Root‐derived auxin contributes to the phosphorus‐deficiency‐induced cluster‐root formation in white lupin (Lupinus albus)

Abstract

Formation of cluster roots is a typical morphological response to phosphorus (P) deficiency in white lupin (Lupinus albus), but its physiological and molecular mechanisms are still unclear. We investigated the role of auxin in the initiation of cluster roots by distinguishing the sources of auxin, measuring the longitudinal distribution patterns of free indole-3-acetic acid (IAA) along the root and the related gene expressions responsible for polar auxin transport (PAT) in different developmental stages of cluster roots. We found that removal of shoot apex or primary root apex and application of auxin-influx or -efflux transport inhibitors, 3-chloro-4-hydroxyphenylacetic acid, N-1-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid, to the stem did not affect the number of cluster roots and the free-IAA concentration in the roots of P-deficient plants, but when these inhibitors were applied directly to the growth media, the cluster-root formation was greatly suppressed, suggesting the fundamental role of root-derived IAA in cluster-root formation. The concentration of free IAA in the roots was higher in P-deficient plants than in P-adequate ones, and the highest in the lateral-root apex and the lowest in the mature cluster roots. Meanwhile the expression patterns of LaAUX1, LaPIN1 and LaPIN3 transcripts related to PAT was consistent with concentrations of free IAA along the lateral root, indicating the contribution of IAA redistribution in the cluster-root development. We proposed that root-derived IAA plays a direct and important role in the P-deficiency-induced formation of cluster roots.

Interactive effects of phosphorus deficiency and exogenous auxin on root morphological and physiological traits in white lupin (Lupinus albus L.)

“…It has been suggested that TIBA acts as a competitive inhibitor of auxin transport [52]. Our results showing TIBA significantly inhibiting LR and CR formation ( Figures 3D-F and 4) are consistent with previous observations that TIBA can interfere with NAA activity [15,35,36]. Taken together, these results confirm that auxin plays an essential role in regulating root development of white lupin.…”

“…In rice, exogenous auxin can amplify P starvation signaling [38], and a cross-talk exists between auxin and P starvation [39]. Application of exogenous auxin increases LR formation in Arabidopsis and CR proliferation in white lupin, whereas auxin antagonists reduce these effects [35,36,50,51]. These findings support the view that most effects of P deficiency on root morphological responses depend primarily on auxin signaling.…”

“…The mechanisms regulating lupin root system response to P deficiency are not fully understood, however. Increasing evidence suggests that auxin plays a key role in the regulation of root morphology in response to P starvation [34][35][36]48,49]. In Arabidopsis, P deficiency leads to auxin redistribution and an increase in auxin sensitivity [15,34,47,49].…”

“…In Arabidopsis, P deficiency leads to auxin redistribution and an increase in auxin sensitivity [15,34,47,49]. In white lupin, root-derived auxin plays a key role in P deficiency-induced CR formation [36]. In rice, exogenous auxin can amplify P starvation signaling [38], and a cross-talk exists between auxin and P starvation [39].…”

“…Auxin plays a crucial role in mediating P-starvation effects on root-hair elongation and LR initiation in Arabidopsis [34] and CR formation in white lupin [35,36]. Phosphorus-deficient Arabidopsis plants can form more LRs than P-sufficient plants when exposed to exogenous auxin [34].…”