Warm temperature modifies cell fates to reduce stomata production in Arabidopsis.

Stomatal abundance decrease in Arabidopsis triggered by warm temperature is attributed to PIF4-mediated repression of SPEECHLESS (SPCH) expression. We identified the unknown developmental and transcriptional basis of this adaptive response. We traced stomatal lineages in vivo using cell-identity marker lines and mutants, quantified epidermal traits, and conducted RNA sequencing under oscillating temperatures. Prolonged warm temperature or PIF4-overexpression altered cell fates, inducing diverted stomatal precursors (DPs) that lacked stomatal fate, contributing to stomata reduction. DPs originated from meristemoids that lost SPCH expression, lacked MUTE expression, and exited the cell cycle. Short warm-temperature pulses allowed later recovery of SPCH expression and did not induce DPs or stomata reduction. Comparison of transcriptomes obtained during warm-temperature pulses with stomatal lineage cell-specific profiles identified gene expression changes and contrasted their reversibility. Though at warm temperatures, key stomatal drivers were downregulated, most lineages formed stomata through partly modified transcriptional landscapes that promoted uncommitted cell identities and could include noncanonical pathways. Expression changes in stomatal regulators and cell-fate changes explain lineage progression under fluctuating temperatures. Since short-term temperature oscillations prevail in natural conditions, the requirement of long warm-temperature exposure to trigger DPs would prevent stomata reduction by occasional temperature rises. Promoting uncommitted lineage stages provides flexibility to stomatal development under environmental changes.