Abstract
Plants use energy from sunlight to transform carbon dioxide from the air into complex organic molecules, ultimately producing much of the food we eat. To make this complex chemistry more efficient, plant leaves are intricately constructed in 3 dimensions: They are flat to maximise light capture and contain extensive internal air spaces to increase gas exchange for photosynthesis. Many years of work has built up an understanding of how leaves form flat blades, but the molecular mechanisms that control air space formation are poorly understood. Here, I review our current understanding of air space formation and outline how recent advances can be harnessed to answer key questions and take the field forward. Increasing our understanding of plant air spaces will not only allow us to understand a fundamental aspect of plant development, but also unlock the potential to engineer the internal structure of crops to make them more efficient at photosynthesis with lower water requirements and more resilient in the face of a changing environment.
Highlights
Further investigation is needed to understand exactly how chloroplasts signal to mesophyll cells to regulate cell identity and growth, but emerging evidence suggests that plastid localised proteins, such as ENLARGED FIL EXPRESSING DOMAIN 2 (ENF2) interact with adaxial/abaxial patterning factors to position spongy versus palisade mesophyll cell identity along the adaxial/abaxial axis, providing a tantalising link to wellknown regulators of leaf development [45]
How are environmental and molecular signals integrated in air space development? The data above suggest that the physiological state of the leaf regulates air space formation, via stomata and chloroplast function
The above discussion makes clear that the formation of air spaces in plant leaves is a complex developmental process
Summary
These data suggest a role for chloroplast signalling in both cell identity and air space formation in the mesophyll. Several of the genes mutated in reticulate mutants are expressed preferentially or exclusively in bundle sheath cells surrounding the vasculature, adding a spatial element to chloroplast regulation of air space patterning [29,39] (reviewed in [35]; Fig 3).
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