WT, Wild type. The expression levels of two salt-responsive genes,OsDREB2AandRab16A, also were monitored by real-time Q-PCR analysis. led to insensitive seed germination, enhanced growth inhibition, and improved salt stress tolerance to NaCl than in untransgenic plants. These results indicate that plant apoplastic proteins may have important roles in the plant salt stress response. Salt stress is one of the most significant abiotic stresses and affects every aspect of plant physiology and metabolism. During salt stress, Na+enters the cells and accumulates to a concentration that induces ionic and osmotic stress in plants. Plant cells respond and adapt to these adverse conditions through signaling networks (Lee et al., 2004). Understanding the signaling pathway of plant salt resistance is important for improving plant salt tolerance, especially for improving agricultural productivity in irrigated land. To study the signaling network of plant salt adaptation, the most AZ 3146 important thing is identification of the components involved. Much effort has been made to discover components or elements of signaling pathways involved in plant salt stress responses. Of all the molecular components and signaling pathways known so far, the best understood signaling pathway in salt ionic stress is the SOS pathway (Zhu, 2002). In addition to the SOS pathway, other signaling pathways and components have also been suggested to be involved in salt osmotic stress signaling. Several plant protein kinases have been found to be activated by osmotic stress (Liu et al., 2000;Zhu, 2002). Osmotic stress can increase the transcription levels of a number of protein kinase genes, including genes for a two-component His kinase, mitogen-activated protein kinase kinase kinase, mitogen-activated protein kinase kinase, and mitogen-activated protein kinase (Munnik et al., 1999;Mikolajczyk et al., 2000;Mizoguchi et al., 2000). Also, membrane phospholipids make up a dynamic system that generates a multitude of signaling molecules in addition to serving important structural roles during stress responses (Munnik et al., 2000;DeWald et al., 2001;Zhu, 2002). Recently, a new Ca2+-dependent membrane-binding protein (AnnAt1) involved in salt stress was identified by a proteomic approach (Lee et al., 2004). To date, most identified salt stress resistance-related signaling AZ 3146 components have been localized to the cytosol or cell membrane. Although it is an important component of the plant cell, the plant apoplast has been ignored in studies of the salt stress response. The apoplast is the portion of the plant cell outside the cell membrane. This region includes the cell walls and intercellular space of the plant (Dietz, 1997). The apoplast, which plays important roles in regulating plant physiological and developmental processes, is not only a barrier but also a linker between the environment and the protoplast; AZ 3146 there are many organic and inorganic molecules, as well as enzymes, present in the plant apoplast. These molecules have crucial functions in plant cell metabolism (Nielsen and Schjoerring, 1998), including responses to pathogen stress (del Carmen Crdoba-Pedregosa et al., 2003;Misas-Villamil and van der Hoorn, 2008), cell division and proliferation, cell differentiation (Takeda et al., 2003), and, especially, responses to drought and salt stress (Brune et al., 1994;Dietz, 1997;Ramanjulu et al., 1999). Recently, there has been increasing evidence that plants use apoplastic peptide signals to regulate different plant developmental and other physiological processes, such as the systemin (Pearce et al., 1991), phytosulfokines (Matsubayashi and Sakagami, 1996), CLAVATA3 (Clark et al., 1997), and S-locus Cys-rich protein (Schopfer et al., 1999). A bioinformatics AZ 3146 approach was taken to discover putative secreted peptides in Arabidopsis (Arabidopsis thaliana); 33,809 open up reading structures (ORFs) had been HBEGF deduced to code for putative secreted peptides, and regarding to microarray details, most of them may be portrayed (Rent and Walker, 2006). To time, however, biological features have already been reported for just a few apoplastic proteins. These protein, likened with the real variety of putative secreted protein forecasted by bioinformatics, will be the suggestion from the iceberg just. Furthermore, the sequencing from the Arabidopsis and grain (Oryza sativa) genomes provides revealed many putative receptor-like kinase genes. The ligands of several of.