Abstract
The OCP3 gene encodes a transcriptional factor from the Homeobox family. Previous studies have demonstrated that the loss of function in Arabidopsis thaliana, provokes a notable increase in the resistance to necrotrophic fungi such as Botrytis cinerea or Plectosphaerella cucumerina. Furthermore, that resistance is dependent on a correct jasmonic acid (JA)-mediated signaling. JA, together with salicylic acid (SA), has been widely related with the establishment of the plant defense responses. Through a finely tuned balance of the signaling of these two regulators (JA and SA), plants are able to activate the most appropriated defense response depending on the challenging pathogen. Thus, the study of the relationships between JA and SA and the components of these signaling could be a useful tool to dissect the complex signaling network composing the plant immune response.
Through the study of some double mutants, has been demonstrated a new role for OCP3 in the crosstalk between these two regulators (JA and SA) in response to Pseudomonas syringae and Hyaloperonospora arabidopsidis, two biotrophic pathogens. These results also suggest a possible function for OCP3 as a new component of the Induced Systemic Resistance (ISR).
There is some overlap between the response of the plants towards biotic and abiotic stresses. The identification of the common components to these two pathways is a very valuable knowledge to understand how plants adapt to different stress situations. Through a yeast two hybrid screen we identify four proteins that interact with OCP3. All of them are implicated in the ABA signaling, a fitohormone that regulates the response to hydric stress. In the present study, we give several evidences indicating that OCP3 regulates ABA-mediated stomatal closure and thus, drought tolerance. 
Furthermore, by some double mutant characterization, we have demonstrated that while the resistance to necrotrophic fungi associated to the ocp3 mutation is MeJA-dependent but ABA-independent, the drought tolerance is ABA-dependent and MeJA-independent. In sum, OCP3 is acting as a nexus between the adaptive responses of the plants to apparently different stresses such as drought and pathogen infection.