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Stomatal Responses to Pathogens
Plants must undergo photosynthesis to produce their own energy. Crucial to
the photosynthetic process is gas exchange. The plant requires CO2, and gives off O2
throughout the sugar production pathway, most actively during the daytime hours
when there is an abundance of sunlight. The plant will facilitate gas exchange
through small pores on the undersurface of its leaves, called stomata. Two
specialized cells known as guard cells, which regulate opening and closing, surround
each stoma. Guard cells respond to various abiotic cues in their control of the size of
the opening. For example, during the day, when photosynthesis is occurring, the
stomata will be open to allow for gas exchange. However, at night, when the lack of
sunlight halts photosynthesis, the guard cells will close the stomata to prevent water
loss. Turgor pressure of the guard cells (regulated by osmotic uptake of water) also
dictates the stomatal status. A plant that is water-deprived will close its stomata in
order to prevent further water loss from the leaves. This occurs as a result of low
turgor pressure in the guard cells that causes them to go flaccid and the opening to
close. Relative solute concentrations also have an impact on the opening and closing
due to the concept of water potential (Reece et al., 2011). Ion and solute flux
therefore has a major effect on the status of the stomata. Stomatal pores open when
the plasma membrane H+-ATPase of guard cells is allowed to be active. The activity
of this proton pump generates a large transmembrane electrochemical gradient that
drives the uptake of charged solutes and, as a consequence, water, which causes the
guard cells to swell and the pore between them to open. This mechanism can be
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seen in Figure 1 below, which illustrates some of these key points.
Figure 1: Depiction of stomata. A) An electron micrograph of a stoma. B) An open stoma, as a result of
water moving into the guard cells due to the water potential gradient. C) A closed stoma, as a result of
flaccid guard cells from a net movement of water out of the cells. Image from: http://ap-bio-chsplants.wikispaces.com/Control+of+Stomata
Recent studies suggest that plant stomata also respond to biotic cues. For
example, plants interact with pathogens just as animals do, but clearly in different
mechanisms. Major plant pathogens include fungi, bacteria, viruses, and insect pests.
Previously it was thought that pathogens infected plants passively, merely taking
advantage of open stomata or wounds in the plant’s surface, and that the plants had
little in the way of defenses. However, recent studies suggest a more active
relationship between the pathogen and the plant. Plants are in fact better adapted
for defense than scientists had thought for a long time. It turns out that plants have
preformed systems in place to prevent the entrance of pathogens, and then have
inducible responses that can also prevent them from spreading (Reece et al., 2011).
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Preformed defenses include chemical secretions, a Casparian strip in the
roots, thick lignified cell walls, and various other generalized defenses that prevent
easy access for pathogenic organisms. Other methods of defense include
nonspecific-triggered immunity, in response to molecules secreted by the attacking
pathogen. These molecular signals perceived by the plant cells are known as
pathogen associated molecular patterns (PAMPs), and they induce a response
known as PAMP triggered immunity, or PTI (Anderson et al., 2010). These PAMPs
are recognized on the part of the plant by pattern recognition receptors, or PRRs,
which then induce the PTI response through a signal transduction pathway
(Chisholm et al., 2006). Plants with the PRR corresponding to the produced PAMP
are able to detect the presence of the pathogen, even at very small concentrations. In
many cases, PTI responses involve rapid closure of stomata to prevent the
pathogens from entering via those openings (Melotto et al., 2006). Given that the
PTI response is widespread among plant genera, the question arises, how do
phytopathogens still manage to cause disease?
Recent inquiries into this topic have shown that certain pathogens, bacteria
in particular, have evolved methods of circumventing the PTI in order to gain access
to the plant. The focus is primarily on bacteria as they lack compounds that can
degrade plant cell walls, or mechanical means of gaining access that other pests
such as fungi or insects may have. Bacteria have developed compounds that
specifically inhibit the ability of plants to close their stomata. For example, in their
review on the role of stomata in bacterial invasion, (Underwood et al., 2007)
mentioned that coronatine produced by several pathovars of P. syringae functions in
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reopening the stomata of Arabidopsis. This again shows a more active back and forth
relationship than researchers previously suspected. There have been various other
related studies done that serve to generalize these results to many varieties of plant
pathogens (Gudesblat et al., 2009). Additionally, the ability to reopen stomata is
seen as an important adaptation that enables the transition from an epiphytic to an
endophytic parasitism (Melotto et al., 2008). Some research has been done on the
role of proteins that may act in conjunction with the plasma membrane H+-ATPase
and have effects on stomatal action. Particularly, (Liu et al., 2009) showed in their
study that the protein RIN4 acts in conjunction with that ATPase to regulate
stomatal closing during the PTI response. The figure below is from their paper, and
illustrates the aforementioned interaction.
Figure 2: Model of PAMP induced stomatal closure. RIN4 acts in conjunction with other membrane
components and the H+-ATPase to regulate stomatal openings in response to pathogenic attack during
the PTI response. A) Virulent pathogens overcome the PTI and induce reopening of stomata. Activation
of the H+-ATPase leads to an efflux of H+ ions and a subsequent influx of K+ ions, which then causes
water uptake, leading to higher guard cell turgor and opening the stoma. B) RIN4 is a negative regulator
of innate immunity. Removal or inactivation of RIN4 inhibits the association between RIN4 and the H+ATPase, keeping it from being activation. The lack of H+ ion efflux prevents the reaction in A, and the
stoma stays closed (Liu et al., 2009).
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Despite these pathogenic adaptations to cause disease through reversal of
stomatal closure, induced susceptibility and successful infection is relatively rare. As
one would expect, the continued selective pressure on each side has caused a
coevolution process to be maintained. This coevolution has led to the development
of more specific, and more rapid targeted defense mechanisms by plants against
invasion. Plant-pathogen interactions are governed by specific interactions between
corresponding sets of genes in the plant (resistance or R genes) and pathogen
(avirulence or avr genes). When the corresponding genes are present in both
organisms, disease resistance (on the part of the plant) results. If either is absent or
switched off, disease results (Dangl and Jones, 2001). This process is known as
effector triggered immunity, or ETI. Stomatal closure has been shown to be a part of
this response as well (Freeman and Beattie, 2009). When a host plant recognizes the
pathogen, often a response known as the hypersensitive response (HR) occurs. This
generally includes programmed cell death to limit the spread of the pathogen, and
also precedes a systemic response in the rest of the plant, that leads to systemic
acquired resistance.
Other pathogens, such as fungi, infect plants as well. In their study on the
fungal foliar pathogen Cercospora zeaemaydis (Kim et al., 2011) provide the first
molecular confirmation that the fungus does not randomly encounter the stomatal
pore, but rather exhibits a tropism towards it. However, the exact mechanisms are
still unknown, but it appears to be through a gene CRP1. Interestingly, they also
found little to no defensive response in the plant cells, indicating that it may not
recognize the fungus’s presence. However, another group working with different
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systems, namely, cereal mildew and rust, found some other intriguing results.
Research done in barley by (Prats et al., 2007) found that in these systems, when the
R-avr gene interaction causes the hypersensitive response permanent, drastic
stomatal dysfunction results: stomata become locked open in response to powdery
mildew attack, and locked shut after rust attack. This would of course be a negative
cost of conferring disease resistance on these crops. Consequently, this would be an
issue in fields that are being treated with new R genes in order to confer better
disease resistance. As stated in their article, more research needs to be done on this
topic to further understand the mechanisms involved and other potential risks of
genetically altering plants.
Proper stomatal function is essential to the life of a photosynthesizing plant.
Pathogens have evolved various ways to take advantage of these openings in the
surface of a leaf as opportunities for endophytic infection. Consequently, plants have
in turn developed various methods to prevent pathogens from gaining access. The
evolutionary “arms race” then favored pathogenic evolution to develop a
mechanism to induce stomatal reopening. This back and forth is a much more active
relationship than previously thought, and is a prime example of the pressures of
natural selection. Applications of research on how plants regulate their stomata in
response to pathogens include agriculture, conservation efforts, and even animal
biology. For example, agriculture is a vastly important part of the global economy,
and of course it feeds the seven billion people on our planet. Anything that sheds
light on how plants respond to pathogens and suggests mechanisms for overcoming
virulence will be helpful. This is the focus of many of the genetic engineering efforts
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currently underway. As for conservation efforts, this research may be of use to some
endangered species of plants. Applications to animal biology are less obvious, but if
we can shed light on how pathogens infect plants, there will almost certainly be
things we learn that can be applied to animals. For these reasons, among many
others, stomatal responses to pathogenic invasion is an area of research that merits
widespread interest, and funding, so that we may continue to feed a growing,
changing world, and cure its diseases.
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References
Anderson J.P., Gleason C.A., Foley R.C., Thrall P.H., Burdon J.B., Singh K.B. (2010)
Plants versus pathogens: an evolutionary arms race. Functional Plant Biology
37, 499–512.
Chisholm S.T., Coaker G., Day B., Staskawicz B.J. Host-microbe interactions: shaping
the evolution of the plant immune response. Cell. 2006 Feb 24;124(4):803-14.
Review. PubMed PMID: 16497589.
Dangl J.L., Jones J.D. Plant pathogens and integrated defence responses to infection.
Nature. 2001 Jun 14;411(6839):826-33. Review. PubMed PMID: 11459065.
Freeman B.C., and Beattie G.A. (2009) Bacterial Growth Restriction During Host
Resistance to Pseudomonas syringae Is Associated with Leaf Water Loss and
Localized Cessation of Vascular Activity in Arabidopsis thaliana. Molecular
Plant-Microbe Interactions, 22: 857-867.
Gudesblat G.E., Torres P.S., Vojnov A.A. Stomata and pathogens: Warfare at the gates.
Plant Signal Behav. 2009 Dec;4(12):1114-6. Review. PubMed PMID:
20514224; PubMed Central PMCID: PMC2819434.
Kim H., Ridenour J.B., Dunkle L.D., Bluhm B.H. (2011) Regulation of Stomatal
Tropism and Infection by Light in Cercospora zeaemaydis: Evidence for
Coordinated Host/Pathogen Responses to Photoperiod? PLoS Pathog 7(7):
e1002113. doi:10.1371/journal.ppat.1002113
Liu J., Elmore J.M., Fuglsang A.T., Palmgren M.G., Staskawicz B.J., et al. (2009) RIN4
Functions with Plasma Membrane H+-ATPases to Regulate Stomatal
Apertures during Pathogen Attack. PLoS Biol 7(6): e1000139.
doi:10.1371/journal.pbio.1000139
Melotto M., Underwood W., He S.Y. Role of stomata in plant innate immunity and
foliar bacterial diseases. Annu Rev Phytopathol. 2008;46:101-22. doi:
10.1146/annurev.phyto.121107.104959. Review. PubMed PMID: 18422426;
PubMedCentral PMCID: PMC2613263.
Melotto M., Underwood W., Koczan J., Nomura K., He S.Y. Plant stomata function in
innate immunity against bacterial invasion. Cell. 2006 Sep 8;126(5):969-80.
PubMed PMID: 16959575.
Prats E., Carver T.L., Gay A.P., Mur L.A. Enemy at the gates: interaction-specific
stomatal responses to pathogenic challenge. Plant Signal Behav. 2007
Jul;2(4):275-7. PubMed PMID: 19704679; PubMed Central PMCID:
PMC2634148.
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Reece, Jane B., Lisa A. Urry, Michael L. Cain, Steven A. Wasserman, Peter V. Minorsky,
and Robert B. Jackson. Campbell Biology. 9th ed. San Francisco: Pearson
Benjamin Cummings, 2011. Print. pp. 822-824.
Underwood W, Melotto M, He S.Y. Role of plant stomata in bacterial invasion. Cell
Microbiol. 2007 Jul;9(7):1621-9. Epub 2007 Apr 5. Review. PubMed PMID:
17419713.
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