Gibberellins
From Purdue Genomics Database Facility
Aldwin Anterola, Southern Illinois Unviversity Carbondale (anterola@siu.edu)
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Evolution of the Gibberellin Biosynthetic Pathway
Gibberellins have been found in lycophytes (Hirano et al. 2007), ferns ( Macmillan 2001) and angiosperms (Macmillan 2001), but not in bryophytes (Hirano et al. 2007). Hence the current view is that the gibberellins may have appeared in the common ancestor of lycophytes and ferns (Hirano et al. 2007; Yasumura et al. 2007). However, the genome sequence of Physcomitrella patens revealed that this moss has at least some of the genes required to produce gibberellins (Anterola and Shanle 2008), which suggests that the pathway may have evolved gradually by duplication and subsequent specialization of pre-existing genes. One evidence that supports this hypothesis is that P. patens has a bifunctional diterpene synthase that can catalyze the first two steps in the gibberellin pathway (Hayashi et al. 2006). In angiosperms, these two steps require two enzymes, copalyl pyrophosphate synthase (CPS) and kaurene synthase (KS) (Sun and Kamiya 1994). Thus, it appears that the ancient gibberellin pathway may have used a bifunctional enzyme, which later duplicated to give rise to two distinct enzymes (Figure 1).
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Bifunctional CPS/KS in Selaginella
The recently sequenced Selaginella genome showed two CPS-like genes which are both very similar to the bifunctional diterpene synthase of P. patens. Based on their sequence similarity, it can be hypothesized that both of these Selaginella genes are bifunctional, rather than each having distinct catalytic activities, such as those found in CPS and KS genes of angiosperms, although this will have to be verified by biochemical characterization of the corresponding proteins. If this proved to be the case, then this is further evidence for the hypothesized evolution of the first step of gibberellins as described above, which I have previously proposed last year in two talks that I gave in the Plant Biology and Botany Joint Congress in Chicago, IL and the International Plant Growth Substances Association Meeting in Puerto Vallarta, Mexico, repectively.
Kaurene Oxidases in Selaginella
There appears to be two kaurene oxidases in Selaginella (421373/424982 and 74427/119177) each having two alleles. This is consistent with my hypothesis regarding the evolutionary expansion of the gibberellin biosynthetic pathway by duplication (and subsequent specialization). In the case of P450s, however, it seems that members of this family (to which kaurene oxidases belong) continue to diverge in the course of plant evolution, giving rise to more and more P450 genes (Nelson, 2006). It is possible in fact that kaurene oxidases gave rise to kaurenoic acid oxidases (KAO), since KAO is not found in mosses (i.e. P. patens). However, my opinion is that KAO has to be investigated in other mosses (not just P. patens) before we can truly say that KAO is absent in mosses. The reason I say this is because KAO appears to be present in liverworts (see David Nelson's homepage on P450).
Kaurenoic Acid Oxidases in Selaginella
There are three putative KAO genes in the Selaginella genome (127900/419505, 86321/115926, 103517/137198) each having two alleles. There are only two KAO genes in A. thaliana and one in rice, so it appears that Selaginella has more KAO genes than angiosperms. I take this as further evidence of the continuing duplication and specialization of P450 genes that occurs during plant evolution It is possible that there could have been more KAO-like genes in the common ancestor of lycophytes and seed-bearing plants but then some of these evolved to become more specialized P450 genes. Indeed you will find more KAO-like genes in Selaginella if you perform an advanced search with "kaurenoic" as query. This is in sharp contrast to P. patens that do not even have a KAO.
Gibberellin Oxidases in Selaginella
In the Selaginella genome, there is one gibberellin 20-oxidase (134937/115673) and at least one GA 3-oxidase (63397), both of which have been shown to be active when heterologously expressed in E. coli (Hirano et al. 2007). There are several putative GA 2-oxidases (about fifteen of them), including the GA20ox-like genes studied by Hirano et al. for which no (GA 20-oxidase) activity was detected. It may be best not to assign any function to these GA oxidase like genes until their activity has been confirmed biochemically since they may in fact have other physiological roles not related to gibberellin biosynthesis. Indeed, my hypothesis is that these 2-oxoglutarate dependent enzymes have been recruited from other pathways that are more ancient that gibberellin biosynthesis.
References
ANTEROLA, A. M. and E. K. SHANLE. 2008. Genomic Insights in Moss Gibberellin Biosynthesis. Bryologist 111 (2), 218-230.
HAYASHI, KAWAIDE, NOTOMI, SAKIGI, MATSUO, and NOZAKI. 2006. Identification and functional analysis of bifunctional ent-kaurene synthase from the moss Physcomitrella patens. FEBS Letters 580: 6175-6181.
HIRANO, K., M. NAKAJIMA, K. ASANO, T. NISHIYAMA, H. SAKAKIBARA, M. KOJIMA, E. KATOH, H. XIANG, T. TANAHASHI, M. HASEBE, J. A. BANKS, M. ASHIKARI, H. KITANO, M. UEGUCHI-TANAKA, and M. MATSUOKA. 2007. The GID1-mediated gibberellin perception mechanism is conserved in the lycophyte Selaginella moellendorffii but not in the bryophyte Physcomitrella patens. Plant Cell 19, 3058-3079.
MACMILLAN, J. 2001. Occurrence of Gibberellins in Vascular Plants, Fungi, and Bacteria. Journal of Plant Growth Regulation 20, 387-442.
SUN, T. P. and Y. KAMIYA. 1994. The Arabidopsis GA1 locus encodes the cyclase ent-kaurene synthetase A of gibberellin biosynthesis. Plant Cell 6, 1509-18.
YASUMURA, Y., M. CRUMPTON-TAYLOR, S. FUENTES, and N. P. HARBERD. 2007. Step-by-Step Acquisition of the Gibberellin-DELLA Growth-Regulatory Mechanism during Land-Plant Evolution. Current Biology 17, 1225-1230.
