Abiotic and Biotic Stress Response Crosstalk in Plants

In the course of its evolution, plants have developed mechanisms to cope with and adapt to different types of abiotic and biotic stress imposed by the frequently adverse environment. The biology of a cell or cells in tissues is so complicated that with any given stimulus from the environment, multiple pathways of cellular signaling that have complex interactions or crosstalk are activated; these interactions probably evolved as mechanisms to enable the live systems to respond to stress with minimal and appropriate biological processes. The sensing of biotic and abiotic stress induces signaling cascades that activate ion channels, kinase cascades, production of reactive oxygen species (ROS), accumulation of hormones such as salicylic acid (SA), ethylene (ET), jasmonic acid (JA) and abscisic acid (ABA). These signals ultimately induce expression of specific sub-sets of defense genes that lead to the assembly of the overall defense reaction. In plants, defense response genes are transcriptionally activated by different forms of environmental stress or by pathogens. The induction of expression of defense genes in the response against certain pathogens is further dependent on temperature and humidity, suggesting the existence of a complex signaling network that allows the plant to recognize and protect itself against pathogens and environmental stress. A body of research has shown that calcium and reactive oxygen species are second messengers in the early response to abiotic and biotic stress. For example, cytosolic calcium (Ca2+) levels increase in plant cells in response to various harsh environmental conditions, including pathogen attack, osmotic stress, water stress, cold and wounding. After the increase of Ca2+ concentration in the intracellular space, several simultaneous pathways are activated by calcium-interacting proteins such as Ca2+-dependent protein kinases (CDPKs), calmodulin and calcineurin Blike proteins (CBLs), all proteins with the structural ‘EF-hand’ calcium-binding motif. It is also known that plants respond with an oxidative burst to avirulent microbial intruders or to the previously mentioned abiotic stress factors. In this response, NADPH oxidases generate O2– that is rapidly converted to H2O2. Recent evidence demonstrated that the NADPH oxidases are activated by Ca2+ signatures. ROS are generated by NADPH oxidases