Brassinosteroid controls leaf air space patterning non-cell autonomously by promoting epidermal growth

Plant leaves contain a complex network of intercellular air spaces, which allow efficient photosynthesis. However, how leaf air spaces are patterned is poorly understood. It has been proposed for almost a century that air spaces form by faster-growing epidermal tissue pulling slower growing mesophyll cells apart, but this has never been tested. Here, we characterise air space morphogenesis throughout in the first leaf of Arabidopsis thaliana and show that the plant hormone brassinosteroid is required for air space expansion in the palisade, but not in the spongy mesophyll. We also show that epidermal brassinosteroid perception is sufficient to promote air space expansion in the palisade non-cell autonomously and propose that this non-cell-autonomous effect is due to altered epidermal growth. To test whether epidermal growth affects air space patterning we reduce growth specifically in the epidermis using inducible expression of the growth repressor BIG BROTHER and show that an epidermal growth restriction reduces air space expansion in the palisade mesophyll. Overall, we propose that brassinosteroid signalling promotes growth in the epidermis to pattern air spaces in the palisade mesophyll.