-Chirino ^1,6 Christopher Wood ⁷ Joseph G. Dubrovsky ¹

¹Departamento de Biología Molecular de Plantas, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), Apartado Postal 510-3, 62250 Cuernavaca, Morelos, Mexico
²Laboratory of Plant Science, MIGAL–Galilee Research Institute, Kiryat Shmona 11016, Israel
³Tel-Hai College, Upper Galilee 11016, Israel
⁴Departamento de Estudios para el Desarrollo Sustentable de Zonas Costeras, Universidad de Guadalajara, Gómez Farías, 82, San Patricio-Melaque, Cihuatlán, Jalisco, 48980, Mexico
⁵Department of Molecular Biology and Genetics, Centre for Carbohydrate Recognition and Signaling, Aarhus University, Aarhus C 8000, Denmark
⁶Laboratorio de Genética Molecular, Epigenética, Desarrollo y Evolución de Plantas, Instituto de Ecología, Universidad Nacional Autónoma de México (UNAM), Ciudad Universitaria, Av. Universidad 3000, Coyoacán, CDMX. 04510, Mexico
⁷Laboratorio Nacional de Microscopía Avanzada, Instituto de Biotecnología, Universidad Nacional Autónoma de México (UNAM), Apartado Postal 510-3, 62250, Cuernavaca, Morelos

Indeterminate root growth depends on the maintenance and activity of the root apical meristem (RAM) and its stem (initial) cells. In search of new regulatory pathways involved in maintenance of the root growth we have identified a mutant (moots koom1 [`short root` in Mayan], mko1) that exhibits complete exhaustion of the primary root meristem resulting in determinate growth. Using Next Generation Secuencing it was identified that a mutation in the THREONINE SYNTHASE1 (TS1) encoding gene was responsible for this phenotype. Similar to the allelic methionine overaccumulator2-1 mutant, mko1 accumulates more methionine than wild type. Nevertheless, we found that non-allelic methionine overaccumulator mutants did not show the RAM exhaustion. Treatment with polyamines and ACC, synthesized from methionine, did not induce the RAM exhaustion either. During the RAM exhaustion the quiescent center (QC) cells start to divide and stem cell activity is abolished. This phenotype was reverted with threonine treatment which is capable of inducing de-novo stem cell niche establishment and recovery of growth. In the root pTS1:TS1-GFP was expressed predominantly in the RAM, presumably in the pro-plastids. In the root cap it was partially co-localized with starch grains of amyloplasts. Analysis of the QC-specific markers, pWOX5::GFP and QC25, and pPLETHORA:CFP and pSCARECROW:H2B-YFP, involved in RAM maintenance, showed that their expression was maintained in the mko1. Auxin response and transport markers were expressed normally and an inhibitor of auxin synthesis, kynurenine, did not abolish the RAM exhaustion in mko1, pointing to independence of the process from auxin. Our results suggest that the THREONINE SYNTHASE1-dependent pathway could be a new regulatory mechanism involved in maintenance of the RAM and that correct amino acid balance is a critical factor for stem cell and RAM activity, and root growth. The work was funded by UNAM-DGAPA-PAPIIT (IN200818, IN201318) and CONACyT (237430, 240055).