Lam_whole_thesis.pdf (5.12 MB)
Chlorinated auxin in the Fabaceae : distribution, evolutionary origin and genetic aspects
thesisposted on 2023-05-27, 10:00 authored by Lam, HK
The ubiquitous presence in plants of a group of hormones called auxins, which includes indole-3-acetic acid (IAA), 4-chloroindole-3-acetic acid (4-Cl-IAA), indole-3-butyric acid (IBA) and phenylacetic acid (PAA), reflects their pivotal roles in mediating various aspects of plant growth and morphogenesis. However, the level of 4-Cl-IAA greatly exceeds the level of primary auxin, IAA in maturing pea (Pisum sativum) seeds. This highly potent chlorinated auxin, 4-Cl-IAA, and its precursor, chlorinated tryptophan (4-Cl-Trp) are mainly found in seeds but were once thought to be restricted to species of the tribe Fabeae within the Fabaceae. One curious exception was Pinus sylvestris, (Scots pine), which was also reported to contain chlorinated auxin in seeds. The action and activity of halogenating enzymes, however, remains unknown in plants. This thesis reports a comprehensive study on the distribution of the chlorinated auxin in the Fabaceae, by screening key species in the Fabaceae and the genus Pinus. For the first time, 4-Cl-IAA was detected in the reproductive structures of species in the genera Trifolium, Melilotus, Trigonella, Medicago and Ononis, all beyond the tribe Fabeae. Scots pine and four other Pinus species, including P. flexilis, P. pinea, P. radiata and P. parviflora, were retested but none contained detectable levels of 4-Cl-IAA. This supports that 4-Cl-IAA is unique to the Fabaceae in all plant species. The notion that chlorinated auxin is restricted to reproductive structures was supported by the evidence of much higher levels of the hormone in the seeds than vegetative tissues in broad bean (Vicia faba) in this study. No detectable level of 4-Cl-IAA was found in the seeds of the cultivated species Cicer arietinum as well as its wild relative, C. echinospermum, both from the tribe Cicerae, immediately basal to the tribe Fabeae and Trifolieae. Five species (Galega officinalis, Galega bicolor, Parochetus communis, Astragalus propinquus and A. sinicus) from tribes and clades basal or sister to the tribe Cicerae were also found to not contain detectable level of 4-Cl-IAA. The absence of 4-Cl-IAA in these species suggests a single evolutionary origin of 4-Cl-IAA, which can be used as a phylogenetically informative trait within the Fabaceae. Interestingly, the evidence indicates that some Trigonella species do not produce 4-Cl-IAA in their seeds. This loss of chlorinating ability is thought to be a derived state in the genus and unique to this genus within the Trifolieae, shedding light on interspecific relationships within the genus. A previously published putative vanadium dependent haloperoxidase gene sequence was used to generate pea haloperoxidase mutants by Targeting Induced Local Lesions in Genomes (TILLING). No obvious phenotypes were observed among the mutants in relation to 4-Cl-IAA synthesis and hormone levels, suggesting the possibility of gene redundancy.
Rights statementCopyright 2017 the author Chapter 3 appears to be, in part, the equivalent of an accepted manuscript of an article published by Taylor & Francis in Plant signaling and behavior on 14 June 2016, available online: http://www.tandfonline.com/10.1080/15592324.2016.1197467