This book provides an overview of the recent advancements for plant scientists with a research focus on phytohormones and their responses (nature, occurrence, and functions) in plant cells. This book focuses on the role of phytohormones in biosynthesis, plant sexual reproduction, seed germination and fruit development and ripening. It further highlights the roles of different phytohormones on signaling pathways as well as on photoperiodism/Gravitropism/Thigmotropism. The volume also explores the role of phytohormones in gene expression and plant melatonin and serotonin and covers how plant hormones react in case of stress/defence response (metals/metalloids/pathogen). Last but not least, this volume also discusses phytohormones in the context of new regulatory molecules such as Nitric oxide, hydrogen sulfide, melatonin.