Molecular Mechanisms for LOT1 Regulation of Plant Drought Stress Tolerance

Drought is a common adverse environmental condition that seriously reduces crop productivity worldwide. Plants lose water mainly through transpiration; accordingly, efficient control of transpiration is a major strategy for plants to increase their drought tolerance. Revealing the genetic basis underlying transpiration efficiency will thus be essential for molecular breeding or genetic engineering of hardy crop varieties. To identify genes that control transpiration, we conducted large-scale genetic screens using an improved infrared imaging method to isolate mutants in Arabidopsis with altered transpiration patterns. The first gene we identified is a novel gene we named LOT1 (Lower Temperature 1). The leaves of lot1 mutant plants are cooler than those of the wild type and they have a much higher transpiration rate. The mutant plants are also more susceptible to drought whereas LOT1-overexpressing plants are significantly more tolerant to the stress than the wild type plants. It is known that upon perceived by its receptors the phytohormone abscisic acid (ABA) will trigger a signaling network leading to the closure of stomata to minimize transpiration. Consistently, the lot1 leaves are insensitive to ABA in inducing stomata closure. ABA signaling involves a group of C2- domain lipid- and Ca2+-binding proteins (CAR) that recruit ABA receptors to the plasma membrane for ABA perception. We recently found that under unstressed conditions, LOT1 interacts with and retains the CAR proteins in the nucleus (a CAR- ‘parking’ state), but ABA (via Ca2+) disrupts this interaction and the released CAR proteins then move to the plasma membrane. Nonetheless, the mechanisms for LOT1 regulation of CAR trafficking and function are unknown. Given that LOT1 protein harbors both an ubiquitination-related domain and a poly-ADP-ribosylation (PARylation) polymerase regulatory domain and interacts with proteins in both of these pathways, we hypothesize that LOT1 may involve in PARylation and ubiquitination of CAR and other ABA signaling proteins to regulate their trafficking, targeting, function, and degradation. In this proposal, we will test this hypothesis with the following three objectives: 1) Isolating the LOT1-containing complexes to uncover the full range of LOT1-mediated signaling pathways. 2) Investigating LOT1-dependent ubiquitination and PARylation of CAR and other ABA signaling proteins and the biological significance of these posttranslational modifications. 3) Revealing the relationship between PARylation and ubiquitination of CAR and other ABA signaling components. The present study will not only uncover novel mechanisms of stress and hormonal signaling in plants, it will also have immediate implications for breeding and bioengineering drought-tolerant crop plants.