Abstract
Analysis of mutants with increased branching has revealed the strigolactone synthesis/perception pathway which regulates branching in plants. However, whether variation in this well conserved developmental signaling system contributes to the unique plant architectures of different species is yet to be determined. We examined petunia orthologs of the Arabidopsis MAX1 and MAX2 genes to characterize their role in petunia architecture. A single ortholog of MAX1, PhMAX1 which encodes a cytochrome P450, was identified and was able to complement the max1 mutant of Arabidopsis. Petunia has two copies of the MAX2 gene, PhMAX2A and PhMAX2B which encode F-Box proteins. Differences in the transcript levels of these two MAX2-like genes suggest diverging functions. Unlike PhMAX2B, PhMAX2A mRNA levels change in leaves of differing age/position on the plant. Nonetheless, this gene functionally complements the Arabidopsis max2 mutant indicating that the biochemical activity of the PhMAX2A protein is not significantly different from MAX2. The expression of the petunia strigolactone pathway genes (PhCCD7, PhCCD8, PhMAX1, PhMAX2A, and PhMAX2B) was then further investigated throughout the development of wild-type petunia plants. Three of these genes showed changes in mRNA levels over a development series. Alterations to the expression patterns of these genes may influence the branching growth habit of plants by changing strigolactone production and/or sensitivity. These changes could allow both subtle and dramatic changes to branching within and between species.
Highlights
Vegetative branching involves the production of new growth axes from axillary meristems
PhMAX1 IS ORTHOLOGOUS TO MAX1 (CYP711A1) Degenerate primers designed to conserved regions of AtMAX1 were used to isolate a fragment of PhMAX1 from petunia genomic DNA by PCR
Probe 2 made to a sequence conserved across a wider range of cytochrome P450 genes detected up to three bands in a single digest, suggesting that petunia does have a significant number of cytochrome P450 genes more distantly related to PhMAX1 (Figure 2C)
Summary
Vegetative branching involves the production of new growth axes from axillary meristems. In petunia (Petunia hybrida), branching occurs in two distinct phases (Snowden and Napoli, 2003). During vegetative development branches are generally produced acropetally from nodes 3–8 on the main stem. Basal branching ceases at or before the floral transition, with the axillary meristems above node eight not developing beyond small buds. The petunia inflorescence is produced by a series of sympodial branches, while at the same time additional lateral branches develop in a basipetal wave down the main stem from the node immediately below the first flower (Snowden and Napoli, 2003; Drummond et al, 2009b)
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