Comparative lndole-3-Acetic Acid Levels in the Slender Pea and Other Pea Phenotypes1

Free indole-3-acetic acid levels were measured by gas chromatography-mass spectrometry in three ultra-tall 'slender' Pisum sativum L. lines differing in gibberellin content. Measurements were made for apices and stem elongation zones of light-grown plants and values were compared with wild-type, dwarf, and nana phenotypes in which internode length is genetically regulated, purportedly via the gibberellin level. lndole-3-acetic acid levels of growing stems paralleled growth rates in all lines, and were high in all three slender genotypes. Growth was inhibited by pchlorophenoxyisobutyric acid, demonstrating the requirement of auxin activity for stem elongation, and also by the ethylene precursor 1-aminocyclopropane-1 -carboxylic acid. It is concluded that the slender phenotype may arise from constant activation of a gibberellin receptor or transduction chain event leading directly or indirectly to elevated levels of indole-3-acetic acid, and that increased indole-3-acetic acid levels are a significant factor in the promotion of stem elongation. In many pea genotypes internode length is correlated with the content of GA1 in the growing region (23). This, however, is not the case in ultra-tall 'slender' peas. The slender phenotype in pea (Pisum sativum L.) is conferred by the gene combination la cry' (17). This phenotype has been described as strongly resembling a dwarf plant treated with a saturating dose of active GA2, e.g. plants display thin elongated internodes, lighter green shoots, parthenocarpy, and reduced leaflet size (22). However, internode elongation is not inhibited by GA-biosynthesis inhibitors (18), and indeed the phenotype is not altered by insertion of the recessive alleles le or na, which inhibit conversion of GA20 to GA1, and the synthesis of all GAs, respectively (7, 8, 22). It has been postulated that the gene combination la cry5 may affect the GA receptor site or some other early step in GA-induced elongation (22). Thus, even in the absence of endogenous GAs, growth and morphogenesis may be affected as if GAs were present and nonlim'This research was supported by National Science Foundation grant No. DCB 88-01880, and, in part, by Hatch grant 185-7408 from the College of Agriculture and Life Sciences at Cornell University. The GC-MS used in this work was funded by NSF equipment grant no. DMB-8505974 and funds from the College of Agriculture and Life Sciences at Cornell University. 2Abbreviations: GA, gibberellin; ACC, 1-aminocyclopropane-lcarboxylic acid; PCIB, p-chlorophenoxyisobutyric acid; SIM, selective ion monitoring; TIBA, triiodobenzoic acid; BHT, butylated hydroxytoluene; R, retention time. iting. In a mutant like slender, then, one might not find a good correlation between stem growth and the levels of extractable hormone(s) which regulates stem elongation (5). Auxin levels are often higher in apices and elongating shoots of tall plants than in dwarfs (e.g. 12, 15, 16, 20). Treatment with GAs also frequently increases IAA levels in growing stems (6, 10, 15, 21, 26) and there is considerable evidence that GA3 can induce increased IAA biosynthesis (10, 11, 13, 26, 28). Thus, ifthe slender phenotype arises from an activated GA-receptor then it is possible that free IAA levels might be elevated through a similar process to GA-increased auxin biosynthesis. We examined IAA contents of the stem elongation region and of apical buds with enclosed meristems (the presumed sites of IAA synthesis) in three light-grown slender pea lines differing dramatically in active GA content. Results were compared with individual lines ofthree other stem length phenotypes in a genetic background closely related to the slender lines. These included a tall line (allele Le) with a high GA, level, a dwarf (allele le) with a low GA, level, and the extreme dwarf 'nana' (allele na) in which GAs are virtually absent. MATERIALS AND METHODS