Cyanobacteria Respond to Low Levels of Ethylene

Abstract

Ethylene is a gas that has long been known to act as a plant hormone. We recently showed that a cyanobacterium, Synechocystis sp. PCC 6803 (Synechocystis) contains an ethylene receptor (SynEtr1) that regulates cell surface and extracellular components leading to altered phototaxis and biofilm formation. To determine whether other cyanobacteria respond to ethylene, we examined the effects of exogenous ethylene on phototaxis of the filamentous cyanobacterium, Geitlerinema sp. PCC 7105 (Geitlerinema). A search of the Geitlerinema genome suggests that two genes encode proteins that contain an ethylene binding domain and Geitlerinema cells have previously been shown to bind ethylene. We call these genes GeiEtr1 and GeiEtr2 and show that in air both are expressed. Treatment with ethylene decreases the abundance of GeiEtr1 transcripts. Treatment of Geitlerinema with 1000 nL L –1 ethylene affected the phototaxis response to white light as well as monochromatic red light, but not blue or green light. This is in contrast to Synechocystis where we previously found ethylene affected phototaxis to all three colors. We also demonstrate that application of ethylene down to 8 nL L –1 stimulates phototaxis of both cyanobacteria as well as biofilm formation of Synechocystis. We formerly demonstrated that the transcript levels of slr1214 and CsiR1 in Synechocystis are reduced by treatment with 1000 nL L –1 ethylene. Here we show that application of ethylene down to 1 nL L –1 causes a reduction in CsiR1 abundance. This is below the threshold for most ethylene responses documented in plants. By contrast, slr1214 is unaffected by this low level of ethylene and only shows a reduction in transcript abundance at the highest ethylene level used. Thus, cyanobacteria are very sensitive to ethylene. However, the dose-binding characteristics of ethylene binding to Geitlerinema and Synechocystis cells as well as to the ethylene binding domain of SynEtr1 heterologously expressed in yeast, are similar to what has been reported for plants and exogenously expressed ethylene receptors from plants. These data are consistent with a model where signal amplification is occurring at the level of the receptors.

Ethylene signaling in plants

“…It is thus interesting to note that ethylene binding has been observed in diverse cyanobacteria and at least one cyanobacterium, Synechocystis, has a functional ethylene receptor that regulates cell surface properties to affect biofilm formation and phototaxis (20)(21)(22). Additionally, ethylene binding affinities to some of these cyanobacteria and the heterologously expressed Synechocystis ethylene receptor are similar to what has been observed in plants (23) showing a conservation of this domain between these organisms. However, the organism where ethylene receptors first arose remains unknown.…”

Two Hybrid Histidine Kinases Involved in the Ethylene Regulation of the Mycelial Growth and Postharvest Fruiting Body Maturation and Senescence of Agaricus bisporus

“…PCC 6803 with the NCBI accession number NP_440714 ; GeiEtr1 and GeiEtr2, the ethylene receptors of Geitlerinema sp. PCC 7105 ( 14 ); LeETR1, LeETR2, LeETR4, LeETR5, LeETR6, LeETR7, and LeNR, the ethylene receptors of Lycopersicon esculentum with the NCBI accession numbers NP_001234149 , NP_001234153 , NP_001234205 , NP_001234212 , NP_001234150 ( 77 ), and NP_001233894 , respectively.…”

“…strain PCC 6803 ( 12 , 13 ) and Geitlerinema sp. strain PCC 7105 ( 14 ) can sense and respond to ethylene. Several putative ethylene receptors are present in early diverging fungi, Mesomycetozoa, Amoebozoa, diatoms, Zooxanthellae, and Chromerida ( 15 , 16 ), but whether they sense and respond to ethylene is unknown.…”

First characterization of cultivable extremophile Chroococcidiopsis isolates from a solar panel

“…Biofilm formation by Chroococcidiopsis sp. Helios was assayed using crystal violet staining described previously with slight modifications (Allen et al, 2019). Briefly, three milliliters of 2-to 3-week-old Chroococcidiopsis sp.…”