Plant Responses to Metal Stress

Plant growth hormones

Plant hormones (also known as phytohormones) are organic substances that regulate plant growth and development. Plants produce a wide variety of hormones, including auxins, gibberellins (GA), abscisic acid (ABA), cytokinins (CK), salicylic acid (SA), ethylene (ET), jasmonates (JA), brassinosteroids (BR), and peptides. A large number of related synthetic chemical compounds are used to regulate the growth of cultivated plants, weeds, and in vitro grown plants and plant cells. These human-made compounds are called plant growth regulators (PGRs).

Plant hormones may be part of a signal-transduction pathway, or their presence may stimulate reactions that are signal and/or causative agents for stress responses (Argueso et al., 2010; Leyser, 2010; Qin et al., 2011). Plant hormones as signal molecules regulate cellular processes in targeted cells locally and when moved to other locations of the plant. They also determine the formation of the root, stem, leaf, and flower and facilitate the shedding of leaves and the development and ripening of fruits. Hormones shape the plant and affect seed growth, time of flowering, sex of flowers, and senescence of leaves and fruits. They affect which tissues grow upward and which grow downward and even plant death. Hormones are vital to plant growth and lacking them, plants would be mostly a mass of undifferentiated cells. Plant hormones play important roles in diverse growth and developmental processes as well as various biotic and abiotic stress responses in plants.

Studies revealed that endogenous regulations (e.g., biosynthesis, transport, redistribution, and conjugation of plant hormones) play a crucial role during the acclimation process against stress (Du et al., 2012; He et al., 2012a; Wilkinson et al., 2012; Krishnamurthy and Rathinasabapathi, 2013; Srivastava et al., 2013). Besides this, exogenous application of plant hormones has also been reported to enhance stress tolerance in plants affected by heavy metals (Quint and Gray, 2006; Koprivova et al., 2008; Gangwar et al., 2011a,b; Peto et al., 2011; Rubio-Wilhelmi et al., 2011; Claeys et al., 2012; Elobeid et al., 2012; Nam et al., 2012; Zhu et al., 2012; Krishnamurthy and Rathinasabapathi, 2013; Srivastava et al., 2013). During the last decade, extensive work has been carried out to understand plant hormone-mediated enhancement in stress tolerance using physiological, biochemical, genetic, molecular, and genomic approaches for crop breeding and management. Next in this chapter, we summarize the roles and mechanisms of auxins, cytokinins, and gibberellic acids in enhancing tolerance of heavy metals including other stresses.

Regards

Joseph Mareddy

Assistant Managing Editor

Journal of Plant Biochemistry and physiology