Histidine kinase MHZ1/OsHK1 interacts with ethylene receptors to regulate root growth in rice

Abstract

Ethylene plays essential roles during adaptive responses to water-saturating environments in rice, but knowledge of its signaling mechanism remains limited. Here, through an analysis of a rice ethylene-response mutant mhz1, we show that MHZ1 positively modulates root ethylene responses. MHZ1 encodes the rice histidine kinase OsHK1. MHZ1/OsHK1 is autophosphorylated at a conserved histidine residue and can transfer the phosphoryl signal to the response regulator OsRR21 via the phosphotransfer proteins OsAHP1/2. This phosphorelay pathway is required for root ethylene responses. Ethylene receptor OsERS2, via its GAF domain, physically interacts with MHZ1/OsHK1 and inhibits its kinase activity. Genetic analyses suggest that MHZ1/OsHK1 acts at the level of ethylene perception and works together with the OsEIN2-mediated pathway to regulate root growth. Our results suggest that MHZ1/OsHK1 mediates the ethylene response partially independently of OsEIN2, and is directly inhibited by ethylene receptors, thus revealing mechanistic details of ethylene signaling for root growth regulation.

Barley HISTIDINE KINASE 1 (HvHK1) coordinates transfer cell specification in the young endosperm

“…Among histidine kinases, three putative ethylene receptors (ERS/ETR), two HvHPs and several type-B RRs are repressed, indicating that different TCS modules are functionally coupled by HvHK1 in directing ETC specification. This coincides with recent results showing that the rice OsHK1 interacts with OsERS2 and phosphorylates OsAHP1 and -2 and OsRR21 (type-B) to regulate root growth in seedlings (Zhao et al, 2020). Transcriptional activation of type-B RRs was also rationalized for AtCKI1 using the synthetic TCSn sensor (Yuan et al, 2016) coinciding with the prediction of HvHP and HvRR14 as direct targets and hub genes in the GRN.…”

Ethylene signaling in plants

“…There is some overlap between transcriptional changes caused by ethylene and cytokinin (174), raising the possibility that there are both overlapping and non-overlapping targets of transcriptional control from this signaling pathway involving ETR1 histidine kinase and the well-known pathway involving EIN3 and EILs. It is interesting to note that in rice, a histidine kinase (MHZ1/OsHK1) that may have a role in cytokinin signaling functions downstream of the OsERS2 ethylene receptor and signals independently of OsEIN2 (175). Thus, our model for canonical ethylene signaling probably needs to be expanded to include secondary pathways such as phosphorelay from some of the ethylene receptors to the AHPs and ARRs.…”

Ethylene signaling in rice and Arabidopsis: New regulators and mechanisms

“…Except OsETR4, all have been proven to be functional and play redundant roles in regulating ethylene signaling (Wuriyanghan et al, 2009; Ma et al, 2014; Yang et al, 2015a). The rice OsERS2 gain‐of‐function mutant Osers2 d /mhz12 exhibits a strong ethylene‐insensitive phenotype, suggesting an important role of subfamily I receptors in regulating the rice ethylene response (Zhao et al, 2020a). Compared with the control rice cultivar, single loss‐of‐function mutants of ethylene receptors, that is, Osers1 , Osers2 , Osetr2 , and Osetr3 , exhibited mild ethylene hypersensitivity in etiolated seedlings but showed largely normal growth in the field (Wuriyanghan et al, 2009; Ma et al, 2013; Yin et al, 2015).…”

“…This MHZ1‐mediated phosphor‐relay is essential for ethylene signaling, since mutations at the three sites in MHZ1 (the conserved His‐accepting phosphoryl group, the G1 box of the kinase domain, and the conserved Asp‐accepting phosphoryl group in the receiver domain) disrupted the ability of MHZ1 to rescue the mhz1 ethylene response in roots. The Osahp1 Osahp2 double mutant also showed an ethylene‐insensitive phenotype, and OsRR21 overexpression enhanced the ethylene response of etiolated seedlings (Zhao et al, 2020a), indicating that these downstream components are involved in ethylene signaling.…”

“…We screened a set of rice ethylene‐insensitive mutants named Mao Huzi ( mhz , meaning “cat whiskers” based on the appearance of their roots). Conserved components, such as MHZ7/OsEIN2 and MHZ6/OsEIL1 (Ma et al, 2013; Yang et al, 2015b), and novel regulators and/or mechanisms have been identified to play roles in ethylene signaling in rice (Chen et al, 2018; Ma et al, 2018; Zhao et al, 2020a, 2020b). Based on these studies, an ethylene signaling pathway in rice was proposed (Figure 2).…”