Changes after Decapitation in Concentrations of Indole-3-Acetic Acid and Abscisic Acid in the Larger Axillary Bud of Phaseolus vulgaris L. cv Tender Green

Abstract

Early changes in the concentrations of indole-3-acetic acid (IAA) and abscisic acid (ABA) were investigated in the larger axillary bud of 2-week-old Phaseolus vulgaris L. cv Tender Green seedlings after removal of the dominant apical bud. Concentrations of these two hormones were measured at 4,6,8,12 and 24 hours following decapitation of the apical bud and its subtending shoot. Quantitations were accomplished using either gas chromatography-mass spectrometry-selected ion monitoring (GC-MS- MATERIALS AND METHODS Plant MaterialsIn each set of experiments Phaseolus vulgaris L. cv Tender Green were grown in 30 x 60 x 10-cm plastic trays, filled with a 1:1 mixture of sand and peat moss, under a 16-h photoperiod at 23°C and 60% RH in a growth chamber (Controlled Environment Systems model PGV 36LT Winnipeg, Man.). Lighting was provided by high-pressure mercury lamps 250 ,uE m-2s-' PPFD. Fourteen days after planting, uniform seedlings were selected on the basis of height, leaf size, and size of both apical and axillary buds. These plants were randomly divided into groups of 60 plants (3 trays of 20 plants). In one-half of the plants the apical bud was removed by decapitation and in the other half the plants were left intact. Trays containing decapitated or intact plants were randomized in the chamber. At 2,4,6,8,12, and 24 h after decapitation the larger axillary bud was excised from each of the 60 plants for each of control and decapitated groups, and immediately immersed in liquid N2. All frozen tissues were lyophilized. Duplicate samples (each from 60 plants) of tissue were taken at each harvest for IAA and ABA analysis by GC-MS-SIM.2 In a second complete experiment axillary buds from another group ofplants were harvested solely for analysis of ABA by ELISA. The harvest times for examining ABA by ELISA were every 0.25 h up to 2 h, then every 0.5 h to 12 h, and hourly thereafter. The number of buds harvested for each ELISA was six, for each of decapitated and control plants.

Auxin Acts in Xylem-Associated or Medullary Cells to Mediate Apical Dominance

“…Consistent with this finding, endogenous auxin levels in axillary buds do not correlate with the degree of bud inhibition. In some species, the level of auxin in buds released from apical dominance remains constant or even increases as they grow out (Hillman et al, 1977;Pilate et al, 1989;Gocal et al, 1991), in contrast to the predictions of the Thimann and Skoog model.…”

“…These data do not preclude an auxin relay model, in which apically derived auxin stimulates de novo biosynthesis of auxin in the stem, which then enters the axillary bud. However, because in some species auxin levels remain constant or even increase in buds as they are released from apical dominance, a more likely explanation is that the site of auxin action is remote from the bud (Hillman et al, 1977;Pilate et al, 1989;Gocal et al, 1991).…”

Strigolactone Acts Downstream of Auxin to Regulate Bud Outgrowth in Pea and Arabidopsis

“…Whatever the precise action of strigolactone in preventing the initial bud release, an outcome is that auxin transport from inhibited buds will be reduced compared with growing buds. Once growing, buds synthesize auxin (Gocal et al, 1991) and its export may enhance vascular connections and nutrient flow to further stimulate the growing bud. Consequently, NPA suppresses continued bud growth (Bennett et al, 2006) but does not suppress the earliest bud growth.…”

Control of Outgrowth and Dormancy in Axillary Buds

“…Many of these isolated genes were homologous ABAinducible genes, e.g. LEA (late embryogenesis abundant protein), rd29B (Yamaguchi-Shinozaki and Shinozaki, 1993), and PsDRM2, indicating that the ABA level of dormant axillary buds was higher than that of growing axillary buds after decapitation (Knox and Wareing, 1984;Gocal et al, 1991). The ABA response element sequence was present in the PsAD1 promoter (Y.…”

“…Radiolabeled auxin applied to the stump is not translocated into the axillary buds. The indoleacetic acid (IAA) level of dormant axillary buds is low and that of the axillary buds after decapitation of the terminal shoot actually increases (Gocal et al, 1991).…”