The chloroplast-localised transporter RETICULATA controls palisade mesophyll cell division and expansion

Plant leaves contain extensive intercellular air spaces, which contribute to photosynthetic capacity and water use efficiency in different environments. Despite this importance, the developmental mechanisms that control leaf air space patterning are poorly understood. For over 20 years it has been known that plants with disrupted chloroplast biogenesis and metabolism have increased intercellular air spaces, but the underlying molecular mechanism is unknown. Here, we characterise leaf air space morphogenesis in mutants of the chloroplast amino acid transporter RETICULATA (RE), which has enlarged air spaces. We show that the reticulata-6 (re-6) loss of function mutant exhibits increased air space expansion specifically in the palisade, and not the spongy, mesophyll. We also show that RE does not regulate air space patterning via programmed cell death, via a change in stomatal density or a change in cell identity of the mesophyll tissue. Instead, we quantify mesophyll cell morphology and division rates to show that RE acts by promoting early palisade cell division and suppressing later cell expansion. Overall, we show that RE specifically regulated palisade mesophyll morphogenesis and air space patterning and hypothesise that chloroplasts may act to regulate air space patterning in the palisade mesophyll in response to light.