Plasmodesmal closure elicits stress responses

Background

Plant cells are interconnected by plasmodesmata, which facilitate the exchange of nutrients, hormones, metabolites, and signaling molecules (Sager and Lee, 2014). These membrane-lined channels are crucial for cellular communication and coordination (Lu et al., 2018). Upon pathogen attack, plants can close their plasmodesmata, a process regulated by the deposition of callose. However, the precise role of plasmodesmal closure in the broader context of immune responses remains unclear, raising questions about its overall impact on plant immunity and physiology (Cheval and Faulkner, 2018). This preprint explores how closing plasmodesmata affects plant stress signaling and immune responses.

Key Findings

Transgenic lines reveal plasmodesmal closure

The researchers generated two transgenic Arabidopsis thaliana lines, LexA::icals3m and LexA::PD-Plug. These lines enabled the precise examination of plasmodesmal closure via estradiol treatment, resulting in callose buildup at the plasmodesmata. This novel method allowed for the detailed analysis of plasmodesmal dynamics and their specific impact on plant physiology and immunity in a controlled setting.

Impact on stress response and pathogen resistance

Induced plasmodesmal closure resulted in the upregulation of stress-responsive genes, accumulation of salicylic acid (SA), and enhanced resistance to the bacterial pathogen Pseudomonas syringae DC3000. Interestingly, this enhanced resistance was not associated with alterations in flg22-triggered ROS bursts or MAPK signaling, indicating a distinct pathway for the observed resistance. However, the study also highlighted that enhanced plasmodesmal closure led to hypersusceptibility to the fungal pathogen Botrytis cinerea, illustrating that plasmodesmal closure does not uniformly enhance resistance against different pathogens. This suggests pathogen-specific interactions with plasmodesmal closure.

Physiological and transcriptional changes

Beyond immune responses, the increased plasmodesmal closure induced by LexA::icals3m led to noticeable physiological alterations such as starch and sugar buildup, reduced leaf growth, and induced leaf senescence. RNAseq analysis showed that plasmodesmal closure triggers specific stress responses, particularly involving SA biosynthesis and signaling pathways. This transcriptional shift suggests a connection between plasmodesmal closure and systemic acquired resistance, driven by SA accumulation. Additionally, the researchers discovered that plasmodesmal closure caused significant sugar accumulation in source leaves without affecting photosynthetic efficiency. This sugar accumulation likely contributes to the observed growth defects and senescence, linking plasmodesmal regulation to metabolic changes in plants and highlighting its broader impact on plant growth and development.

 

What I Like About This Preprint

In this preprint authors employ transgenic lines to control plasmodesmal closure without relying on external signals. The extensive analysis demonstrates that plasmodesmal closure triggers SA accumulation and stress responses, indicating a direct role for plasmodesmata in detecting cellular stress. Furthermore, the observed variations in pathogen resistance highlight the complexity of plant-pathogen interactions and the subtle role of plasmodesmal regulation, providing new insights into plant immunity and development. These discoveries pave the way for further investigation into how plasmodesmal closure is achieved, the signals involved in this process, and whether these responses occur universally.

 

Open Questions to Authors

1. Mechanisms of SA Synthesis Induction:

Considering that plasmodesmal closure leads to the accumulation of SA, do you propose that this accumulation is a direct consequence of plasmodesmal signaling, or is it an indirect result of osmotic stress or other cellular changes induced? How might these different mechanisms be experimentally distinguished?

 Authors’ reply:

We observed that soluble sugars accumulate in response to plasmodesmal closure and this might induce osmotic stress, which might in turn be the trigger for SA production. Like us, Wang et al. 2013 observed that plasmodesmal closure is correlated with SA production, but they found that degrading SA via the activity of NahG lead to re-opening of the plasmodesmata, suggesting a positive feedback-loop between SA and PD closure.

In our system, we observed the correlation between sugars and SA in the LexA::icals3m line, but not  in the LexA::PD-Plug line which exhibited only an increase in SA. This suggests the possibility that SA is not induced by sugar-mediated osmotic stress. How to experimentally disentangle these processes and directly test the dependency is challenging. Maybe the ‘simplest’ first experiment would be to test whether plasmodesmal closure in roots induces SA since they won’t produce excess sugars.

2. Pathogen-Specific Responses:

Given that enhanced plasmodesmal closure led to increased resistance to Pseudomonas syringae DC3000 but heightened susceptibility to Botrytis cinerea, what specific factors or mechanisms do you think account for these differing responses? How might these findings influence strategies for improving plant resistance to a range of pathogens?

Authors’ reply:

We hypothesize that the different pathogen lifestyle may explain our pathoassay data. The increased  sugars in the induced LexA::icals3m line might be accessible and beneficial to the necrotrophic pathogen B. cinerea, but not to Pst DC3000. Further, Pst DC3000 is more likely impaired by the increase in the SA defence pathways. We know that plasmodesmal closure is required for the full immune response; for example, we observed that mutants defective in plasmodesmal closure are more susceptible to B. cinerea, so it’s clear that you cannot simply open or close plasmodesmata to gain a positive outcome. These findings indicate that its difficult to have one strategy that fits a range of pathogens – dynamic and multi-component strategies are definitely necessary.

3. Cellular and Tissue-Level Responses:

   Your study suggests that the homogeneity of plasmodesmal closure in different transgenic lines correlates with the magnitude of physiological responses. Could you elaborate on the potential threshold effect of isolated cells and how this might influence tissue-level responses? What experimental approaches could further elucidate whether plasmodesmal responses are indeed tissue-level parameters?

 Authors’ reply:

We observed that the frequency of plasmodesmal closure induced in the LexA::PD-Plug line more closely mimics physiological responses to elicitors, such as chitin and flg22. In all these cases, some cells respond by closing their plasmodesmata, but others don’t (we expand on this heterogenous concept in our recent review, Tee & Faulkner, 2024). In comparison, the LexA::icals3m line executes homogenous plasmodesmal closure response, i.e., almost all cells close their plasmodesmata. Both patterns are correlated with an increase in SA, but the magnitude is far higher in the LexA::icals3m. There appears to be a benefit to maintaining a measure of connection to your neighbour, as while both patterns led to an increase in defence transcripts and resistance to Pst DC3000, the homogenous plasmodesmal closure pattern in the LexA::icals3m line led to severe growth phenotypes and tissue senescence. Therefore, we reason that plant tissues constantly balance the trade-offs between the benefits and costs of cell isolation. While it is an ambitious aim, we need to identify markers of cell connectivity states (e.g. possibly SA production). Then, highly resolved spatio-temporal experimental approaches can reveal the dynamics and distribution of cell connectivity states in a tissue under different conditions.

References

C Cheval, C Faulkner (2018) Plasmodesmal regulation during plant-pathogen interactions. New Phytol 217:62-67

EE Tee and C Faulkner (2024) Plasmodesmata and intercellular molecular traffic control. New Phytol 243: 32-47.

KJ Lu, FR Danila, Y Cho, C Faulkner (2018) Peeking at a plant through the holes in the wall – exploring the roles of plasmodesmata. New Phytol 218: 1310-1314

R Sager, J-Y Lee (2014) Plasmodesmata in integrated cell signaling: insights from development and environmental signals and stresses. Journal of Experimental Botany 65: 6337-6358

Xu Wang, et al (2013) Salicylic acid regulates plasmodesmata closure during innate immune responses in Arabidopsis. The Plant Cell 25: 2315-2329.

 

 

doi: https://doi.org/10.1242/prelights.37696

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