Diversity in plant architecture is largely generated by the post-embryonic regulation of meristem initiation and activity. In a phenomenon known as apical dominance, the active growth of the shoot apical meristem (SAM) exerts significant inhibitory force on the outgrowth of axillary meristems (AMs) into shoot branches. The degree of branching in plants is a determinant of yield in many crop species and is carefully regulated to ensure that plants only branch at specific stages of development or in response to their environment. Apical dominance has been attributed to the action of the hormone auxin, produced in SAM tissues and transported downwards. A second hormone, cytokinin, acts antagonistically to auxin to promote branching. Nonetheless, the exact mechanism by which these hormones operate is still being elucidated and continued research suggested that novel signals are involved. The recent discovery that strigolactones, previously implicated in parasitic weed germination and mycorrhizal associations, are branching inhibitors supports the existence of additional signals controlling branching in plants. In garden pea (Pisum sativum) strigolactones are synthesised by the coordinated action of the carotenoid cleavage dioxygenase (CCD) family enzymes, RMS1 (RAMOSUS1) and RMS5. These are encoded by MAX4 (MORE AXILLARY GROWTH4) and MAX3 in Arabidopsis thaliana respectively. Mutants for MAX genes have increased amounts of auxin travelling in the polar auxin transport stream (PATS) of inflorescence stems but exhibit increased branching that is insensitive to inhibition by this auxin. Two hypotheses for the action of strigolactones have been presented. The first is that strigolactones modulate the levels of auxin transport proteins, preventing axillary buds from establishing an active auxin transport flow into the primary stem, which inhibits growth. The second is that strigolactones act downstream of auxin signalling to inhibit the action of outgrowth-promoters. Consistent with this latter hypothesis, in pea, rice (Oryza sativa) and petunia (Petunia hybrida), the expression of RMS1/MAX4 orthologues is auxin regulated. These genes are also regulated by feedback signalling in strigolactone pathway mutants and this is proposed to involve an additional novel signal. In Arabidopsis, however, research showed that MAX4 is not regulated by feedback or auxin in the shoot and placed doubt on the importance of this regulation for branching control. The strigolactone biosynthetic pathway offers a novel target for the manipulation of plant architecture and yield while controlling the germination of parasitic weed species that are detrimental to agriculture. Therefore, a greater understanding of the pathway and its regulators is beneficial. The majority of the research in this thesis pre-dates the discovery of strigolactones as the RMS/MAX-derived branching inhibitor, yet aimed to clarify the evolutionary conservation and functional importance of the regulation of strigolactone biosynthetic genes by auxin and feedback signalling in Arabidopsis. Quantitative real-time PCR analysis demonstrated that MAX3 and MAX4 are co-ordinately and systemically regulated by auxin and by feedback throughout development. Both auxin and feedback regulation required the AXR1/TIR1 auxin response pathway, which targets Aux/IAA transcriptional repressors for proteasomal degradation. In particular, correct degradation of the Aux/IAA protein IAA12 appears to be necessary for optimal MAX3 and MAX4 expression. Moreover this regulation affects strigolactone-dependent branching inhibition. Therefore it is proposed that auxin inhibits branching, in part, by positively regulating strigolactone synthesis. As feedback requires AXR1, this also suggests that increased auxin level and/or signalling in the PATS in conditions of reduced strigolactone signalling mediates feedback regulation of the strigolactone pathway. Consistent with this, microarray analysis revealed that in addition to the inflorescence, max mutants have increased global auxin-responsive gene expression associated with the PATS in the vegetative stage. The pea RMS1 gene was the first strigolactone pathway gene demonstrated to be auxin-regulated. Sequencing of the RMS1 promoter and comparative bioinformatic analysis with promoters of other strigolactone synthesis genes revealed a number of conserved, putative regulatory cis-elements that could mediate this regulation and cross-talk with additional branching cues. However a 2.5 kb fragment of the RMS1 promoter was not sufficient to drive transcriptional and translational fusions with GFP and the RMS1 coding region in Arabidopsis. The RMS1 coding region driven by the CAMV 35S promoter complemented the max4 mutant but did not affect branching induced by auxin-depleting treatments. Grafting studies with axr1 and iaa12 mutants, and decapitation and auxin-transport inhibition in max4 mutants, demonstrated that auxin signalling has a function in branching control independent from the regulation of strigolactone synthesis genes. Overall, data obtained herein was incorporated into current models for the interaction of the strigolactone pathway with auxin and cytokinin in the control of shoot branching. It is suggested that both strigolactone and auxin have the capacity to regulate the levels or distribution of each other in interlocking feedback loop that intersects with additional developmental, physiological and environmental cues for the precise control of axillary branching in plants.
| Identifer | oai:union.ndltd.org:ADTP/254247 |
| Creators | Alice Hayward |
| Source Sets | Australiasian Digital Theses Program |
| Detected Language | English |
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