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 ϫ 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.Leaf senescence is the final stage in leaf development, and understanding senescence is important not only for purely scientific reasons, but also for practical purposes. Premature senescence leads, for example, to decreased photosynthetic capacity, and consequently lower yield. Senescence is not simply the passive death of a leaf because of aging, but is a tightly controlled process during which cell components are degraded in a coordinated fashion and, when nutrients have been relocated to other parts of the plant body, the cell finally dies (Gan and Amasino, 1997;Nooden et al., 1997). Despite the resemblance with apoptosis of animal cells (Yen and Yang, 1998), a form of programmed cell death, only a few orthologs of genes regulating apoptosis have been found in plants, indicating that there are significant differences between the processes (Koonin and Aravind, 2002). Plant cells respond to some animal apoptosis regulators (e.g. Danon et al., 2000), so there must be common elements between the processes. However, it seems as if plants have developed a unique mode of cell death (Beers, 1997) that, if understood, may give insight into processes that are important for cell integrity and viability. However, very little is known about the details of plant leaf senescence.During the last decade, studies of leaf senescence, focusing especially on Arabidopsis, and other annual species to a lesser extent, have identified a number of senescence-associated genes (SAGs) and cellular mechanisms of senescence have begun to be elucidated, as reviewed by various authors (BuchananWollaston, 1997;Nam, 1997;Quirino et al., 2000). The most obvious visual phenotype of senescen...

Large‐scale identification of leaf senescence‐associated genes

“…Similar to our ®nding in Arabidopsis, the most redundant expressed gene is the metallothionein whose transcript was 13 times more abundant in the autumn leaves as compared to young leaves (Bhalerao et al, 2003). In addition to the metallothionein, genes encoding early-light-inducible proteins (ELIP), proteases and components of the ubiquitin degradation pathway are found at the top of the list as the most abundant expressed genes in the aspen autumn leaf.…”

“…A recent genomic study aimed at understanding the molecular program of leaf senescence in autumn leaves of the aspen tree (Populus) has yielded large-scale sequencing and analysis of expressed sequence tags (ESTs) obtained by comparison of two cDNA libraries prepared from autumn and young leaves (Bhalerao et al, 2003). The information obtained from this study allows us to compare and estimate the degree of similarity between gene expressions in the two extreme variations of leaf senescence systems; monocarpic senescence as represented by Arabidopsis and autumn leaves of perennial trees.…”

“…In addition to the metallothionein, genes encoding early-light-inducible proteins (ELIP), proteases and components of the ubiquitin degradation pathway are found at the top of the list as the most abundant expressed genes in the aspen autumn leaf. In general, stress-related proteins are preferentially expressed and abundant in Arabidopsis and other monocarpic senescing leaf (Quirino et al, 1999) and are similarly highly expressed in the autumn leaf of aspen (Bhalerao et al, 2003). One example is the ELIP, which is known to accumulate in thylakoids during not only early stages of light-induced greening but also stress (Binyamin et al, 2001).…”

Overexpression of Nelumbo nucifera metallothioneins 2a and 3 enhances seed germination vigor in Arabidopsis

“…Many reports have documented that plant MTs function similarly as their mammalian counterparts in metal tolerance and homeostasis and controlling ROS balance (reviewed in Cobbett et al 2002;Akashi et al 2004;Mir et al 2004;Xue et al 2009). It has also been shown that plant MTs are involved in plant stress tolerance and plant growth and development processes (Bhalerao et al 2003;Guo et 20 al. 2003;Yuan et al 2008;Xue et al 2009;Yang et al 2009).…”

“…Noticeably, many studies have reported that plant MTs acted as protectants against oxidative stresses imposed by a dverse environmental conditions (Akashi et al 2004;Mir et al 2004;Wong et al 2004;Xue et al 2009;Samardzic et al 2010). In addition, other studies have highlighted the importance of plant MTs in plant growth and development, such as seed development 15 and germination (Brkljacic et al 2004;Yuan et al 2008), tissue senescence (Bhalerao et al 2003;Guo, Bundithya & Goldsbrough 2003), fruit ripeness (Clendennen & May 1997) and root development (Yuan et al 2008). For example, the expression of most Arabidopsis MT genes was dramatically increased in senescing siliques and leaves (Guo et al 2003).…”