The Auxin Transport Inhibitor N-(1-Naphthyl)phthalamic Acid Elicits Pseudonodules on Nonnodulating Mutants of White Sweetclover

Abstract

The collection of symbiotic (sym) mutants of white sweetclover (Melilotus alba Desr.) provides a developmental sequence of mutants blocked early in infection or nodule organogenesis. Mutant phenotypes include non-nodulating mutants that exhibit root-hair deformations in response to Rhizobium meliloti, mutants that form ineffective nodules lacking infection threads, and mutants that form infection threads and ineffective nodules. Mutant alleles from both the sym-1 and the sym-3 loci exhibited a non-nodulating phenotype in response to R. meliloti, although one allele in the sym-1 locus formed ineffective nodules at a low frequency. Spot-inoculation experiments on a non-nodulating allele in the sym-3 locus indicated that this mutant lacked cortical cell divisions following inoculation with R. meliloti. The auxin transport inhibitor K ( 1 -naphthyl)phthalamic acid elicited development of pseudonodules at a high frequency on all of the sweetclover sym mutants, including the nonnodulating mutants, in which the early nodulin ENODZ was expressed. This suggests that K ( 1 -naphthyl)phthalamic acid activates cortical cell divisions by circumventing a secondary signal transduction event that is lacking in the non-nodulating sweetclover mutants. The sym-3 locus and possibly the sym-1 locus appear to be essential to early host plant responses essential to nodule organogenesis.

Manipulation of Auxin Transport in Plant Roots during Rhizobium Symbiosis and Nematode Parasitism

“…In Arabidopsis, auxin gradients needed for lateral root initiation rely on a functional auxin transport system (Benkovà et al, 2003), which might suggest that nodule development depends on the auxin transport mechanisms of their hosts. This suggestion is supported by the observation that external application of synthetic auxin transport inhibitors to roots inhibits nodulation (van Noorden et al, 2006), although in some legumes, long-term application of auxin transport inhibitors can induce nodule-like structures (Hirsch et al, 1989;Wu et al, 1996). It is likely that temporary inhibition of polar auxin transport at the infection site is a prerequisite for indeterminate nodule formation, as rhizobia and purified Nod factors inhibit the auxin response in white clover roots expressing an auxinresponsive GUS reporter below the site of inoculation, similar to the action of synthetic auxin transport inhibitors (Mathesius et al, 1998a).…”

Nitrate reductase activity and yield of Lens culinaris sprayed with 28-homobrassinolide

“…That is most likely to be the regulation of the hormone level, in particular that of auxin, by acting at the level of its synthesis and/or transport. These indirectly affect nodule formation (Kefford et al, 1960;Hunter, 1987;1989;Hirsch et al, 1989;Fukuhara et al, 1994;Wu et al, 1996) and the partitioning of photosynthates (Brenner and Cheikh, 1995). This is further supported by the synergistic response observed to HBR and auxin (Mandava, 1988).…”

Strigolactones fine-tune the root system

“…Indeed, application of polar auxin transport inhibitors such as NPA and 2,3,5-triiodobenzoic acid (TIBA) can induce spontaneous nodules in some legumes (Allen et al 1953;Hirsch et al 1989;Wu et al 1996;Rightmyer and Long 2011). 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).…”