**3. Phytohormones crosstalk**

Sessile plants should maintain growth plasticity and adaptation ability to severe environ‐ mental conditions. Stress‐responsive hormones assist in the alteration of cellular dynamics and thus regulating plant growth under stress conditions [3, 46]. The interacting actions among hormone signal transduction cascades are called crosstalk and form a signaling net‐ work [2, 3]. In this case, hormones interact by activating a phosphorylation cascade or a common second messenger. Several phytohormones interact together forming a defense network against environmental stresses such as JA, SA, and ABA which play a crucial role in regulating signaling pathways [3]. Understanding the crosstalk between these phytohor‐ mones and defense signaling pathways helps reveal new important targets for developing host resistance mechanisms [3, 4].

A complex signaling network regulates stomatal closure. ABA regulates gene expression which mediates root growth maintenance and water uptake. ABA interacts with signaling molecules and other phytohormones such as nitric oxide and JA to induce stomatal clo‐ sure, as well as to induce genes controlling response to cytokinin, ethylene or auxin [2, 3]. Furthermore, exogenous treatment of ABA down‐regulated the key cytokinin biosynthetic pathway gene, termed isopentenyltransferase, but up‐regulated genes encoding cytokinin dehydrogenases and oxidases [3, 21]. GA is also included in the hormonal crosstalk in envi‐ ronmental signals [47]. In conclusion, like the potential use of molecular and genetic markers in crop improvement [48–57], phytohormones play crucial roles in development and stress tolerance of crops.
