Barley metallothioneins differ in ontogenetic pattern and response to metals

The barley genome encodes a family of 10 metallothioneins ( MTs ) that have not previously been subject to extensive gene expression profiling. We show here that expression of MT1a , MT2b1 , MT2b2 and MT3 in barley leaves increased more than 50‐fold during the first 10 d after germination. Concurrently, the root‐specific gene MT1b1 was 1000‐fold up‐regulated. Immunolocalizations provided the first evidence for accumulation of MT1a and MT2a proteins in planta , with correlation to transcript levels. In developing grains, MT2a and MT4 expression increased 4‐ and 300‐fold over a 28‐day‐period after pollination. However, among the MT grain transcripts MT2c was the most abundant, whereas MT4 was the least abundant. Excess C u up‐regulated three out of the six MT s expressed in leaves of young barley plants. In contrast, most MTs were down‐regulated by excess Zn or C d. Zn starvation led to up‐regulation of MT1a , whereas C u starvation up‐regulated MT2a , which has two copper‐responsive elements in the promoter. Arabidopsis lines constitutively overexpressing barley MT2a showed increased sensitivity to excess C d and Zn but no C u‐induced response. We suggest that barley MTs are differentially involved in intracellular homeostasis of essential metal ions and that a subset of barley MTs is specifically involved in C u detoxification.