The Role Of Nitric Oxide And Auxin In The Formation Of Cluster Roots In White Lupin(Lupinus Albus L.)

Phosphorus is one of the necessary nutrient elements in plant growth. It is also one of the limitations in global agricultural production, because the effectiveness of P is low in most of the soil. Formation of cluster roots is one of the specific root adaptations to nutrient deficiency. In white lupin(Lupinus albus L.), cluster roots can be induced by phosphorus (P) or iron (Fe) deficiency. Most of the previous studies focused on the associated physiology of cluster roots, but little was known of the mechanism of signaling cascade during the initiation of cluster roots. The present work focuses on the role of NO and IAA in inducing the formation of cluster roots in white lupin(Lupinus albus L.).1. Regulation of nitric oxide in inducing the formation of cluster roots under P-or Fe starvationP-and Fe-deficiency induced cluster roots share the same basic structure and physiology, but there are still some quantitative differences. In order to investigate the potential shared signaling pathway in the P-and Fe-deficiency induced cluster-root formation, we treated the plants with four P x Fe combinations, namely,(1)50μM P and10μM Fe (+P+Fe);(2) nil P and10μM Fe (-P+Fe);(3)50μM P and nil Fe (+P-Fe) and (4) nil P and nil Fe (-P-Fe). We determined the internal concentration of P and Fe, levels of nitric oxide (NO), citrate exudation, and expression of LaSCR genes under the P×Fe combinations, and linked these with the formation of cluster roots. The deficiency of either P or Fe or both increased the cluster-root number and cluster zones. It also enhanced NO accumulation in pericycle cells and rootlet primordia at various stages of cluster-root development. The formation of cluster roots and rootlet primordia, together with the expression of LaSCR1and LaSCR2 which is crucial in cluster-root formation, were induced by exogenous NO donor: S-nitrosoglutathione (GSNO) under the+P+Fe condition, but inhibited by NO-specific endogenous scavenger2-(4-carboxyphenyl)-4,4,5.5-tetramethylimidazoline-l-oxyl-3-oxide (cPTIO) under-P+Fe,+P-Fe and-P-Fe conditions. However, cluster roots induced by an exogenous supply of the NO donor did not secrete citrate unlike those formed under-P or-Fe conditions. In conclusion, NO plays an important role in the shared signaling pathway of the P-and Fe-deficiency-induced formation of cluster roots in white lupin.2.Regulation of auxin in P-deficiency induced formation of cluster rootsExogenous applied NAA promotes the formation of cluster roots. To understand the mechanism of auxin in phosphorus (P)-deficiency-induced formation of cluster roots, we investigated the role of auxin derived from shoot and root, longitudinal distribution patterns of free IAA along the root and the related gene expressions responsible for polar auxin transport (PAT) in different developmental stages of cluster root in white lupin(Lupinus albus L.). We found that the removal of young leaves or primary root apex and the application of auxin influx or efflux transport inhibitors,3-chloro-4-hydroxyphenylacetic acid (CHPAA), N-1-naphthylphthalamic acid (NPA) and2,3,5-triiodobenzoic acid (TIBA) did not affect the number of cluster roots and the free IAA concentration in the roots of P-deficient plants. The concentration of free IAA in the roots was higher in P-deficient plants than in P-adequate ones, and decreased with the aging of cluster roots along the lateral root. The distribution pattern of free IAA was consistent with the expression patterns of LaAUX1, LaPIN1and LaPIN3transcripts related to PAT. It indicated that root-derived auxin contributes to the P-deficiency-induced formation of cluster roots.