Abstract
Heterostyly distinct hermaphroditic floral morphs enforce outbreeding. Morphs differ structurally, promote cross-pollination, and physiologically block self-fertilization. In Turnera the self-incompatibility (S)-locus controlling heterostyly possesses three genes specific to short-styled morph genomes. Only one gene, TsBAHD, is expressed in pistils and this has been hypothesized to possess brassinosteroid (BR)-inactivating activity. We tested this hypothesis using heterologous expression in Arabidopsis thaliana as a bioassay, thereby assessing growth phenotype, and the impacts on the expression of endogenous genes involved in BR homeostasis and seedling photomorphogenesis. Transgenic A. thaliana expressing TsBAHD displayed phenotypes typical of BR-deficient mutants, with phenotype severity dependent on TsBAHD expression level. BAS1, which encodes an enzyme involved in BR inactivation, was downregulated in TsBAHD-expressing lines. CPD and DWF, which encode enzymes involved in BR biosynthesis, were upregulated. Hypocotyl growth of TsBAHD dwarfs responded to application of brassinolide in light and dark in a manner typical of plants over-expressing genes encoding BR-inactivating activity. These results provide empirical support for the hypothesis that TsBAHD possesses BR-inactivating activity. Further this suggests that style length in Turnera is controlled by the same mechanism (BR inactivation) as that reported for Primula, but using a different class of enzyme. This reveals interesting convergent evolution in a biochemical mechanism to regulate floral form in heterostyly.
Highlights
Most flowering plants bear flowers with male and female reproductive structures in close physical proximity
We investigated whether TsBAHD encodes, as hypothesized, a BR degrading acyltransferase [11], by employing a bioassay in which TsBAHD was over-expressed in A. thaliana
We show that TsBAHD caused dwarf phenotypes consistent with BR-deficiency
Summary
Most flowering plants bear flowers with male and female reproductive structures in close physical proximity This should lead to an inherent tendency to inbreed, but most angiosperm species have evolved mechanisms that promote outbreeding and/or prevent self-fertilization [1,2]. In species that possess heterostyly, individual plants bear flowers of one of two (distyly) or three (tristyly) floral morphs, between which male and female organs are reciprocally positioned, promoting cross-pollination (Figure 1). These morphological differences are often combined with within-morph biochemical incompatibility, forming a breeding system that acts to prevent selfand intra-morph fertilization in addition to promoting out-crossing. The significance and widespread polyphyletic occurrence of heterostyly has garnered the attentions of many researchers seeking to unravel the evolutionary and genetic mechanisms of the polymorphism
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