The plant cuticle, often perceived as merely a passive barrier against the environment, plays a far more dynamic role in plant defense. Recent research illuminates its crucial involvement in regulating the active transport of salicylic acid (SA), a vital hormone that triggers systemic acquired resistance (SAR). SAR is a sophisticated defense mechanism that empowers plants to protect themselves from secondary infections, and Apoplastic Transport, the movement of substances through the cell walls and intercellular spaces, is at its heart. This article delves into the fascinating world of apoplastic transport, exploring how it facilitates the long-distance movement of SA and why this process is essential for plant immunity.
Understanding Apoplastic Transport and its Role in Plants
Apoplastic transport is one of the two major pathways for movement of water and solutes in plants, the other being symplastic transport. The apoplast encompasses the cell walls and the intercellular spaces, forming a continuous system throughout the plant. This pathway is distinct from the symplast, which involves movement through the cytoplasm and plasmodesmata, the channels that connect plant cells.
For efficient systemic immunity, plants need to rapidly communicate threats and mobilize defenses across their entire structure. This is where apoplastic transport becomes indispensable, especially for signaling molecules like SA. SA, a key player in SAR, is synthesized at the site of infection and must be transported to distal, uninfected parts to activate defense responses. Research has shown that apoplastic transport is the primary route for this long-distance SA movement, driven by the plant’s own physiological processes and environmental factors.
The Journey of Salicylic Acid: Apoplastic Route Preferred
Scientists have discovered that SA is preferentially transported from infected areas to healthy parts of the plant through the apoplast. This apoplastic accumulation of SA is not a random event; it’s a carefully orchestrated process driven by the pH gradient between the apoplast and the inside of plant cells (cytosol).
SA is a weak acid, and at the slightly alkaline pH of the cytosol, it tends to lose a proton (H+) and become deprotonated. In contrast, the apoplast has a more acidic pH. This pH difference creates a gradient that favors the movement of deprotonated SA from the cytosol into the apoplast. Once in the apoplast, SA can move more freely towards distant tissues, effectively acting as a mobile signal of impending danger.
Fig. 1
Figure 1: ACP4 and MOD1 are required for distal transport of SA. (A) SA and SAG levels in local tissues after mock and pathogen inoculations. (B) G3P levels in local tissues after mock and pathogen inoculations. (C) G3P levels in Petiole Exudates (PEX) collected from mock and pathogen-inoculated plants. (D) Plasmodesmata permeability in wild-type and mutant leaves. (E) SA levels in PEX collected from mock and pathogen-inoculated plants. (F) Quantification of radioactivity transported to distal tissues using 14C-SA. (G) Autoradiograph showing transport of 14C-SA from local to distal leaves.
Cuticle’s Role: More Than Just a Barrier
The cuticle, the outermost layer covering plant aerial parts, is not just a passive shield. Its integrity directly impacts apoplastic transport and, consequently, systemic immunity. Plants with defects in their cuticle, often resulting from mutations affecting fatty acid synthesis (key components of the cuticle), exhibit compromised SAR.
Studies on cuticle-defective mutants, like acp4 and mod1 in Arabidopsis, have revealed a fascinating link between cuticle integrity and SA transport. These mutants, while capable of producing SA at the infection site, are deficient in transporting it systemically. This deficiency is not due to problems in symplastic transport pathways, but rather a disruption in apoplastic SA movement.
Transpiration and Water Potential: Regulators of Apoplastic SA Flow
The reason behind impaired apoplastic transport in cuticle mutants lies in altered transpiration rates and water potential. Cuticle defects lead to increased water loss through transpiration. This heightened transpiration reduces the water potential within the plant, essentially changing the pressure dynamics that influence fluid movement.
In normal plants, transpiration pull aids in drawing water and dissolved substances, including SA, through the apoplast towards distal parts. However, in cuticle-defective mutants with excessive transpiration, this flow is disrupted. Instead of SA being effectively transported to distal tissues via the apoplast, it gets diverted towards the cuticle wax layer.
This misdirection of SA towards the cuticle wax, rather than systemic apoplastic transport, results in a failure to activate SAR in distant tissues. The crucial systemic signaling is weakened because the mobile signal, SA, is not reaching its destination in sufficient amounts.
Humidity as a Restorative Force: Re-establishing Systemic SA Transport
Interestingly, scientists found that manipulating the environment can restore systemic SA transport and SAR in cuticle mutants. Growing cuticle-defective plants under high humidity conditions reduces transpiration. This reduction in transpiration normalizes water potential and redirects SA flow back to the apoplast, facilitating its long-distance transport.
Under high humidity, cuticle mutants regain their ability to effectively transport SA systemically, subsequently restoring their SAR capability. This highlights the critical interplay between environmental factors, cuticle integrity, and apoplastic transport in regulating plant immunity.
Fig. 2
Figure 2: MOD1 is required for normal cuticle development. (A) Morphological phenotype of Col-0 and mod1 plants. (B) Toluidine blue staining showing cuticle permeability in Col-0, mod1, and acp4 leaves. (C) Transmission electron micrographs of the cuticle layer in Col-0 and mod1 plants. (D) Scanning electron micrographs of the abaxial surface of Col-0 and mod1 leaves.
Apoplastic pH and SA Deprotonation: Fine-Tuning Transport
The pH gradient between the cytosol and apoplast is not the only factor influencing apoplastic SA transport; the chemical properties of SA itself play a role. At the pH of the cytosol, SA primarily exists in its deprotonated form. This deprotonated SA is more membrane-permeable, allowing it to readily cross cell membranes and enter the apoplast.
Experiments with protoplasts (plant cells without cell walls) have demonstrated that deprotonated SA exhibits higher membrane permeability, especially at the acidic pH of the apoplast. Furthermore, inhibitors of proton pumps, which maintain pH gradients across membranes, can reduce SA transport. This suggests that proton pumps are involved in facilitating SA movement across the plasma membrane, potentially contributing to the apoplast-cytosol pH gradient that drives apoplastic transport.
SA Partitioning: A Balancing Act Between Apoplast, Symplast, and Cuticle
SA doesn’t just flow unidirectionally through the apoplast. It’s dynamically partitioned between different compartments within the plant: the symplast (cytoplasm), apoplast, and even the cuticle wax. In healthy plants, there’s a balance in this partitioning that ensures efficient systemic signaling.
However, in cuticle-defective mutants, this balance is disrupted. The increased transpiration and altered water potential favor SA accumulation in the cuticle wax at the expense of apoplastic and, potentially, symplastic SA levels in distal tissues. This shift in SA partitioning is a key reason why these mutants are SAR-compromised.
SA in Cuticle Wax: A Role in Stomatal Regulation?
The discovery of SA accumulating in cuticle wax raises intriguing questions about its function there. While it might seem like a sink, diverting SA from its signaling role, evidence suggests that cuticle wax SA could be involved in regulating stomatal opening.
Stomata, the tiny pores on plant leaves, control gas exchange and water transpiration. Research indicates that SA in cuticle wax might contribute to stomatal closure, especially in response to pathogen infection. This stomatal closure could be a mechanism to limit pathogen entry and reduce water loss during defense responses.
Mutants deficient in SA biosynthesis (sid2) exhibit increased stomatal aperture and reduced water potential. Exogenous application of SA can restore normal stomatal function in these mutants. This suggests that SA, potentially acting from its location in the cuticle wax, plays a role in fine-tuning stomatal behavior and water balance in plants, particularly during immune responses.
Fig. 5
Figure 5: Distal transport of SA is associated with water potential. (A) SA levels in PEX collected from plants grown under normal and high humidity. (B) SAR response in plants grown under normal and high humidity. (C) Leaf water potential in wild-type and mutant plants. (D) Stomatal apertures in wild-type and mutant plants. (E) Water Use Efficiency (WUE) in wild-type and mutant plants.
Implications for Plant Immunity and Beyond
This research underscores the critical role of apoplastic transport in plant systemic immunity. It reveals that the cuticle is not merely a passive barrier but an active participant in regulating SA transport and SAR. The interplay between cuticle integrity, transpiration, water potential, and apoplastic pH creates a complex and finely tuned system for mobilizing plant defenses.
Understanding the mechanisms governing apoplastic transport of SA has significant implications for enhancing plant immunity. Strategies to improve cuticle integrity or manipulate transpiration and water potential could potentially boost SAR and enhance plant disease resistance.
Further research into the specific transporters involved in SA movement across membranes and the precise role of cuticle wax SA in stomatal regulation will undoubtedly deepen our understanding of plant immunity and apoplastic transport. This knowledge can pave the way for developing innovative approaches to protect crops and ensure global food security in the face of increasing pathogen threats and changing environmental conditions.
References
Original research article cited throughout this text: Transport of salicylic acid is important for systemic immunity and is regulated by osmotic pressure and cuticle permeability. Science Advances, 6(19), eaaz0478.
