Ectopic Expression of miR160 Results in Auxin Hypersensitivity, Cytokinin Hyposensitivity, and Inhibition of Symbiotic Nodule Development in Soybean    

Abstract

Symbiotic root nodules in leguminous plants result from interaction between the plant and nitrogen-fixing rhizobia bacteria. There are two major types of legume nodules, determinate and indeterminate. Determinate nodules do not have a persistent meristem, while indeterminate nodules have a persistent meristem. Auxin is thought to play a role in the development of both these types of nodules. However, inhibition of rootward auxin transport at the site of nodule initiation is crucial for the development of indeterminate nodules but not determinate nodules. Using the synthetic auxin-responsive DR5 promoter in soybean (Glycine max), we show that there is relatively low auxin activity during determinate nodule initiation and that it is restricted to the nodule periphery subsequently during development. To examine if and what role auxin plays in determinate nodule development, we generated soybean composite plants with altered sensitivity to auxin. We overexpressed microRNA393 to silence the auxin receptor gene family, and these roots were hyposensitive to auxin. These roots nodulated normally, suggesting that only minimal/reduced auxin signaling is required for determinate nodule development. We overexpressed microRNA160 to silence a set of repressor auxin response factor transcription factors, and these roots were hypersensitive to auxin. These roots were not impaired in epidermal responses to rhizobia but had significantly reduced nodule primordium formation, suggesting that auxin hypersensitivity inhibits nodule development. These roots were also hyposensitive to cytokinin and had attenuated expression of key nodulation-associated transcription factors known to be regulated by cytokinin. We propose a regulatory feedback loop involving auxin and cytokinin during nodulation.

Flavonoids and Auxin Transport Inhibitors Rescue Symbiotic Nodulation in the Medicago truncatula Cytokinin Perception Mutant cre1

“…In addition, it is likely that the GH3:GUS reporter construct does not reflect the exact localization and activity of all different auxins in the root. Nevertheless, auxin activity analyzed through GH3:GUS expression agrees with the induction of the DR5 promoter in nodule primordia of L. japonicus (Suzaki et al, 2012) and soybean (Turner et al, 2013) and in the cortex of M. truncatula (Breakspear et al, 2014). Interestingly, an auxin response in root hairs at the infection site was visible after 24 hpi in both genotypes.…”

MicroRNA dynamics in a wild and cultivated species of Convolvulaceae exposed to drought stress

“…In another study, miR394 was shown to be downregulated in high-tolerant sugarcane cultivar but upregulated in the sensitive cultivars (Gentile et al 2013). In these cultivars, (Liu et al 2007;Ballen-Taborda et al 2013;Turner et al 2013). In Arabidopsis, miR160 appears to regulate auxin-ABA crosstalk, playing crucial role in seed germination (Liu et al 2007).…”

Soybean miR172c Targets the Repressive AP2 Transcription Factor NNC1 to Activate ENOD40 Expression and Regulate Nodule Initiation

“…In soybean, numerous miRNAs are differentially expressed during nodule organogenesis (Subramanian et al, 2008;Wang et al, 2009;Dong et al, 2013). Studies have shown that overexpression of several of these soybean miRNAs, including isoforms of miR160, miR156, miR1515, and miR172, can affect nodule numbers Turner et al, 2013;Yan et al, 2013). However, to date, there are no reports on the consequences of reducing endogenous miRNA activity on soybean nodule development.…”