Long-Term Inhibition by Auxin of Leaf Blade Expansion in Bean and Arabidopsis

Abstract

The role of auxin in controlling leaf expansion remains unclear. Experimental increases to normal auxin levels in expanding leaves have shown conflicting results, with both increases and decreases in leaf growth having been measured. Therefore, the effects of both auxin application and adjustment of endogenous leaf auxin levels on midrib elongation and final leaf size (fresh weight and area) were examined in attached primary monofoliate leaves of the common bean (Phaseolus vulgaris) and in early Arabidopsis rosette leaves. Aqueous auxin application inhibited long-term leaf blade elongation. Bean leaves, initially 40 to 50 mm in length, treated once with α-naphthalene acetic acid (1.0 mm), were, after 6 d, approximately 80% the length and weight of controls. When applied at 1.0 and 0.1 mm, α-naphthalene acetic acid significantly inhibited long-term leaf growth. The weak auxin, β-naphthalene acetic acid, was effective at 1.0 mm; and a weak acid control, benzoic acid, was ineffective. Indole-3-acetic acid (1 μm, 10 μm, 0.1 mm, and 1 mm) required daily application to be effective at any concentration. Application of the auxin transport inhibitor, 1-N-naphthylphthalamic acid (1% [w/w] in lanolin), to petioles also inhibited long-term leaf growth. This treatment also was found to lead to a sustained elevation of leaf free indole-3-acetic acid content relative to untreated control leaves. Auxin-induced inhibition of leaf growth appeared not to be mediated by auxin-induced ethylene synthesis because growth inhibition was not rescued by inhibition of ethylene synthesis. Also, petiole treatment of Arabidopsis with 1-N-naphthylphthalamic acid similarly inhibited leaf growth of both wild-type plants and ethylene-insensitive ein4 mutants.

Auxin-induced leaf blade expansion in Arabidopsis requires both wounding and detachment

“…10 Here (Fig. 3C) this epinastic effect (measured as the area of the flattened leaf minus the area presented by the leaf unflattened) was particularly dramatic with intact leaves treated with high concentrations of IAA.…”

“…For our experiments we chose to experiment with young soil-grown Arabidopsis seedlings. This system offered the same advantage as young Phaseolus plants used in previous studies 10,14 as the paired first and second true leaves in both plant species develop simultaneously allowing opposite untreated leaves to serve as a within plant control. Unlike Phaseolus, however, the first and second true leaves emerge nearly flat allowing for nondestructive initial sampling of leaf size.…”

“…As leaves expand auxin levels fall. 9,10 It has been suggested that cell expansion in leaves is promoted by auxin only at lower concentrations. 5 Indeed, in the intact plant sustaining high levels of leaf auxin by various means has consistently resulted in inhibition of leaf expansion elevation of leaf auxin (indole-3-acetic acid; Iaa) levels in intact plants has been consistently found to inhibit leaf expansion whereas excised leaf strips grow faster when treated with Iaa.…”

A constant production hypothesis guides leaf venation patterning

“…On one side, application of the hormone appears to accelerate growth, e.g., leaf expansion (25) or branching and phyllotaxis (26). On the other, increased concentration of the substance on the leaf due to transport inhibition limits leaf blade expansion (27,28). Our model predicts both types of experimental results on leaves, thereby removing the dilemma.…”

Pertumbuhan dan Perkembangan Jaringan Meristem Bawang Merah (Allium ascalonicum L.) Kultivar Katumi Secara In Vitro

“…Keadaan ini diduga disebabkan peningkatan konsentrasi NAA dapat menghambat pertumbuhan daun (Keller et al, 2004), sedangkan penggunaan sitokinin dengan konsentrasi tinggi yang dikombinasikan dengan auksin konsentrasi rendah sangat penting dalam pembentukan daun (Hartmann et al, 1997). Daun baru yang terbentuk terlihat lebih tebal dan hijau dibandingkan dengan perlakuan tanpa kenetin dan NAA.…”