Ontogenetic Changes in the Transport of Indol-3yl-acetic Acid into Maize Roots from the Shoot and Caryopsis

Abstract

The quantities of endogenous indol-3y1-acetic acid (IAA) in endosperms and scutella of 6-day-old maize seedlings (Zea mays L. cv Giant White Horsetooth) were determined by a fluorimetric method. Endosperms were found to contain 33A nanograms IAA per plant, and scutella 7.5 nanograms IAA per plant. [5-3HIIAA applied to endosperms of 6-day-old seedlings moved into the roots and radioactivity accumulated at the apex of the primary root within 8 hours. Two to 7-day-old seedlings were treated simultaneously with IS3HIIAA in the endosperm and 12-'4C1 IAA on the shoot apex. The patterns of transport into the root were found to change during ontogeny: in successively older plants, transport from the shoot into the roots increased relative to transport from the endosperm into the roots. The auxin required for the growth of maize roots could, therefore, partially be contributed by the shoot and endosperm. Ontogenetic changes in the relative importance of these two supplies could be of significance for the integration of growth and development between shoot and root.It has been suggested (7,(23)(24)(25) (1,2,5,12,15,29). The caryopses therefore represent a potential supply of auxin for the roots and shoots. Radioactive IAA applied to maize endosperms has been shown to be transported into the shoot (3, 9, 11, 21) and into the root (3). In Vicia faba, IAA is transported into the root from the epicotyl and from the seed (28). It has recently been shown (4) that the maize endosperm and coleoptile both influence the growth and gravireaction of primary roots, although the way in which control is achieved is not yet clear.The purposes of the present investigation were first to determine the distribution of free IAA in caryopses of 6-d-old maize seedlings, second to determine whether IAA is transported from the caryopsis into the roots, and third to investigate the relative contributions of IAA supplied by the shoot and caryopsis to the roots of plants of different ages. The last of these questions was to be resolved using a double-label technique whereby shoot-toroot transport and endosperm-to-root transport could be measured simultaneously in the same plants. ' Extraction and Assay of IAA. Five g of lyophilized plant material were finely ground and homogenized for 3 min in 37.5 ml of freshly redistilled methanol using an M.S.E. homogenizer (Measuring and Scientific Equipment, London, U.K.) fitted with an ice bath. The slurry was filtered through a glass-fiber pad on a sintered glass funnel, with suction, and the residue resuspended in 25 ml of methanol, homogenized for 3 min, and filtered. The extraction was repeated and the free extracts pooled. [2-'4C]IAA (2.1 ng; 1500 dpm) (55 Ci mol'; Radiochemical Centre, Amersham, U.K.) were added to permit the estimation oflosses during purification. The same method was used for endosperms and scutella; the quantities of solvents, etc., given above are those used when 5 g dry weight of endosperm were extracted; when scutella (which weigh less per unit) were used, the volumes of ...

Root Growth and 14C‐Translocation into the Roots of Maize (Zea mays L.) as influenced by local Nitrate Supply

“…As not only assimilates but also phytohormones are translocated in the phloem (Ziegler, 1975), particularly auxins from the shoots into the roots (Goldsmith et al, 1974;Murtin et al, 1978;Martin and Elliot, 1984), an increased phloem unloading in the zone of nitrate supply could lead to endogenous shifts in the phytohormone balance responsible for the increased formation of higher order laterals observed in our experiments (Fig. 2).…”

Water stress and indol-3yl-acetic acid content of maize roots

“…Considering the polarity of IAA transport in maize plants (basipetal in the aerial parts and acropetal in the root; Torrey 1976), and that the endosperm appears to be a major source of IAA for the shoot and the young root (Momonoki et al 1983;Martin and Elliott 1984), it was of interest to quantify IAA in excised maize roots after a water stress (1 h) to examine the capacity for local synthesis. Under these conditions, IAA accumulation (free form) was as marked as in intact roots ( Table 2).…”

“…In whole maize plants, IAA has been shown to move into the root tips from the shoot and/or from the endosperm (Torrey 1976;Momonoki et al 1983;Martin and Elliott 1984). From these results, it is generally assumed that most of the IAA present in the maize root plants is imported.…”

Plant–rhizobacteria interactions alleviate abiotic stress conditions

“…2007, and references therein). Auxins, specifically indole acetic acid (IAA), are produced in the plant shoot and transported basipetally to the root tips (Martin & Elliott 1984), where, in low concentrations, they enhance cell elongation, resulting in enhanced root growth. Furthermore, auxin promotes the initiation of lateral roots.…”