**2. Conclusions**

Plant hormones play an important role in the growth and development of plants and also represent an important line of defense against abiotic stress. Hormones change the pattern of growth to enable the plants to withstand stress. The plant stress response involves many hormones, their downstream response factors, associated gene networks, and transcription factors. The crosstalk between hormones and their synergistic or antagonistic interactions play central role in phytohormone-mediated abiotic stress tolerance [165]. Understanding the molecular level interaction between elements of different pathways controlling stress response is critical to allow their manipulation to improve stress tolerance. This is important, as the diversity, duration, and intensity of abiotic stresses are increasing in the changing global climate scenario. Plant hormones are an important target for better management of abiotic stress, especially, in view of the limited success of conventional breeding techniques in dealing with it. Phytohormone pathways and the intermediaries therein can go a long way in the production of climate-resilient crops.

New technologies to bioengineer plants have proven useful in achieving this end; examples include soybean [166], maize [167], rice [168], and potato [169]. Techniques including transcriptome analysis, next-generation sequencing analysis, transgenic plants, genome editing, etc. are being used to identify the hormone-mediated regulatory mechanisms of the plant stress response. Transcriptome analysis using microarrays, a survey of transcriptome profiles, and levels of microRNAs in plants under stress using RNA-seq have helped understand the mechanism of stress tolerance in plants [170]. With genome editing technology, genomes can now be modified in a site-specific manner using specifically designed endonucleases like zinc finger nucleases (ZFN) or TAL effector nucleases (TALEN; [49, 171]) and the CRISPR/CAS system [49, 172].

In a nutshell, new pathways are already emerging. However, the complex interactions between the hormones and their ability to regulate a wide array of plant developmental and physiological processes complicate teasing out the effect of an individual hormone. Lack of information about the tissue-specific stress response and genetic plasticity as well as the extreme complexity of thresholds for different stress responses makes mechanistic understanding of abiotic stress tolerance difficult [173]. In order to better understand the hormone mediated abiotic stress response, the future research should focus on identifying the antagonistic and synergistic interactions between various hormones and the critical regulatory junctures in the hormone crosstalk.
