Wax ester synthase overexpression affects stomatal development, water consumption and growth of poplars

Poplars are important fast-growing biomass crops. Their water-spending lifestyle renders them susceptible to drought and threatens plantations under global climate change with extended periods of water deprivation. The cuticle and stomatal regulation are major traits to protect plants from uncontrolled water loss. Here, we targeted the wax biosynthesis pathway of Populus x canescens by overexpressing jojoba (Simmondsia chinensis) wax ester synthase (ScWS) to improve cuticular properties. ScWS expression caused accumulation of lipid droplets inside the cells, decreased transcript levels of endogenous wax biosynthetic genes, and moderate shifts in surface wax composition but did not affect non-stomatal water loss. During short- and long-term drought scenarios under greenhouse and outdoor conditions, ScWS lines showed decreased stomatal conductance and increased water-use-efficiencies leading to a water-saving phenotype and delayed leaf shedding. This phenotype was caused by a high fraction (80%) of wax-occluded or semi-occluded stomata, and was accompanied by suppression of OCCLUDED STOMATAL PORE1 (OSP1), known to cause abberant wax accumulation at the stomatal ledges as found here. Occluded stomata limited poplar photosynthesis under high but not under low light intensities. Leaf damage and insect scores did not reveal differences compared with wild-type plants. Biomass production of ScWS lines was unaffected in short-term experiments but dropped below that of wild-type poplars at the end of two field seasons, indicating a growth trade-off. In conclusion, our study pinpoints a tight connection between wax biosynthesis and stomatal features and opens a new avenue to improve poplar water consumption by optimizing stomatal ledges with refined biotechnological approaches.