Here are some examples of plants in which senescence has gone wrong because one or more genes have become altered.

Genetic interference with senescence often results in plants which turn yellow abnormally slowly, or late, or not at all.  Such plants are sometimes called staygreen. 

Staygreen is a useful trait in a number of agricultural and horticultural species, either because it is associated with improved yield or quality, or because the appearance of staygreen plants appeals to consumers.

A. Flageolet beans.  The seeds and pods of this variety do not go yellow like those of normal (wildtype) beans.  Leaves of flageolet beans also remain green when wildtype leaves go yellow.  This tells us that the same gene controlling yellowing is normally active in all green tissues of the plant, and has been inactivated in flageolet.

B. Wildtype and staygreen lines of soybean.  Both pigment loss and the shedding of leaves are delayed in the staygreen type.  Quite a few different staygreen genes are known in soybean, affecting seeds, pods and leaves in different ways.

C. Staygreen and wildtype sorghum.  Staygreen is an important trait in sorghum agriculture, associated with improved yield, drought tolerance and disease resistance in this key crop of developing countries in the dry tropics.

D. Staygreen and wildtype maize.  Extended greenness of foliage has been one of the characters of advanced maize varieties contributing to the huge increase in yields achieved over the last century or so.  Several different types of staygreen are known.  The illustration is of an experimentally-created transgenic line that overproduces an anti-senescence hormone.  Leaves of this line pass directly from green to dead without going through an intervening senescence period.

E. Green and yellow peas.  These have an honoured position in the history of biology because seed colour was one of the seven characters studied by Gregor Mendel (that's his portrait), the father of genetics.  Just like flageolet bean (which probably has the bean version of Mendel's green pea gene), leaf colour follows seed colour.  Incidentally, the picture on this page was taken in Mendel's garden at the Abbey in Brno.

F. Tomato.  Ripening of fruits that start off green and go through a yellowing process (such as banana) followed by development of attractive orange and red colours (tomato, pepper) is essentially a senescence process and many mutants that block the sequence of events are known and available at your greengrocer.

G. Staygreen and wildtype meadow fescue.  This senescence mutant was discovered around 1970 and has kept your humble and obedient servant busy ever since.  We now know which gene has been altered and how, and found that it turned out to be the grass equivalent of Mendel's green/yellow pea gene.  Staygreen has been bred into turf grasses to please gardeners and sports enthusiasts who like their lawns to be green at all times.

H. Arabidopsis is the model species used for plant molecular biology.  Here it has been made into a staygreen by silencing the equivalent of the grass/Mendel gene.  Many other senescence mutants have been discovered and generated by scientists working on Arabidopsis.

You can read more about senescence mutants here:

H Thomas, C M Smart (1993) Crops that stay green. Annals of Applied Biology 123: 193-219

H Thomas, C J Howarth (2000) Five ways to stay green. Journal of Experimental Botany 51: 329-337

PRH Robson, IS Donnison, Kan Wang, B Frame, SEl Pegg, A Thomas, H Thomas (2004) Leaf senescence is delayed in maize expressing the Agrobacterium IPT gene under the control of a novel maize senescence-enhanced promoter. Plant Biotechnology Journal 2: 101-112

I Armstead, I Donnison, S Aubry, J Harper, S Hörtensteiner, C James, J Mani, M Moffet, H Ougham, L Roberts, A Thomas, N Weeden, H Thomas, I King (2007) Cross-species identification of Mendel's I locus. Science 315: 73