Arabidopsis XPD functions upstream of CDKA;1 to regulate stomatal development.

In eukaryotes, XERODERMA PIGMENTOSUM GROUP D (XPD) is an integral subunit of the DNA repair/transcription complex TFIIH. In animals, XPD has been implicated in TFIIH-independent complexes regulating cell division, which, however, remains poorly understood in plants. Here, we identified XPD as a novel regulator of stomatal development in Arabidopsis. Its loss-of-function mutants exhibited increased stomatal precursor cells and formed stomatal clusters. Genetic analysis showed that XPD functions upstream of SPEECHLESS (SPCH) to control stomatal lineage entry, coordinates with MUTE to regulate meristemoid division and works together with FLP and FAMA to restrict GMC division. In a search of XPD interactors, we identified CDKA;1, which serves as both an essential cyclin-dependent kinase and a key SPCH activator. Consistently, xpd mutants exhibited enhanced stomatal lineage cell divisions and elevated SPCH protein levels. Furthermore, XPD acts upstream of CDKA;1, as expression of the dominant-negative CDKA;1.N146 allele significantly suppressed the excessive cell division and stomatal development defects in xpd plants. Our data highlight the precise regulation of stomatal development by XPD, expanding its critical TFIIH-independent roles in plant cell division and fate specification.