Plant Growth Regulation 21: 183–187, 1997. 183 c 1997 Kluwer Academic Publishers. Printed in the Netherlands. NO –releasing substances that induce growth elongation in maize root segments C.M.C.P. Gouvˆ ea 1 , J.F. Souza 1 , A.C.N. Magalh˜ aes 2 & I.S. Martins 1 1 Departamento de Bioqu´ ımica, 2 Departamento de Fisiologia Vegetal, Universidade Estadual de Campinas, Caixa Postal 6109, Campinas, 13083-970, Brazil ( corresponding author). Received 27 September 1996; accepted in revised form 9 February 1997 Key words: growth elongation, nitric oxide, NO –releasing substances, IAA, nitrite, Zea mays Abstract Root segments of maize were incubated in different solutions containing substances that non-enzymatically release nitric oxide, such as sodium nitrite (SN), sodium nitroprusside (SNP), nitrosoglutathione (NGLU) and nitrosocys- teine (NCYS). We found that all of these substances induced root tip expansion in a dose-dependent manner. The decreasing order of potency for root-induced elongation was: 10 7 M SN, pH 4.5; 10 11 M NCYS, 10 10 M SNP, 10 9 M NGLU and 10 7 M SN, pH 7.0. Nitric oxide scavenger such as methylene blue prevented the elongation induced by NO –releasing substances, but had no effect on indole-3-acetic acid (IAA)-induced cell expansion. Our results suggest that nitric oxide is the putative elongation inducer and that IAA and NO –releasing substances conceivably share common steps in the signal transduction pathway, since both elicited the same plant response. Vanadate, a plasmamembrane ATPase inhibitor, significantly reversed IAA-induced elongation when supplied at 10 M concentration. IAA-induced elongation was strongly enhanced by 10 nM BAY K 8644, an agonist of voltage dependent Ca 2 channels. Promotion of root elongation in the absence of IAA occurred only at higher concentrations of BAY K. Vanadate and BAY K had no influence on the NCYS-induced elongation suggesting that the common steps in the signalling of IAA and NCYS are not at the level of the plasmamembrane. Abbreviations: BAY K 8644, (1,4-Dihydro-2,6-dimethyl-5-nitro-4-[2 -(trifluoromethyl)phenyl]-3-pyridine- carboxylic acid methyl ester); carboxy-PTIO, Carboxy-2-phenyl-4,4,5,5-tertramethylimidazoline-1-0xyl-3-oxide; EGTA, ethylene glycol-bis( -aminoethyl ether) N,N,N ,N -tetraacetic acid; HEPES, N-(2-hydroxymethyl) piperazine-N -2-ethanesulfonic acid; IAA, indole-3-acetic acid; NCYS, nitrosocysteine; NGLU, nitrosoglu- tathione; SN, sodium nitrite; SNP, sodium nitroprusside. 1. Introduction Plant cell expansion is known to be limited by a fairly rigid cell wall so that growth becomes controlled by the rate at which the wall yields and expands in response to turgor pressure [5]. Indeed, the rate of cell elongation is determined by two nearly simultane- ous auxin-modulated physical processes, such as water uptake and irreversible cell wall extension [3]. It has been reported that auxin does not directly govern cell wall loosening, but rather intervenes in both short-term growth events associated with wall acidification and relaxation, and long-term episodes leading to deposi- tion of wall materials [5]. Several experimental evidences has demonstrated that the association of soil-borne bacteria of the genus Azospirullum with wheat roots promotes stimulation of lateral root differentiation [4, 26], a response that has been credited to the production of indole-3-acetic acid by the bacteria [11]. More recently, Bothe et al. [4] and Didonet and Magalh˜ aes [7] suggested that the ion nitrite, produced by the bacteria under semi-anaerobic conditions, play a role in the induction of lateral root formation in wheat roots; similar to the effect of auxin.