Active ROP2 triggers leaf senescence and orchestrates the growth-senescence trade-off under nitrogen starvation

Nitrogen (N) deficiency-induced leaf senescence is a genetically programmed process that facilitates plant adaptation to nutrient-limited conditions. Although numerous transcription factors (TFs) involved in N deficiency-induced leaf senescence have been identified, how they are regulated remains largely unknown. Here, we discovered that a plant-specific small GTPase, RHO of Plant 2 (ROP2), acts as an upstream positive regulator that bridges N starvation to leaf senescence by modulating key leaf senescence-associated TFs. Plants expressing constitutively active ROP2 (CA-ROP2) promote cotyledon early senescence under N-deficient conditions, by inducing the expression of key senescence-promoting NAC family TFs (ORE1, NAP, ANAC005), while strongly suppressing the expression of these TFs' inhibitory regulators (HASTY and NLA). On the contrary, CA-ROP2 suppresses the expression of the negative regulation TFs of N limitation-induced leaf senescence, like WRKY53 and NIGT1. The two opposing regulatory axes of CA-ROP2 coordinately accelerate premature cotyledon senescence under N starvation. Furthermore, phenotypic and time-series transcriptomic analysis revealed that CA-ROP2, relative to the wild type (WT), maintains shoot growth and elevated expression of growth- and development-related genes during early N starvation, yet later induces premature cotyledon senescence and drives a pronounced transcriptional switch to upregulate senescence-associated genes. Together, ROP2 converts from a "growth promoter" to a "senescence-initiating regulator" depending on environmental nitrogen status, unveiling a novel mechanism by which plants balance growth and senescence to adapt to nutrient stress.