AKR-deficiency disturbs the balance of some signal transduction pathways in Arabidopsis thaliana

Abstract

High-intensity light induces anthocyanin production in wild-type Arabidopsis leaves, but this induction is largely abolished in the chlorotic leaf tissues of AKR (ankyrin repeat-containing protein)-deficient plants. The steady-state mRNA levels of three anthocyanin biosynthetic genes, CHI, CHS and DFR, did not increase in response to high-intensity light treatment in chlorotic leaf tissues, whereas they increased several fold in green leaf tissues. There is a good correlation between anthocyanin production and transcript levels of anthocyanin biosynthetic genes, especially DFR, in green leaf tissues. In contrast, the transcripts of TCH2 and TCH3 that encode for calmodulin-related proteins and GPA that encodes for the α subunit of the trimeric G protein were much higher in chlorotic leaf tissues than those in green leaf tissues. These data suggest that the AKR-deficiency could increase gene expression in one signal transduction pathway and at the same time repress gene expression in another signal transduction pathway, which alludes to a possible mechanism for AKR involvement in chloroplast development.

Introduction

The Arabidopsis nuclear gene AKR was implicated in playing an important role in chloroplast development or chloroplast structure maintenance, because its absence from plant cells (caused by antisense and sense transgenes) can lead to chlorosis [27]. Further studies with the AKR-GUS gene fusion (AKR 5'-sequence fused to the β-glucuronidase gene) indicated that AKR expression coincides with chloroplast development and it occurs only in chloroplast-containing tissues [28], suggesting that AKR might play a regulatory role in the process of chloroplast development from proplastid. However, there is no direct evidence to support the above suggestions.

In an effort to understand the biochemical and physiological consequences of AKR-deficiency-caused chlorosis, we studied the responses of these plants to several stress conditions and examined the gene expression patterns of several signal transduction pathways. We noticed a dramatic difference in the stress-induced production of anthocyanin pigments between chlorotic tissues and green tissues. For example, after chlorotic plants were exposed to a chilly temperature, anthocyanin pigments would accumulate in green tissues, but not in the white tissues of chlorotic plants. Under normal conditions, the anthocyanin production is developmentally regulated in Arabidopsis plants. It was shown that the purple pigmentation transiently occurs on the cotyledon and hypocotyl of young seedling plants, but is not normally visible on mature rosette leaves [17]. That purple color is due to the accumulation of anthocyanins in cotyledon and hypocotyl tissues. These anthocyanin pigments are thought to play a role in the protection of photosynthetic membrane structures from photodamage during early stages of plant development [1], [5], [11]. Anthocyanin production can be induced under several environmental stress conditions [10]. One of the most effective stresses for triggering a high production of anthocyanins in Arabidopsis is high-intensity light irradiation [12], [13], [25]. The reason for the mass production of anthocyanin pigments under high-intensity light is likely due to the activation of gene expression in anthocyanin biosynthetic pathways [12]; however, the molecular mechanism of gene activation under high-intensity light has not been studied in detail.

By studying the AKR-deficient chlorotic plants, we found that the gene expression patterns in response to high-intensity light treatment were altered in the AKR-deficient chlorotic tissues. The high-intensity light could not activate gene expression in the anthocyanin biosynthetic pathway in chlorotic tissues, resulting in no anthocyanin production under this condition. Furthermore, the transcripts of three signaling genes (two encoding for calmodulin-related proteins and one encoding for the α subunit of the trimeric G protein) were increased in chlorotic tissues, suggesting that AKR may play a role in these signal transduction pathways, which opens up new opportunities to explore the in vivo function of the AKR gene.