Auxin acts in xylem-associated or medullary cells to mediate apical dominance

Abstract

A role for auxin in the regulation of shoot branching was described originally in the Thimann and Skoog model, which proposes that apically derived auxin is transported basipetally directly into the axillary buds, where it inhibits their growth. Subsequent observations in several species have shown that auxin does not enter axillary buds directly. We have found similar results in Arabidopsis. Grafting studies indicated that auxin acts in the aerial tissue; hence, the principal site of auxin action is the shoot. To delineate the site of auxin action, the wild-type AXR1 coding sequence, which is required for normal auxin sensitivity, was expressed under the control of several tissue-specific promoters in the auxin-resistant, highly branched axr1-12 mutant background. AXR1 expression in the xylem and interfascicular schlerenchyma was found to restore the mutant branching to wild-type levels in both intact plants and isolated nodes, whereas expression in the phloem did not. Therefore, apically derived auxin can suppress branching by acting in the xylem and interfascicular schlerenchyma, or in a subset of these cells.

Figure 1.

Effect of Reciprocal Grafting between Col and Both axr1-12 and IAAL. Graft designation is scion/rootstock. (A) Visible phenotypes of reciprocal grafts between Col and axr1-12. Plants are Col/Col, Col/axr1-12, axr1-12/Col, and axr1-12/axr1-12 (left to right). (B) Visible phenotypes of reciprocal grafts between Col and IAAL. Plants are Col/Col, Col/IAAL, IAAL/Col, and IAAL/IAAL (left to right). (C) Number of secondary rosette branches developed by grafting between Col and axr1-12 (n = 8 to 16). (D) Number of secondary rosette branches developed by grafting between Col and IAAL (n = 5 to 17). All values are means ± se.

Figure 2.

Patterns of Expression Driven by the rolC and 4CL1 Promoters in Arabidopsis. GUS activity appears as a blue precipitate. (A) Hand-cut section of stem showing rolC-GUS expression in the phloem. (B) Hand-cut section of stem showing 4CL1-GUS expression in the xylem. (C) Fine section of rolC-GUS stem tissue. Staining is present in the primary phloem as well as in a random selection of cortical cells. (D) Fine section of 4CL1-GUS stem showing GUS activity in the parenchymatous cells surrounding the xylem, in the interfascicular schlerenchyma, and irregularly in the pith. (E) rolC-GUS flower showing staining in the anther filaments, style, and pedicel. (F) rolC-GUS root tissue showing expression in the vasculature. Af, anther filaments; C, cortex; I, interfascicular area; P, phloem; Pd, pedicel; T, pith; Tt, transmitting tissue; X, xylem; Xp, xylem parenchymatous cell; Xv, xylem vessel element. Bars in (C) and (D) = 100 μM.

Figure 3.

Total Number of Branches of T1 Transformants after the Termination of Flowering on the Primary Inflorescence. All values are means ± se. For all lines, n = 40, except for BIAn (axr1-12) and GL2-AXR1, for which n = 37.

Figure 4.

Number of Cauline Nodes Developed on T1 Transformants. All values are means ± se. For all lines, n = 40, except for BIAn (axr1-12) and GL2-AXR1, for which n = 37.

Figure 5.

Branching in T3 Homozygous Transformants of axr1-12 Containing the 4CL-AXR1 or rolD-AXR1 Construct after 7 Weeks of Growth under Long-Day Conditions. All values are means ± se. For all lines, n = 15, except for rolD-AXR1(50), for which n = 10.

Figure 6.

Inhibition of Root Growth in axr1-12 Plants Transformed with the 4CL-AXR1 Construct. Plants were germinated for 3 days on ATS and then transferred to ATS supplemented with IAA and grown for another 7 days. Values are means ± se, n = 10 to 12.

Figure 7.

Restoration of the in Vitro Auxin Response of Isolated axr1-12 Nodes to That of the Wild Type by the 4CL1-AXR1 Construct. Nodes were excised from aseptically grown plants after bolting but before bud outgrowth and placed on split plates supplemented with (+NAA) or without (−NAA) 1 μM 1-NAA. For all lines, n = 12 to 15.