Red Light-Regulated Growth

Abstract

We examined the changes in the levels of indoleacetic acid (IAA), IAA esters, and a 22-kilodalton subunit auxin-binding protein (ABP1) in apical mesocotyl tissue of maize (Zea mays L.) during continuous red light (R) irradiation. These changes were compared with the kinetics of R-induced growth inhibition in the same tissue. Upon the onset of continuous irradiation, growth decreased in a continuous manner following a brief lag period. The decrease in growth continued for 5 hours, then remained constant at 25% of the dark rate. The abundance of ABP1 and the level of free IAA both decreased in the mesocotyl. Only the kinetics of the decrease in IAA within the apical mesocotyl correlated with the initial change in growth, although growth continued to decrease even after IAA content reached its final level, 50% of the dark control. This decrease in IAA within the mesocotyl probably occurs primarily by a change in its transport within the shoot since auxin applied as a pulse moved basipetally in R-irradiated tissue at the same rate but with half the area as dark control tissue. In situ localization of auxin in etiolated maize shoots revealed that R-irradiated shoots contained less auxin in the epidermis than the dark controls. Irradiated mesocotyl grew 50% less than the dark controls even when incubated in an optimal level of auxin. However, irradiated and dark tissue contained essentially the same amount of radioactivity after incubation in [14C]IAA indicating that the light treatment does not affect the uptake into the tissue through the cut end, although it is possible that a small subset of cells within the mesocotyl is affected. These observations support the hypothesis that R causes a decrease in the level of auxin in epidermal cells of the mesocotyl, consequently constraining the growth of the entire mesocotyl. R-causes the elongation rate of the cells in the apical 1 cm of the etiolated maize mesocotyl (e.g. 16, 29) and cell Administration gr;ant (NAGW-297) to M.L.E. Abbreviations: R. red light: ABPI, maize 22-kD subunit auxin-bindinig protein: FR. far-red light: NAA. naphthalene-l-acetic acid: [ H]5-N IAA. tritiated 5-azidoindole-3-acetic acid: SIM. selected ion monitoring. division just below the node (25) to decrease in a fluence-dependent manner. Type I and probably type II phyto-chromes (27) mediate these light responses. How phyto-chrome mediates the decrease in growth is not known, although there are several hypotheses. One hypothesis (12, 30) is that the level of the growth promoting hormone, IAA, decreases in the mesocotyl and there is evidence for such a decrease (6. 11). However, it has also been noted that a decrease in hormone alone can not account entirely for the observed change in growth rate (11, 26). It is also unclear how a decrease in the auxin pool might occur since either decrease in its synthesis or transport (6) or increase in its degradation or conjugation (2) could serve to decrease the free auxin pool. Other hypotheses on the mechanism of light-regulated growth include a role for inhibitors (19), calcium (9), cell wall enzymes (34), cell wall properties (35), and the auxin receptor (16, 33). Although it is likely that each of these molecular changes is involved, it is unclear which of these changes is directly mediated by phytochrome. We have reexamined this problem by looking at the kinetics of the R-induced changes in IAA level, ABP1 level (18, 23), and growth kinetics. Because there is some indirect evidence that ABP1 may mediate auxin effects on cell walls (e.g. 3), it was of interest to determine if changes in its abundance could be a primary step in light-regulated growth. MATERIALS AND METHODS Plant Tissue, Chemicals and Light Treatments Maize (Zea mays L.) caryopses (B73 x Mo 17) (Jacque Seed Co., Lincoln, IL) were soaked in tap water for 2 h, then sown in wet vermiculite and grown in darkness at 26°C for 3.5 days.