A tomato strigolactone-impaired mutant displays aberrant shoot morphology and plant interactions

Abstract

Strigolactones are considered a new group of plant hormones. Their role as modulators of plant growth and signalling molecules for plant interactions first became evident in Arabidopsis, pea, and rice mutants that were flawed in strigolactone production, release, or perception. The first evidence in tomato (Solanum lycopersicon) of strigolactone deficiency is presented here. Sl-ORT1, previously identified as resistant to the parasitic plant Orobanche, had lower levels of arbuscular mycorrhizal fungus (Glomus intraradices) colonization, possibly as a result of its reduced ability to induce mycorrhizal hyphal branching. Biochemical analysis of mutant root extracts suggested that it produces only minute amounts of two of the tomato strigolactones: solanacol and didehydro-orobanchol. Accordingly, the transcription level of a key enzyme (CCD7) putatively involved in strigolactone synthesis in tomato was reduced in Sl-ORT1 compared with the wild type (WT). Sl-ORT1 shoots exhibited increased lateral shoot branching, whereas exogenous application of the synthetic strigolactone GR24 to the mutant restored the WT phenotype by reducing the number of lateral branches. Reduced lateral shoot branching was also evident in grafted plants which included a WT interstock, which was grafted between the mutant rootstock and the scion. In roots of these grafted plants, the CCD7 transcription level was not significantly induced, nor was mycorrhizal sensitivity restored. Hence, WT-interstock grafting, which restores mutant shoot morphology to WT, does not restore mutant root properties to WT. Characterization of the first tomato strigolactone-deficient mutant supports the putative general role of strigolactones as messengers of suppression of lateral shoot branching in a diversity of plant species.

Fig. 1.

(A) Colonization rate by arbuscular mycorrhizal fungi of WT (Solanum lycopersicon cv. M82) and Sl-ORT1 plants. Six replicates were done for each treatment, and in each replicate 10 plants were examined. Means and standard error were determined for all replicates; means of replicates were subjected to statistical analysis by multiple-range test. (B) Examples of arbuscular mycorrhiza hyphal branching following exposure to WT or Sl-ORT1 root exudates, or to GR24 as a positive control or sterile distilled water as a negative control. Two experiments were conducted, with 20 replicates each; in each replicate, branching of at least two spores was examined. Means and standard error were determined for all replicates; means of replicates were subjected to statistical analysis by multiple-range test. Different lowercase letters (a, b) above bars represent significantly different means (P ≤0.05). Dashed arrows point to AMF spores; black arrows point to AMF hyphal branching sites.

Fig. 2.

Results of LC-MS analysis of strigolactones in WT (Solanum lycopersicon cv. M82) and Sl-ORT1 tomato root extracts. Two replicates were performed for each line, WT or Sl-ORT1. Each replicate contained approximately 500 individual plants. One of the replicates is presented in the figure, whereas the second replicate demonstrated similar data. (This figure is available in colour at JXB online.)

Fig. 3.

Morphological analysis of shoots of WT (Solanum lycopersicon cv. M82) and Sl-ORT1 plants. (A) An example of Sl-ORT1 and WT shoots. Arrows denote sites of lateral shoot branching (B) Lateral shoot number per plant, with or without (control) exogenous application of GR24 (0.027 μM). (C) Lateral shoot fresh weight, with or without (control) exogenous application of GR24 (0.027 μM). Number and weight of lateral shoots were measured for eight plants for each WT and Sl-ORT1 strains. The experiment was repeated four times. Means and standard deviations are shown; means of replicates were subjected to statistical analysis by multiple-range test. Different lowercase letters (a, b) above bars represent significantly different means (P ≤0.05). (This figure is available in colour at JXB online.)

Fig. 4.

Morphological analysis of WT (Solanum lycopersicon cv. M82), Sl-ORT1 and hypocotyl grafted plants. (A) Lateral shoot number per plant. (B) Lateral shoot fresh weight. (C) Total (main and lateral) shoot fresh weight. WT/WT: WT_scion-WT_stock; O/O: Sl-ORT1_scion-Sl-ORT1_stock; WT/O: WT_scion-Sl-ORT1_stock; O/WT: Sl-ORT1_scion-WT_stock. Number and weight of lateral shoots were measured for eight plants for each WT and Sl-ORT1 strain. The experiment was repeated three times. Means and standard deviations are shown; means of replicates were subjected to statistical analysis by multiple-range test. Different lowercase letters (a, b, c) above bars represent significantly different means (P ≤0.05).

Fig. 5.

Morphological analysis of WT (Solanum lycopersicon cv. M82), Sl-ORT1 and interstock grafted plants. (A) An example of grafted plant morphology. (B) Lateral shoot number. (C) Lateral shoot fresh weight. (D) Total (main and lateral) shoot fresh weight. WT/WT/WT: WT_scion-WT_interstock-WT_stock; O/O/O: Sl-ORT1_scion-Sl-ORT1_interstock-Sl-ORT1_stock; WT/O/WT: WT_scion-Sl-ORT1_interstock-WT_stock; O/WT/O: Sl-ORT1_scion-WT_interstock-Sl-ORT1_stock. Number and weight of lateral shoots were measured for eight plants for each WT and Sl-ORT1 strain. The experiment was repeated three times. Means and standard deviations are shown; means of replicates were subjected to statistical analysis by multiple-range test. Different lowercase letters (a, b, c) above bars represent significantly different means (P ≤0.05). (This figure is available in colour at JXB online.)

Fig. 6.

Relative transcription level of Sl-CCD7 and Sl-CCD8 in WT (Solanum lycopersicon cv. M82), Sl-ORT1 and interstock grafted plant whole-roots. WT/WT/WT: WT_scion-WT_interstock-WT_stock; O/O/O: Sl-ORT1_scion-Sl-ORT1_interstock-Sl-ORT1_stock; WT/O/WT: WT_scion-Sl-ORT1_interstock-WT_stock; O/WT/O: Sl-ORT1_scion-WT_interstock-Sl-ORT1_stock. The experiment was performed in eight replicates; in each replicate, the resulting amount-of-amplification product values were normalized to that of the WT. Means and standard error were determined from all replicates; means of replicates were subjected to statistical analysis by multiple-range test. Different lowercase letters (a, b) above bars represent significantly different means (P ≤0.05).