Gene expression in leaves ofArabidopsis thalianainduced to senesce by nutrient deprivation

Abstract

Senescence was induced in leaves of Arabidopsis thaliana by transferring rosettes from a nutrient-sufficient medium to water. A visible gradient of yellowing along the shoot axis became apparent within 5 d. Leaves were grouped into four age-classes, representing fully green (nodes 17-20) through about one-third (nodes 13-16) and one-half (9-12) to more than 80% yellow (5-8). The decrease in chlorophyll and carotenoid content with increasing tissue senescence was accompanied by a loss of total protein and of specific polypeptides identified by electrophoresis and Western blotting. Heterogeneity in the rates at which photosynthetic proteins decreased during senescence could be related to differences in susceptibility to proteolysis conferred by location (soluble proteins are more labile than membrane components) and association with stabilizing components such as pigments. The abundance of senescence-related mRNAs was determined by Northern analysis using heterologous DNA probes and also cDNAs isolated from Arabidopsis by differential library screening. Expression of the plastid gene psbA increased until the most extreme stage of senescence, in contrast to the pattern of the protein it encodes, D1, which was almost undetectable in all but mature green tissue. Transcripts corresponding to two senescence-enhanced cDNAs from Brassica napus, LSC54 and LSC94, were strongly up-regulated in senescing Arabidopsis leaves. Two Arabidopsis cDNAs, LdeVA8 and LdeVA32, detected mRNAs of increasing abundance up to mid-senescence and decreasing thereafter. Another homologous clone, LdeVA43, hybridized with a transcript showing some enhancement in early senescence. Partial base sequences of LdeVA8 and LdeVA32 revealed homologies with genes encoding a metallothienin-like protein and catalase, respectively. LSC54 also encodes a putative metallothionein. The possible significance of the patterns of gene expression in senescing rosette leaves of A. thaliana is discussed.