Auxin, One Major Plant Hormone, in Soil

Abstract

This chapter describes “auxin, one major plant hormone, in soil,” especially focusing on “outline of plant hormones” including the brief history of auxin, “auxins in soil,” “polar movement of auxin” including recent findings of its molecular mechanisms, “auxin polar transport inhibitors,” and “important role of auxin polar transport and its inhibitors in plant growth and development.” Unlike other plant hormones, auxin shows a specific movement in plant tissues recognized as auxin polar transport. The process of auxin polar transport generates auxin maxima and gradients within tissues that are instrumental in the diverse regulation of various plant developmental processes. Some novel naturally occurring inhibitors of auxin polar transport such as dehydrocostus lactone (decahydro-3,6,9-tris-methylene-azulenol(4,5-b)furan-2(3H)-one) and artabolide (3-hydroxy-4,6,7(H)-germacra-1(10),11(13)-dien-6,12-olide) with α-methylene-γ-lactone moiety and 4-hydroxy-β-thujone (4-hydroxy-4-methyl-1-(1-methylethyl)-bicyclo[3.1.0]hexan-3-one), as well as potent synthetic products such as 2,3,5-triiodobenzoic acid (TIBA), 1-N-naphthylphthalamic acid (NPA), methyl 2-chloro-9-hydroxyfluorene-9-carboxylate (morphactin IT 3456), and 9-hydroxyfluorene-9-carboxylic acid (HFCA) are intensively described. Auxin polar transport inhibitors interfere with basipetal polar transport of auxin and change auxin distribution, and in consequence perturb plant growth and development. NPA, TIBA, morphactin IT 3456, and HFCA have been expected to show various effects on plant growth and development. Generally, various effects of auxin polar transport inhibitors have been proposed to be caused, at molecular level, by interference with the auxin efflux carrier and/or facilitator(s) located in the polar side of plasma membrane, but detailed mechanism remains still unclear whether or how blockage of auxin efflux by the inhibitors is involved in the effects on physiological and developmental processes in plants.