Jill M. Farrant

Genetic programs underlying seed desiccation-tolerance

The genetic programs responsible for DT in (orthodox) seeds must have arisen from the gametophytic and/or sporophytic tissues of mosses and ferns, the bryophyte and/or pteridophyte ancestors of seed plants [2]. Not all seeds are desiccation tolerant and a number of species produce seeds, termed recalcitrant, that are desiccation sensitive. Such species occur in environments in which seeds are released into conditions that are immediately conducive to germination. It has been proposed that as a consequence of evolution within such environments genes for seed DT in such species either have been lost or are permanently repressed [29]. Comparative studies between recalcitrant and orthodox seeds are required to establish the reason for differences in DT. A number of transcriptomic and proteomic studies have been undertaken in order to investigate the genetic program utilized by orthodox seeds upon acquisition of DT during their development [19,20,30,31] One of the most comprehensive transcriptomic studies in seeds has been performed on Medicago trunculata [19]. Transcript profiling of mature dry seeds have revealed highly abundant transcripts encoding, among others, LEAs, sHSPs and peroxiredoxins. Highly expressed functional protein categories included transcripts involved in signal transduction, development, transcriptional regulation, carbohydrate metabolism, storage protein synthesis, lipid metabolism, ‘abiotic stress response’ and photosynthesis. Proteomics analysis has confirmed that many of the proteins encoded by these transcripts are highly abundant in seeds [20,30,31]. In order to differentiate between genes associated with developmental processes and those associated with acquisition of DT, osmotic stress was reinduced in radicles of germinating M. truncatula seedlings by incubation in polyethylene glycol (PEG) and transcripts upregulated during this process were analyzed [19]. The expression of 16086 M. trunculata genes was followed of which 1300 genes were differentially expressed during PEG treatment. These were grouped into several clusters based on the temporal patterning of the data. Early clustered genes encoded proteins required for initial protection, including many of the LEAs and several genes encoding stress and defence proteins, whereas late clusters consisted of genes involved in sucrose synthesis, storage protein production and downregulation of metabolism. The importance of sucrose was confirmed as 4 a fold increase occurred with reestablishment of DT [19]. A functional proteomics strategy for analyzing M. trunculata seeds was also undertaken on the heat stable proteome extracted [20]. Comparative analysis of desiccation-tolerant versus – sensitive imbibed seeds identified 15 proteins showing seed-specific expression, including 6 LEA proteins. We propose that it was this genetic programming in seeds, probably consisting mainly of the early-stage genes involved in protection that were not de-activated during germination and seedling growth that gave rise to the numerous lineages of resurrection plants during angiosperm evolution and radiation.

Extract from: Programming desiccation-tolerance: from plants to seeds to resurrection plants

 
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