Growth- and stress-related defects associated with wall hypoacetylation are strigolactone-dependent

Abstract

Mutants affected in the Arabidopsis TBL29/ESK1 xylan O-acetyltransferase display a strong reduction in total wall O-acetylation accompanied by a dwarfed plant stature, collapsed xylem morphology, and enhanced freezing tolerance. A newly identified tbl29/esk1 suppressor mutation reduces the expression of the MAX4 gene, affecting the biosynthesis of methyl carlactonoate (MeCLA), an active strigolactone (SL). Genetic and biochemical evidence suggests that blocking the biosynthesis of this SL is sufficient to recover all developmental and stress-related defects associated with the TBL29/ESK1 loss of function without affecting its direct effect-reduced wall O-acetylation. Altered levels of the MAX4 SL biosynthetic gene, reduced branch number, and higher levels of MeCLA, were also found in tbl29/esk1 plants consistent with a constitutive activation of the SL pathway. These results suggest that the reduction in O-acetyl substituents in xylan is not directly responsible for the observed tbl29/esk1 phenotypes. Alternatively, plants may perceive defects in the structure of wall polymers and/or wall architecture activating the SL hormonal pathway as a compensatory mechanism.

Figure 1

tbl29S mutation suppresses tbl29‐associated dwarfism, altered plant architecture and collapsed xylem. (a) Growth phenotypes of 4‐week‐old (upper panel) and 6‐week‐old (lower panel) plants. (b) Primary inflorescence heights (cm). (c) Number of rosette branches. (d) Toluidine‐O‐Blue stained cross section of inflorescence stems. (e) Alkali‐released acetate content of wall material (AIR) from stems. Data are represented as the mean of biological replicates (≥15 plants) with standard deviation. Means with different letters are significantly different (Tukey's HSD, p<0.05)

Figure 2

Effect of strigolactone application. (a) Growth phenotypes of 3‐week‐old (upper panel) and 6‐week‐old (lower panel) plants. (b) Primary inflorescence heights (cm). (c) Number of rosette branches. (d) Toluidine‐O‐Blue stained cross section of inflorescence stems. (e) Alkali‐released acetate content of wall material (AIR) from stems. Data are represented as the mean of biological replicates (N=6 plants) with standard deviation. Means with different letters are significantly different (Tukey's HSD, p<0.05)

Figure 3

Quantification of endogenous MeCLA. Endogenous MeCLA was quan1fied by LC‐MS/MS in Col‐0, tbl29, max4‐7, tbl29 max4‐7, max4‐1 and tbl29 max4‐1 plants using D1‐MeCLA as an internal standard. Data are represented as the mean of biological replicates (3‐4) with standard devia1on. Means with different leLers are significantly different (Tukey's HSD, p<0.05). FW, fresh weight. ND, not detectable

Figure 4

tbl29‐associated freezing tolerance is suppressed by max4‐7. (a) 4‐week‐old plants before (upper panel) and 3 days after freezing treatment (lower panel). (b) Survival rate calculated as the percentage of plants alive after treatment. Error bars represent standard deviation (N=3 experiments, 20 plants per experiment)

Figure 5

Proposed model. Defects in xylan acetylation in the secondary wall are perceived by an unknown mechanism triggering the activation on of a SL‐dependent response. In the tbl29 mutant, the continuous perception of a defective wall would lead to a constitutive activation of this signaling, leading to xylem collapse with the corresponding growth/morphological/ stress response defects. Altering the production of SL (as in the SL‐deficient max4 mutant) blocks the signaling preventing the developmental‐ and stress‐related phenotypes