The Histidine Kinase AHK5 Integrates Endogenous and Environmental Signals in Arabidopsis Guard Cells

Abstract

BackgroundStomatal guard cells monitor and respond to environmental and endogenous signals such that the stomatal aperture is continually optimised for water use efficiency. A key signalling molecule produced in guard cells in response to plant hormones, light, carbon dioxide and pathogen-derived signals is hydrogen peroxide (H2O2). The mechanisms by which H2O2 integrates multiple signals via specific signalling pathways leading to stomatal closure is not known.Principal FindingsHere, we identify a pathway by which H2O2, derived from endogenous and environmental stimuli, is sensed and transduced to effect stomatal closure. Histidine kinases (HK) are part of two-component signal transduction systems that act to integrate environmental stimuli into a cellular response via a phosphotransfer relay mechanism. There is little known about the function of the HK AHK5 in Arabidopsis thaliana. Here we report that in addition to the predicted cytoplasmic localisation of this protein, AHK5 also appears to co-localise to the plasma membrane. Although AHK5 is expressed at low levels in guard cells, we identify a unique role for AHK5 in stomatal signalling. Arabidopsis mutants lacking AHK5 show reduced stomatal closure in response to H2O2, which is reversed by complementation with the wild type gene. Over-expression of AHK5 results in constitutively less stomatal closure. Abiotic stimuli that generate endogenous H2O2, such as darkness, nitric oxide and the phytohormone ethylene, also show reduced stomatal closure in the ahk5 mutants. However, ABA caused closure, dark adaptation induced H2O2 production and H2O2 induced NO synthesis in mutants. Treatment with the bacterial pathogen associated molecular pattern (PAMP) flagellin, but not elf peptide, also exhibited reduced stomatal closure and H2O2 generation in ahk5 mutants.SignificanceOur findings identify an integral signalling function for AHK5 that acts to integrate multiple signals via H2O2 homeostasis and is independent of ABA signalling in guard cells.

A Prominent Role of the Flagellin Receptor FLAGELLIN-SENSING2 in Mediating Stomatal Response to Pseudomonas syringae pv tomato DC3000 in Arabidopsis

“…The double-mutant plants behaved similarly to the fls2 single-mutant plants (Fig. 1, A and B It is known that Arabidopsis stomata respond to several purified bacterial MAMPs, such as flg22, LPS, and elf26 (a bioactive peptide derived from EF-TU; Melotto et al, 2006;Cho et al, 2008;Desikan et al, 2008;Gudesblat et al, 2009), suggesting that Arabidopsis stomatal guard cells can perceive multiple MAMPs. It is surprising, then, that the virulence of Pst DC3000 COR-deficient mutants, which presumably carry multiple MAMPs, is almost fully restored in fls2 mutant plants that are defective in only one MAMP receptor (Fig.…”

“…Both findings suggest that Pst DC3000 produces MAMPs other than flagellin, and that Arabidopsis leaf cells (at least the mesophyll cells inside the apoplast) are capable of perceiving multiple MAMPs. It has also been shown that stomatal guard cells of Arabidopsis can respond to more than one MAMP (Melotto et al, 2006;Cho et al, 2008;Desikan et al, 2008;Gudesblat et al, 2009). Therefore, it was surprising to observe that (1) stomata in fls2 plants are completely defective in response to Pst DC3000 and its COR-deficient mutants (Fig.…”

“…Stomatal guard cells also respond to purified microbe-associated molecular patterns (MAMPs), such as chitosan, a polymer of b-1,4-glucosamine residues derived from fungal chitin (Lee et al, 1999;Amborabe et al, 2008), flg22, a 22-amino acid peptide derived from bacterial flagellin (Melotto et al, 2006;Cho et al, 2008;Desikan et al, 2008;Zhang et al, 2008), and bacterial lipopolysaccharides (LPSs; Melotto et al, 2006;Cho et al, 2008). Peptidoglycan, derived from Gram-positive bacteria, is shown to be able to induce plant innate immune responses (Gust et al, 2007;Erbs et al, 2008).…”

Perturbation of Indole-3-Butyric Acid Homeostasis by the UDP-GlucosyltransferaseUGT74E2ModulatesArabidopsisArchitecture and Water Stress Tolerance

“…al., 2007;Desikan et al, 2008;Galvez-Valdivieso et al, 2009). For example, in guard cells, ABAdependent H 2 O 2 production is essential for stomatal closure (Desikan et al, 2008) and during root growth; gibberellin signaling contributes to the fine-tuning of ROS levels by inactivating DELLA proteins that transcriptionally regulate ROS-scavenging enzymes and hence modulate biotic and abiotic stress tolerance (Achard et al, 2008). The best known case in which ROS directly influence the action of auxins is a H 2 O 2 -dependent mitogenactivated protein kinase cascade that negatively affects auxin sensitivity by downregulation of auxin-inducible gene expression (Nakagami et al, 2006).…”

Classification of the Genes Involved in the Two-Component System of the MossPhyscomitrella patens

“…Notably, P. patens has seven homologues of the ethylene receptor, ETR1, two homologs of a putative osmo-sensor, AHK1, and six homologs of AHK5, which has been found to be involved in ethylene and abscisic acid signaling. 9,10) These results indicate that the fundamental TCS-associated components found in extant seed plants were established prior to divergence of the moss lineage.…”