Mesophyll conductance and cell wall composition: insights from a meta‐analysis across species

Short abstract

This article is a Commentary on Roig‐Oliver et al. (2025), 246: 2384–2391.

Mesophyll conductance (g _m) plays a critical role in plant photosynthesis by regulating the diffusion of CO₂ from substomatal cavities to the site of carbon fixation within the chloroplasts. Despite its importance, our understanding of the factors influencing g _m, particularly the role of cell wall properties, remains incomplete. In this issue of New Phytologist, Roig‐Oliver et al. (2025; pp. 2384–2391) address this knowledge gap through a comprehensive meta‐analysis across multiple species from the major clades of land plants. By analyzing trends in g _m and two other key physiological parameters – the ratio of pectin to cellulose and hemicellulose in the cell wall by weight (P/(C + H)), and cell wall thickness (T _cw) – their findings reveal strong correlations of g _m with the combination of P/(C + H) and T _cw, where g _m is directly proportional to P/(C + H) and inversely proportional to T _cw. This work not only underscores the influence of cell wall composition on CO₂ diffusion but also provides a valuable framework for further experimental and modeling studies. By shedding light on the dynamic properties of cell walls, shaped by both composition and environmental regulation, this study opens new frontiers for understanding and improving photosynthesis.

This highlights the potential of cell wall composition as a measurable proxy for effective porosity, advancing our understanding of the variability of g _m across species.

Mesophyll conductance can be dissected into several components, representing diffusion barriers in the intercellular airspace, the cell wall, the plasma membrane, the cytosol, the chloroplast envelope and the chloroplast stroma. Among these, the cell wall has been generally considered the largest barrier to CO₂ diffusion, in line with the dominant role of cell wall thickness (T _cw) in limiting g _m, as demonstrated by the anatomical limitation analysis introduced by Tomás et al. (2013). Evans et al. (2009) proposed a conceptual model for cell wall conductance (g _cw) based on CO₂ diffusion following Fick's first law. In this model, cellulose and hemicellulose form a fibrous network that influences the porosity and tortuosity of the cell wall. The model defines g _cw as

$$g_{\mathrm{cw}}=\frac{D_{\mathrm{c}}\times p}{T_{\mathrm{cw}}\times \tau }$$ (Eqn 1)

where D _c is the diffusivity of CO₂ in water, $p$ is the cell wall porosity, and $\tau$ is the tortuosity of the CO₂ diffusion path through the cell wall. Earlier studies, such as those by Tosens et al. (2012) and Tomás et al. (2013), demonstrated that accounting for T _cw improved predictions of g _m variation. However, these studies did not investigate the influence of $p$ or $\tau$ on g _m, due to difficulties in measuring and estimating these quantities. Roig‐Oliver et al. add a new layer of insight, showing that incorporating P/(C + H), which approximates the effective porosity ($p/\tau$) factor in Eqn 1, further strengthens correlations between g _m and cell wall properties. This highlights the potential of cell wall composition as a measurable proxy for effective porosity, advancing our understanding of the variability of g _m across species.

Pectin influences cell wall conductance by modulating pore size and the hydrophilic nature of the cell wall matrix. Pectin's hydrocolloid properties enhance effective porosity ($p/\tau$) to water and CO₂. The degree of pectin methylesterification and subsequent de‐esterification play a critical role in altering cell wall stiffness and porosity. The effects of de‐esterification are determined by two mechanisms: blockwise and nonblockwise de‐esterification. Blockwise de‐esterification facilitates cross‐linking with Ca²⁺, leading to gelation and increased cell wall stiffness, while nonblockwise de‐esterification reduces cross‐linking, enhancing porosity and softness. This dynamic process is influenced by factors such as pH, cation concentration and the activity of pectin‐remodeling enzymes. Overall, pectin instigates a complex and responsive system within the cell wall, significantly impacting its mechanical and functional properties. For a more detailed and comprehensive review on this topic, readers are referred to Flexas et al. (2021).

Despite our current understanding of the complexity of pectin's influence on cell wall conductance, the relationship between cell wall composition and g _m still requires deeper exploration. Experimental measurements of these interactions, particularly in vivo, remain challenging. Computational modeling may offer a promising avenue for hypothesis generation and testing. For example, from the shift of an exponential decay between g _m and T _cw to a linear relationship between g _m and 1/T _cw × P/(C + H), effective porosity is one potential explanation. However, how other anatomical components, such as the plasma membrane and chloroplast stroma, influence g _m beyond the cell wall properties in Eqn 1, and whether these effects explain differences among plant groups, could become an immediate research question for modeling to explore. For example, fig. 2 from Roig‐Oliver et al. reports that mosses exhibit wide variation in both T _cw and P/(C + H), yet their 1/T _cw × P/(C + H) and g _m show relatively small variation, raising questions about the underlying mechanisms. Additionally, crops deviate from these regressions, suggesting that they may be governed by different mechanisms. Developing models that account for these dynamics could provide critical insights into the mechanistic basis of g _m sensitivity and help bridge the gap between cell wall properties and photosynthetic efficiency. Furthermore, such models could also provide insights into how cell wall conductance may respond to environmental conditions such as light, CO₂, temperature and drought.

Another complication to understanding the relationship between cell wall composition and conductance to CO₂ lies in the distinction between type I and type II primary cell walls, which differ in their composition. Type I cell walls are found in eudicot angiosperms, noncommelinoid monocot angiosperms and all gymnosperms, whereas type II cell walls are only found in the commelinoid monocot division of angiosperms, which includes key crops such as wheat and rice. Type I cell walls generally contain less pectin (10% vs 35%) and more aromatic molecules compared to type II walls. In addition, proteins comprise only 1% of mass in type II cell walls, in contrast to 10% in type I walls. Differences in hemicelluloses further distinguish the two types, with type I walls containing more xyloglucans, which play a key role in tightening and loosening the cell wall. These compositional differences underscore the importance of considering the type of cell wall (I vs II) when targeting the cell wall to improve CO₂ diffusion and mesophyll conductance. This is particularly relevant because some of our most important crops are commelinoid monocots. A deeper understanding of the structural and functional differences between primary cell wall types is pivotal for advancing efforts to improve crop yield.

Roig‐Oliver et al. suggest that crops may be outliers in how cell wall composition affects photosynthetic traits, potentially due to indirect selection for higher g _m during breeding for improved performance. For instance, a comparative study between an elite soybean cultivar LD11 and four ancestral cultivars found that g _m in the elite cultivar was double that of its ancestral cultivars (Pelech et al., 2025). The study also revealed that improvements in g _m were smaller than improvements in photosynthetic biochemistry, suggesting the potential for further g _m improvements to benefit crop productivity. Together, these findings highlight the importance of studying both crop and noncrop species to gain a more comprehensive understanding of how cell wall composition affects g _m and photosynthetic performance in order to identify traits that could be exploited for crop improvement.

Looking ahead, expanding research across the phylogeny of land plants will be critical for bridging the gap between the mechanistic understanding of mesophyll conductance and photosynthetic efficiency. Exploring natural variation in cell wall composition, as well as leveraging gene editing and transgenic approaches, offers promising strategies to enhance photosynthesis and yield. Recent successes, such as manipulating cell wall thickness and effective porosity (Pathare et al., 2024; Salesse‐Smith et al., 2024), demonstrate the potential of these approaches. Moving forward, a major challenge will be to understand how cell wall composition interacts across plant tissues, developmental stages and in response to different environmental conditions. For example, recent work by Sun et al. (2025) examined how drought conditions influence cell wall properties and mesophyll conductance in cotton. Techniques like glycome profiling, which provides detailed information about cell wall structure and carbohydrate components beyond P/(C + H), will play a key role in addressing these challenges. Future developments in this field will require interdisciplinary research that integrates plant physiology, biophysics, computational modeling and advanced analytical techniques, all of which are essential for advancing our ability to optimize cell wall properties for improved photosynthesis and plant productivity under diverse environmental conditions.

Disclaimer

The New Phytologist Foundation remains neutral with regard to jurisdictional claims in maps and in any institutional affiliations.