Normal root elongation requires arginine produced by argininosuccinate lyase in rice

Plant roots play an important role in the uptake of water and nutrients, structural support and environmental sensing, but the molecular mechanisms involved in root development are poorly understood in rice (Oryza sativa), which is characterized by a dense fibrous root system. Here we report a rice mutant (red1 for root elongation defect 1) with short roots. Morphological and physiological analyses showed that the mutant had a shorter length from the quiescent center (QC) to the starting point of the elongation zone but a similar cell size and number of lateral and crown roots compared with the wild type. Furthermore, the mutant had similar radial structure and nutrient uptake patterns to the wild type. Map-based cloning revealed that the mutant phenotype was caused by a point mutation of a gene encoding an argininosuccinate lyase (ASL), catalyzing the last step of arginine biosynthesis. The OsASL1 gene has two distinct transcripts, OsASL1.1 and OsASL1.2, which result from different transcription start sites, but only OsASL1.1 was able to complement the mutant phenotype. OsASL1.1 was expressed in both the roots and shoots. The protein encoded by OsASL1.1 showed ASL activity in yeast. OsALS1.1 was localized to the plastid. The short root of the mutant was rescued by exogenous addition of arginine, but not by other amino acids. These results indicate that arginine produced by ASL is required for normal root elongation in rice.