The Ethylene-InsensitivesickleMutant ofMedicago truncatulaShows Altered Auxin Transport Regulation during Nodulation

Abstract

We studied the ethylene-insensitive, hypernodulating mutant, sickle (skl), to investigate the interaction of ethylene with auxin transport during root nodulation in Medicago truncatula. Grafting experiments demonstrated that hypernodulation in skl is root controlled. Long distance transport of auxin from shoot to root was reduced by rhizobia after 24 h in wild type but not in skl. Similarly, the ethylene precursor 1-amino cyclopropane-1-carboxylic acid inhibited auxin transport in wild type but not in skl. Auxin transport at the nodule initiation zone was significantly reduced by rhizobia after 4 h in both wild type and skl. After 24 h, auxin transport significantly increased at the nodule initiation zone in skl compared to wild type, accompanied by an increase in the expression of the MtPIN1 and MtPIN2 (pin formed) auxin efflux transporters. Response assays to different auxins did not show any phenotype that would suggest a defect of auxin uptake in skl. The auxin transport inhibitor N-1-naphthylphtalamic acid inhibited nodulation in wild type but not skl, even though N-1-naphthylphtalamic acid still inhibited auxin transport in skl. Our results suggest that ethylene signaling modulates auxin transport regulation at certain stages of nodule development, partially through PIN gene expression, and that an increase in auxin transport relative to the wild type is correlated with higher nodule numbers. We also discuss the regulation of auxin transport in skl in comparison to previously published data on the autoregulation mutant, super numerary nodules (van Noorden et al., 2006).

The Root Hair “Infectome” of Medicago truncatula Uncovers Changes in Cell Cycle Genes and Reveals a Requirement for Auxin Signaling in Rhizobial Infection

“…Four auxin-responsive genes, GH3.1, SAUR1, ARF16a, and IAA9, were more strongly induced by S. meliloti in skl relative to the wild type ( Figure 5), consistent with enhanced auxin transport in skl (Prayitno et al, 2006), and reveals an unexpected role for auxin in root-hair infection. Although S. meliloti induced DR5-GUS in all root hairs in the infection zone, expression of GH3.1, ARF16a, and SAUR1 was restricted to infected cells, indicating that auxin signaling in infected and noninfected root hairs is distinct.…”

Strigolactones fine-tune the root system

“…It has been shown that some genes transcriptionally controlled by both nodulation and auxin transport inhibitors might be important for nodule initiation (Rightmyer and Long 2011). In agreement, physiological experiments in vetch, white clover and M. truncatula have revealed that at the nodule initiation site, a local inhibition of polar auxin transport takes place that might allow auxin accumulation at the site of nodule primordia development (Mathesius et al 1998;Boot et al 1999;Prayitno et al 2006;van Noorden et al 2006). Besides a local influence on auxin transport, the development of the first nodules reduces the long-distance shoot-to-root transport of auxins (van Noorden et al 2006) that might be fundamental to control nodule numbers (van Noorden et al 2006).…”

The Temperature-Sensitive brush Mutant of the Legume Lotus japonicus Reveals a Link between Root Development and Nodule Infection by Rhizobia

“…Nodulation mutants that show pleiotropic phenotypes, such as har1, astray, crinkle, and lot1 in L. japonicus, and sickle and latd in M. truncatula (Krusell et al, 2002;Nishimura et al, 2002;Tansengco et al, 2003;Oka-Kira et al, 2005;Ooki et al, 2005;Prayitno et al, 2006;Liang et al, 2007), provide evidence for this statement. Our analysis of brush reveals a coupled effect on primary root growth and successful rhizobial infection.…”