Abstract
The regulation of arbuscular mycorrhizal development and nodulation involves complex interactions between the plant and its microbial symbionts. In this study, we use the recently identified ethylene-insensitive ein2 mutant in pea (Pisum sativum L.) to explore the role of ethylene in the development of these symbioses. We show that ethylene acts as a strong negative regulator of nodulation, confirming reports in other legumes. Minor changes in gibberellin1 and indole-3-acetic acid levels in ein2 roots appear insufficient to explain the differences in nodulation. Double mutants produced by crosses between ein2 and the severely gibberellin-deficient na and brassinosteroid-deficient lk mutants showed increased nodule numbers and reduced nodule spacing compared with the na and lk single mutants, but nodule numbers and spacing were typical of ein2 plants, suggesting that the reduced number of nodules innaandlkplants is largely due to the elevated ethylene levels previously reported in these mutants. We show that ethylene can also negatively regulate mycorrhizae development when ethylene levels are elevated above basal levels, consistent with a role for ethylene in reducing symbiotic development under stressful conditions. In contrast to the hormone interactions in nodulation, ein2 does not override the effect of lk or na on the development of arbuscular mycorrhizae, suggesting that brassinosteroids and gibberellins influence this process largely independently of ethylene.
Highlights
Symbioses between plants and microorganisms are common and are important for the acquisition of at least two key macronutrients, nitrogen and phosphorus (Ferguson et al, 2010; Foo et al, 2013b; Gu et al, 2011)
Double mutants produced by crosses between ein2 and the severely gibberellin-deficient na and brassinosteroid-deficient lk mutants showed increased nodule numbers and reduced nodule spacing compared with the na and lk single mutants, but nodule numbers and spacing were typical of ein2 plants, suggesting that the reduced number of nodules in na and lk plants is largely due to the elevated ethylene levels previously reported in these mutants
While there has been debate in the literature about the role of ethylene in both rhizobial and arbuscular mycorrhizal symbioses, the results with the Psein2 mutant are clear and generally in agreement with the results seen in the Medicago Mtein2 mutant (Penmetsa and Cook, 1997; Penmetsa et al, 2008)
Summary
Symbioses between plants and microorganisms are common and are important for the acquisition of at least two key macronutrients, nitrogen and phosphorus (Ferguson et al, 2010; Foo et al, 2013b; Gu et al, 2011). The decrease in nodulation seen in gibberellin-deficient na mutants can be reversed by treatment with amino-ethoxyvinyl glycine (Ferguson et al, 2011), and recently it has been shown that the gibberellin biosynthetic pathway is regulated by ethylene during the development of the rhizobial symbiosis (Larrainzar et al, 2015). In this paper we use the recently identified ein mutant of pea (Weller et al, 2015) to explore the role of the hormone ethylene on nodule and arbuscular mycorrhizal development. Ethylene is likely to act independently of gibberellin and brassinosteroids to influence interactions with arbuscular mycorrhizal fungi
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