Convergence of developmental mutants into a single tomato model system: 'Micro-Tom' as an effective toolkit for plant development research

Abstract

Background: The tomato (Solanum lycopersicum L.) plant is both an economically important food crop and an ideal dicot model to investigate various physiological phenomena not possible in Arabidopsis thaliana. Due to the great diversity of tomato cultivars used by the research community, it is often difficult to reliably compare phenotypes. The lack of tomato developmental mutants in a single genetic background prevents the stacking of mutations to facilitate analysis of double and multiple mutants, often required for elucidating developmental pathways.

Results: We took advantage of the small size and rapid life cycle of the tomato cultivar Micro-Tom (MT) to create near-isogenic lines (NILs) by introgressing a suite of hormonal and photomorphogenetic mutations (altered sensitivity or endogenous levels of auxin, ethylene, abscisic acid, gibberellin, brassinosteroid, and light response) into this genetic background. To demonstrate the usefulness of this collection, we compared developmental traits between the produced NILs. All expected mutant phenotypes were expressed in the NILs. We also created NILs harboring the wild type alleles for dwarf, self-pruning and uniform fruit, which are mutations characteristic of MT. This amplified both the applications of the mutant collection presented here and of MT as a genetic model system.

Conclusions: The community resource presented here is a useful toolkit for plant research, particularly for future studies in plant development, which will require the simultaneous observation of the effect of various hormones, signaling pathways and crosstalk.

Figure 1

Phenotype of hormone mutants introgressed into cv. Micro-Tom (MT). Leaf phenotype of MT (A), dgt (B), epi (C), pro (D), gib3 (E) and dpy (F). (G) Reduced gravitropic response and lateral root formation in 10-day old seedlings of the auxin mutant dgt when compared to MT. (H) Severe epinasty of the ethylene overproducer mutant epi in the MT background resulting in a phenotype where stems are hardly observable. (I) When growing in 200 ppm ethrel (an ethylene-releaser), MT seedlings show short roots and hypocotyls with exaggerated hook, a phenotype not observed in the ethylene low sensitive Nr. (J) Nr also shows incomplete ripening, producing yellow fruits. (K) Phenotype of pro with increased stem elongation and navel fruits (L). (M and N) Phenotypes of gib2 (M) and cu3 (N), with severe plant size reduction and leaf expansion inhibition compared to MT. (O) ABA deficiency in notabilis leads to wilting during the hottest hours of the day. (P) Precocious germination (vivipary) in sit seeds within the fruit. A description of hormone alterations involved in each mutant can be found on Table 1. Ruler in (H), (J), (K), (M), (N) and (O) = 15 cm.

Figure 2

Phenotype of photomorphogenic mutants introgressed into cv. Micro-Tom. (A) au, MT and hp1 seedlings grown in the dark (left) or light (right). Note that in the dark, the three genotypes do not differ in hypocotyl length, however, in the light, au appears etiolated and hp1 shows higher de-etiolation than MT. (B) Reduced plant size and dark fruits in hp1. (C) Etiolation in au in the light leads to taller and chlorotic plants. The length of the pipette tip is 8 cm. (D) Anthocyanin accumulation and dark-green pigmentation in hp2 leaves. (E) Anthocyanin accumulation in light-grown hp1 hypocotyls. (F) Increased pigmentation in Ip fruits. (G) Increased pigmentation in hp1 leaves. (H) Increased pigmentation in hp1 fruits is stronger than in Ip (F). (I) Anthocyanin accumulation in atv stems. (J) Absence of visible anthocyanin pigmentation in MT shoots. (K) Decreased chlorophyll pigmentation in both au and yg2 makes the double mutant au yg2 almost albino and lethal (plants usually die before the second pair of true leaves). See Table 2 for a detailed description of gene functions of photomorphogenic mutants.

Figure 3

Seed germination and seedling growth in hormone and photomorphogenic mutants. (A-B) Time (days) to reach 50% germination of seeds in dark (A) or light (B). Final germination percentage is between brackets. (C-D) Hypocotyl and root length of light-grown (open bars) and dark-grown (closed bars) seedlings during 10 days. In (C), numbers between brackets represent the ratio between hypocotyl length in the light and dark. Vertical lines represent standard error (n = 3 × 50 for germination and n = 20 for hypocotyl and root length).

Figure 4

Growth of hormone mutants in the MT background under greenhouse conditions. (A) Growth curves of hormone mutants. (B) Dry weight of shoots (open bars) and roots (closed bars) in 60-day-old plants (n = 7). (C) Schematic representation of a 60-day-old MT plant growing in a 100-mL pot. T-shaped bars represent leaves with five leaflets (except for leaves 1 and 2, which usually have three leaflets) and small circles represent inflorescences with 5-8 flowers. Leaf 9 was highlighted for being the most frequent host of the first inflorescence in MT. This varies between mutants, the most frequent position is shown in the adjacent table with the frequency between brackets. This parameter, as well as time to anthesis in 50% of plants is indicative of developmental alterations that could increase or decrease vegetative development, in opposition to flowering. Note that MT has determinate growth (formation of two consecutive inflorescences) due to the presence of the self-pruning (sp) mutation. None of the introgressed mutants altered this trait in the MT background.

Figure 5

Lines near isogenic to MT harboring wild type alleles for DWARF (D), SELF-PRUNING (SP) and UNIFORM RIPENING (U) genes. (A) MT plant harboring the D allele still shows a reduced size (as compared to common cultivars), but does not show rough leaves and is significantly bigger than MT harboring the d allele. (B) The wild Sp allele in the MT background produces taller plants due to the extended growth of its vegetative apices, which denotes the indeterminate growth habit in opposition to the determinate one conferred by sp. The plants shown are of same age. Note the presence of leaves between inflorescences in Sp. (C) Plants harboring the U allele develop fruits with a green shoulder. The pale bright color of MT fruits is probably a result not only of the u mutation, but also uniform gray-green (ug).