Simultaneous activation of SHR and ATHB8 expression defines switch to preprocambial cell state in Arabidopsis leaf development.

The processes underlying the formation of leaf vascular networks have long captured the attention of developmental biologists, especially because files of elongated vascular-precursor procambial cells seem to differentiate from apparently equivalent, isodiametric ground cells. In Arabidopsis leaves, ground cells that have been specified to vascular fate engage expression of ARABIDOPSIS THALIANA HOMEOBOX8 (ATHB8). While definition of the transcriptional state of ATHB8-expressing ground cells would be particularly informative, no other genes have been identified whose expression is initiated at this stage. Here we show that expression of SHORT-ROOT (SHR) is activated simultaneously with that of ATHB8 in leaf development. Congruence between SHR and ATHB8 expression domains persists under conditions of manipulated vein patterning, suggesting that inception of expression of SHR and ATHB8 identifies transition to a preprocambial cell state that presages vein formation. Our observations further characterize the molecular identity of cells at anatomically inconspicuous stages of leaf vein development.