Approximately 185,000 Gossypium EST sequences comprising >94,800,000 nucleotides were amassed from 30 cDNA libraries constructed from a variety of tissues and organs under a range of conditions, including drought stress and pathogen challenges. These libraries were derived from allopolyploid cotton (Gossypium hirsutum; AT and DT genomes) as well as its two diploid progenitors, Gossypium arboreum (A genome) and Gossypium raimondii (D genome). ESTs were assembled using the Program for Assembling and Viewing ESTs (PAVE), resulting in 22,030 contigs and 29,077 singletons (51,107 unigenes). Further comparisons among the singletons and contigs led to recognition of 33,665 exemplar sequences that represent a nonredundant set of putative Gossypium genes containing partial or full-length coding regions and usually one or two UTRs. The assembly, along with their UniProt BLASTX hits, GO annotation, and Pfam analysis results, are freely accessible as a public resource for cotton genomics. Because ESTs from diploid and allotetraploid Gossypium were combined in a single assembly, we were in many cases able to bioinformatically distinguish duplicated genes in allotetraploid cotton and assign them to either the A or D genome. The assembly and associated information provide a framework for future investigation of cotton functional and evolutionary genomics.
SummaryIn cotton, gossypol and related sesquiterpene aldehydes are present in the glands of aerial tissues and in epidermal cells of roots. A cytochrome P450 was found to be expressed in aerial tissues of glanded cotton cultivars, but not or at an extremely low level in the aerial tissues of a glandless cultivar. Its cDNA was then isolated from Gossypium arboreum L. After expression in Saccharomyces cerevisiae, the P450 was found to catalyse the hydroxylation of (+)-d-cadinene, forming 8-hydroxy-(+)-d-cadinene. This P450 mono-oxygenase has been classi®ed as CYP706B1, and is the ®rst member of the CYP706 family for which a function has been determined. Sesquiterpene aldehydes and CYP706B1 transcripts were detected in roots of both the glanded and glandless cultivars and in aerial tissues of the glanded cultivar. In suspension cultured cells of G. arboreum, elicitors prepared from the phytopathogenic fungus Verticillium dahliae caused a dramatic induction of CYP706B1 expression. The expression pattern of CYP706B1 and the position at which it hydroxylates (+)-d-cadinene suggest that it catalyses an early step in gossypol biosynthesis. Southern blotting revealed a single copy of CYP706B1 in the genome of G. arboreum. CYP706B1 holds good potential for manipulation of gossypol levels in cottonseed via genetic engineering.
Interactions between Gossypium spp. and the bacterial pathogen Xanthomonas campestris pv. malvacearum are understood in the context of the gene-for-gene concept. Reviewed here are the genetic basis for cotton resistance, with reference to resistance genes, resistance gene analogs, and bacterial avirulence genes, together with the physiological mechanisms involved in the hypersensitive response to the pathogen, including production of signaling hormones, synthesis of antimicrobial molecules and alteration of host cell structures. This host-pathogen interaction represents the most complex resistance gene/avr gene system yet known and is one of the few in which phytoalexins are known to be specifically localized in HR cells at anti-microbial concentrations.
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