Wounding induces multilayered barrier formation in mature leaves via phytohormone signalling and ATML1-mediated epidermal specification.

The epidermis of plants forms a protective barrier against biotic and abiotic stress. Little is known about how breaches in the epidermis are repaired, especially in mature leaves. Here we investigated wound healing in the mature leaves of Arabidopsis. We discovered a wound protection mechanism comprising a multilayered ligno-suberized barrier covered with cuticular wax. This barrier is formed by mesophyll cells that adopt an epidermal fate. This cell fate transition is regulated in two steps by ATML1, a key transcription factor in epidermal specification. First, mesophyll cells of protective layer 1, just beneath the wound, transition into epidermal cells and seal the wound by depositing cuticle, a mechanism that involves signalling through ethylene and reactive oxygen species produced by RbohE. This signalling also promotes cell death in protective layer 1, ensuring wax accumulation on the surface. Second, the underlying protective layer 2 undergoes ligno-suberization, driven by jasmonic acid and RbohD, forming a cork-like layer on the leaf surface. ATML1 regulates this process in protective layer 2 as well. Wound healing in mature leaves thus involves the integration of ethylene and jasmonic acid signalling with ATML1-mediated epidermal cell specification to coordinate cell-layer-specific functions, including cuticular wax formation and ligno-suberization. This protective mechanism also occurs in the leaves of tobacco and Capsella, suggesting it is widespread.