page 230. Characteristics Plant Growth Regulators can be of a diverse chemical composition such as gases (ethylene), terpenes (gibberellic acid) or carotenoid derivates (abscisic acid). New York: Plenum Press. Learning Objectives. Describe the roles played by ethylene and nontraditional hormones in plant development; Key Points. Bewley, J. Derek, and Michael Black. A subgroup of protein phosphatase 2C (group A PP2C) or SNF1-related protein kinase 2 (subclass III SnRK2) have been known as major negative or positive regulators of ABA signaling, respectively. It is a conjugate acid of a (+)-abscisate. It is an important sesquiterpenoid plant hormone which acts as a regulator of plant responses to environmental stresses such as drought and cold.