Gene expression in autumn leaves

Abstract

Two cDNA libraries were prepared, one from leaves of a field-grown aspen (Populus tremula) tree, harvested just before any visible sign of leaf senescence in the autumn, and one from young but fully expanded leaves of greenhouse-grown aspen (Populus tremula x tremuloides). Expressed sequence tags (ESTs; 5,128 and 4,841, respectively) were obtained from the two libraries. A semiautomatic method of annotation and functional classification of the ESTs, according to a modified Munich Institute of Protein Sequences classification scheme, was developed, utilizing information from three different databases. The patterns of gene expression in the two libraries were strikingly different. In the autumn leaf library, ESTs encoding metallothionein, early light-inducible proteins, and cysteine proteases were most abundant. Clones encoding other proteases and proteins involved in respiration and breakdown of lipids and pigments, as well as stress-related genes, were also well represented. We identified homologs to many known senescence-associated genes, as well as seven different genes encoding cysteine proteases, two encoding aspartic proteases, five encoding metallothioneins, and 35 additional genes that were up-regulated in autumn leaves. We also indirectly estimated the rate of plastid protein synthesis in the autumn leaves to be less that 10% of that in young leaves.

Figure 1

Autumn senescence in free-growing aspen. The pictures were taken at the time of leaf sampling (11.00) during the autumn of 1999.

Figure 2

Amount of extractable RNA from aspen leaves at different dates during the autumn of 1999. Values are means of three different preparations.

Figure 3

Functional classification (according to the UPSC-MIPS classification scheme) of autumn leaf and young leaf ESTs.

Figure 4

RNA gel-blot analysis of mRNA transcripts during senescence. Total RNA was isolated from leaves at different dates during the autumn of 1999 and hybridized with labeled cDNA probes for Lhcb2, polyubiquitin (UBQ), three Cys proteases, and pathogenesis-related protein 1 (PR1).