Interactions between auxin and strigolactone in shoot branching control

Abstract

In Arabidopsis (Arabidopsis thaliana), the carotenoid cleavage dioxygenases MORE AXILLARY GROWTH3 (MAX3) and MAX4 act together with MAX1 to produce a strigolactone signaling molecule required for the inhibition of axillary bud outgrowth. We show that both MAX3 and MAX4 transcripts are positively auxin regulated in a manner similar to the orthologous genes from pea (Pisum sativum) and rice (Oryza sativa), supporting evolutionary conservation of this regulation in plants. This regulation is important for branching control because large auxin-related reductions in these transcripts are associated with increased axillary branching. Both transcripts are up-regulated in max mutants, and consistent with max mutants having increased auxin in the polar auxin transport stream, this feedback regulation involves auxin signaling. We suggest that both auxin and strigolactone have the capacity to modulate each other's levels and distribution in a dynamic feedback loop required for the coordinated control of axillary branching.

Figure 1.

Auxin dose response of MAX3, MAX4, and IAA1 expression in wild-type (WT) and axr1-3 basal cauline internodes. IAA was applied in a lanolin ring around the primary bolt of 5-week-old plants 15 mm above the base, and the cauline tissue below the application site was collected 3 h post application. Expression is shown relative to wild-type controls for each gene; data are means of two biological pools of six plants ± se. Asterisks indicate significant differences between axr1 and the wild type.

Figure 2.

A, MAX3 expression in response to auxin-depleting treatments in various tissues of 5-week-old wild-type (WT) plants relative to intact hypocotyls. Plants were intact, treated with 3 mg mL⁻¹ NPA in a lanolin ring around the primary bolt 15 mm above the base (NPA-treated bolt) or around the top of the hypocotyl (NPA-treated hypocotyl), or decapitated (Decap) 15 mm above the base of the primary inflorescence with or without young rosette leaves removed. “Bolt” refers to the basal 1 cm of primary cauline stem below the treatment site. B to D, MAX3 (B), MAX4 (C), and IAA1 (D) expression in response to auxin-depleting treatments in the hypocotyls of 3-week-old wild-type and max mutant plants. Plants were intact, treated with 3 mg mL⁻¹ NPA in a lanolin ring around the top of the hypocotyl, or decapitated by removing the entire primary bolt and youngest leaves with or without 19 mm IAA applied to the decapitation site. Tissues were collected 24 h after treatments. Data are means of two to three biological pools of seven to 25 plants ± se. Asterisks indicate significant differences from intact plants per genotype/tissue.

Figure 3.

Rosette branching and gene expression in bdl mutants. A to C, Representative rosette phenotypes of 5-week-old wild-type plants (WT; A), bdl-2 heterozygotes (B), and bdl-2 homozygotes (C). Blue arrows indicate the primary inflorescence stem; white arrows indicate example axillary rosette branches/buds. D, Relative expression levels of MAX3, MAX4, and IAA1 in the basal cauline internodes of 5-week-old bdl-2 homozygotes and heterozygotes. Data are means of two biological pools of eight to 10 plants ± se; asterisks indicate significant differences from the wild type. E, Number of rosette branches 5 mm or longer in reciprocally grafted wild-type plants, max4-1 mutants, and bdl-2 heterozygotes ± se; n = 4 to 10. F, Photographs of representative max4-1 grafted plants (scion/rootstock). G, Number of rosette branches 5 mm or longer per rosette leaf in wild-type plants, max2-1 and max4-1 mutants, and bdl-2 heterozygotes with or without GR24; n = 13 to 21. Data are means ± se; asterisks indicate significant inhibition of branching by GR24. [See online article for color version of this figure.]

Figure 4.

Expression of MAX3, MAX4, and IAA1 in wild-type (WT), axr1-3, max2-1, and axr1-3max2-1 plants. A and B, Expression in the basal cauline internodes (A) and hypocotyls (B) of 5-week-old plants. C, Expression in the hypocotyls of vegetative 2-week-old plants; samples marked with “N” were treated with 3 mg mL⁻¹ NPA around the top of the hypocotyls (note log scale). Data are means of two biological pools of five to 10 plants ± se. Asterisks indicate a significant effect of axr1 on feedback in max2.

Figure 5.

Expression of MAX3, MAX4, and IAA1 in the shoot (A) and the rootstock (B) of grafted wild-type (WT) and max2-1 plants. Expression was measured in vegetative plants 2 weeks after transfer to soil; values are shown relative to wild-type/wild-type (scion/rootstock) rootstocks. Data are means of two biological pools of five to eight plants ± se. Bars with asterisks are significantly different from each other due to a long-distance effect.

Figure 6.

Interactions between auxin and the MAX pathway. Regulatory signals represented in blue were investigated in this study. Step 1, Auxin traveling in the PATS promotes MAX3 and MAX4 expression in an AXR1-dependent manner, prevented if IAA12 is stabilized. Step 2, Auxin-promoted MAX3 and MAX4 transcript levels lead to increased strigolactone production, which leads to reduced bud outgrowth and reduced auxin export from buds into the PATS. Step 3, If strigolactones are low, branching is increased and auxin level increases, feedback up-regulating MAX3 and MAX4 to increase strigolactone levels as in step 1. Step 4, Strigolactone signaling may feedback down-regulate transcript levels independent of auxin; the dashed line indicates weak/putative interaction. Step 5, Auxin also down-regulates cytokinin synthesis, and cytokinin promotes branching. The environment/genotype/developmental program may influence each of these regulatory stages tissue specifically. [See online article for color version of this figure.]