The Expression And Phytohormone Regulation Of ARL1 Controlling Adventitious Root Formation In Rice

Root architecture is a key determinant of nutrient and water use efficiency in crops. The primary root of rice plant is short-lived. Therefore, plant acquisition of water and nutrients during the growing period is mainly dependent on adventitious and lateral roots. Compared with Arabidopsis,, the genetic and molecular information on development of the root system of crops is limited. Due to the simplicity of root structure and availability of root-specific mutants, extensive studies have been conducted in the root development of Arabidopsis, However, only a small number of mutants have been found in monocots, and no genes corresponding to these mutants have been identified. A growing number of studies have focused on the characterization of root development in cereals using mutation analysis.. Here we report findings for a novel gene controlling initiation of adventitious root primordia in rice (Oryza sativa L.). The gene designated ARL1 (Adventitious rootless1) encodes a protein with a LATERAL ORGAN BOUNDARIES (LOB) domain.. To identify the fuction of ARL1, We examine the responses of the arl1 mutant to hormones, and analyse the expression of arl1 gene in rice during adventitious root development.l.Both QHB and OsSCR genes were expressed in the pericycle cells of wild-type plants, but not in those of arl1 mutant plants, indicating that initiation of periclinal division of cells to form primordia is impaired in the arl1 mutant.2. Reduction of ARL1 in the wild-type plant using RNA interfering (RNAi) of ARL1 resulted in a defect in adventitious roots, but not in lateral roots.3ARL1 is expressed in lemma tips, lemma vein, pedicel of young panicle, aurical, base of leaf blade, vascular cylinder tissue of primary root and lateral roots, root tip and vascular tissue of anthe.4. Treatments with 1μM α -NAA and 150μM ethephon significantly enhanced the growth of adventitious roots in wild-type seedlings. However, these treatments did not rescue adventitious root formation in the arl1 mutant. Lateral root growth in both wild-type and arl1 mutant seedlings was repressed by both treatments. Similar responses of wild-type and arl1 mutant plants were observed to NPA , an inhibitor of auxin polar transport, and NBD5.ARL1 was remarkably induced by1 μM α -NAA within 30 min, while the induction by ethephon was detectable after 6 hr of the treatment. To confirm the direct effect of auxin and ethephon on the expression of ARL1, an inhibitor of protein synthesis, cycloheximide at 20μM,was used together with a -NAA or ethephon for a time course experiment. The result shown thar the direct response of ARLI to auxin.,but indirect response of ARLI to ethephon.Based on these facts, we propose a hypothesis that ARLI involves in an auxin-mediated cell dedifferentiation and entering initial division through an interaction with other regulatory protein(s) restricted to the pericyle cells or the adjacent peripheral vascular cylinder in stem.