Howard Sid Thomas | Senescence and genes

Senescence and genes
	Genes make proteins and proteins make cells and cells make tissues and tissues make organs and organs make whole plants. The best way to watch genes at work during senescence is to look inside the senescing cell and see what’s happening. Here we’re interested primarily in the senescence of foliage, so we’ll focus on the cells of the major green tissue of leaves, called mesophyll. Mesophyll cells do not normally change in number or size during senescence. But inside these cells are organelles , structures that do change, and in a distinctive way.
	< The characteristic organelles of mesophyll cells are the plastids. For a very nice interactive view of a typical green plant cell, go here. During senescence the plastids are transformed from the green organelles we know as chloroplasts (responsible for photosynthesis and the green colour of leaves) into structures within the senescent cell that have been called gerontoplasts. Senescence is when recycling happens. Most of the proteins, lipids, DNA and RNA that get dismantled for relocation from senescing mesophyll cells to new cells and tissues elsewhere are part of the fabric of the chloroplasts. Gerontoplasts are chloroplasts in the process of mobilising their structures and contents into forms suitable for recycling.
	< To make a gerontoplast from a chloroplast requires a lot of new biochemistry which in turn means a lot of new genes become active.
	< As well as the genes that make the recycling machinery, there are regulatory genes that determine when the chloroplast-to-gerontoplast transition begins in plant development, how fast it proceeds and how the various cell processes interact and integrate.
	< We know there must be specific genes responsible for plastid senescence because there are mutants in which the transformation from chloroplast to gerontoplast is partly or completely blocked.
	< Senescence genes have also been identified by genetic mapping – a way of relating variation in particular traits (such as yellowing, or protein recycling) to particular regions of the plant’s DNA.
	< SAGs (senescence-associated genes) are specifically activated during senescence. They can be isolated using molecular biology methods and identified by DNA sequencing. By comparing sequences with those of known genes in DNA databases, senescence genes can be sorted into different functional categories. Because it’s clearly under the direct control of specific genes with particular functions, senescence is regarded as being genetically programmed.
	< This, in turn, suggests senescence could be re-programmed. There are many good agricultural, environmental or economic reasons why you might want to do this.
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