Regulation of early tomato fruit development by the diageotropica gene

Abstract

The vegetative phenotype of the auxin-resistant diageotropica (dgt) mutant of tomato (Lycopersicon esculentum Mill.) includes reduced gravitropic response, shortened internodes, lack of lateral roots, and retarded vascular development. Here, we report that early fruit development is also dramatically altered by the single-gene dgt lesion. Fruit weight, fruit set, and numbers of locules and seeds are reduced in dgt. In addition, time to flowering and time from anthesis to the onset of fruit ripening are increased by the dgt lesion, whereas ripening is normal. The dgt mutation appears to affect only the early stages of fruit development, irrespective of allele or genetic background. Expression of members of the LeACS (1-aminocyclopropane-1-carboxylic acid synthase, a key regulatory enzyme of ethylene biosynthesis) and LeIAA (Aux/IAA, auxin-responsive) gene families were quantified via real-time reverse transcriptase-polymerase chain reaction in both dgt and wild-type fruits, providing the first analysis of Aux/IAA gene expression in fruit. The dgt lesion affects the expression of only certain members of both the LeACS and LeIAA multigene families. Different subsets of LeIAA gene family members are affected by the dgt mutation in fruits and hypocotyls, indicating that the DGT gene product functions in a developmentally specific manner. The differential expression of subsets of LeIAA and LeACS gene family members as well as the alterations in dgt fruit morphology and growth suggest that the early stages of fruit development in tomato are regulated, at least in part, by auxin- and ethylene-mediated gene expression.

Figure 1

The dgt mutation affects fruits characteristics irrespective of genetic background. Top, Ripe wild-type tomato fruits (Alisa Craig [AC]). Bottom, Ripe dgt tomato fruits in corresponding genetic backgrounds. Fruits were obtained from the greenhouse experiment.

Figure 2

The dgt mutation delays the onset of reproductive development and reduces fruit size. Fruit diameters were measured three times per week from the time of fruit set until ripeness and increases were plotted against time from planting to ripeness. Values represent means of at least six fruits. Diameters at anthesis (A), breaker (B), and red ripe (R) are shown. —▪—, VFN8, greenhouse; —□—, VFN8, growth chamber; --▪--, dgt ^1-1/VFN8, greenhouse; --□--, dgt ^1-1/VFN8, growth chamber.

Figure 3

Differential expression of LeACS genes during fruit development in wild-type and dgt plants. Fruits of both dgt (dotted lines) and wild-type (solid lines) were harvested at the following stages: 15 DPA, IG, MG, B, O, and R. Expression levels of LeACS transcripts relative to RPL2 were measured via real-time RT-PCR as described in “Materials and Methods.” A, LeACS2. B, LeACS4. C, LeACS6. D, LeACS7. Note different scales.

Figure 4

Differential expression of members of the LeIAA gene family during fruit development in wild-type and dgt plants. Fruits were harvested at the following stages: IG, MG, B, O, and R. A, Gene expression was analyzed by RT-PCR. Total RNA (2.5 μg) was used in the RT reaction in a final volume of 20 μL. The cDNAs generated were subsequently used in a 25-μL PCR reaction in the presence of specific primers for each LeIAA gene as well as the RPL2 control. The RT-PCR products were separated on a 1.5% (w/v) agarose gel stained with ethidium bromide. A representative experiment is shown. B, Presence (+) or absence (−) of LeIAA transcripts at different stages of fruit development in wild-type and dgt tomatoes. Data was obtained as described for A. No cDNA clone has been isolated for LeIAA7. LeIAA9 exhibited very low and erratic expression levels, precluding accurate analysis.

Figure 5

Differential expression of four members of the LeIAA gene family during fruit development in wild-type and dgt plants. Fruits of both dgt (dotted lines) and wild type (solid lines) were harvested at the same stages as those in Figure 3. Expression levels of LeIAA transcripts relative to RPL2 were measured via real-time RT-PCR as described in “Materials and Methods.” A, LeIAA2. B, LeIAA8. C, LeIAA10. D, LeIAA11. Note different scales.

Figure 6

Diagram of gene expression of members of the LeIAA and LeACS gene families in tomato seedlings and fruits. Note the differential developmental specificity in the expression of members of the LeIAA gene family during hypocotyl elongation and fruit development in wild-type and dgt plants. The broken arrows labeled dgt indicate that the expression of the genes in that pathway is either enhanced or lowered by the dgt lesion.