Starvation‐induced expression of autophagy‐related genes in Arabidopsis

Abstract

Background information. Autophagy is a catabolic process for degradation of cytoplasmic components in the vacuolar apparatus. A genome-wide survey recently showed evolutionary conservation among autophagy genes in yeast, mammals and plants. To elucidate the molecular and subcellular machinery responsible for the sequestration and subsequent digestion of intracellular material in plants, we utilized a combination of morphological and molecular methods (confocal laser-scanning microscopy, transmission electron microscopy and real-time PCR respectively).Results. Autophagy in Arabidopsis thaliana suspension-cultured cells was induced by carbon starvation, which triggered an immediate arrest of cell growth together with a rapid degradation of cellular proteins. We followed the onset of these responses and, in this report, provide a clear functional classification for the highly polymorphic autophagosomes by which the cell sequesters and degrades a portion of its own cytoplasm. Quantification of autophagy-related structures shows that cells respond to the stress signal by a rapid and massive, but transient burst of autophagic activity, which adapts to the stress signal. We also monitored the real-time expressions of AtATG3, AtATG4a, AtATG4b, AtATG7 and AtATG8a-AtATG8i genes, which are orthologues of yeast genes involved in the Atg8 ubiquitination-like conjugation pathway and are linked to autophagosome formation. We show that these autophagy-related genes are transiently up-regulated in a co-ordinated manner at the onset of starvation.Conclusions. Sucrose starvation induces autophagy and up-regulates orthologues of the yeast Atg8 conjugation pathway genes in Arabidopsis cultured cells. The AtATG3, AtATG4a, AtATG4b, AtATG7 and AtATG8a-AtATG8i genes are expressed in successive waves that parallel the biochemical and cytological remodelling that takes place. These genes thus serve as early markers for autophagy in plants.

Role of plant autophagy in stress response

“…The transcripts of some AtATG genes are induced during starvation. Arabidopsis autophagy-related genes AtATG4a, AtATG4b, AtATG8a-AtATG8i, AtATG3, and AtATG7 are transiently up-regulated in a coordinated manner at the onset of starvation (Rose et al, 2006). This supports the notion that plant autophagy plays an important role in nutrient recycling, especially under nutrient-starved conditions.…”

“…Upon fusion, the internal vesicle is released into the vacuole (now called an autophagic body) where the degradation of its membrane and internal remnants is completed. The degradation of the cell components recycles the intracellular substrates, making resources available for the cell to survive (Aubert et al, 1996;Rose et al, 2006). Starvation-induced autophagy in BY-2 cells, as in Arabidopsis, is perturbed by bafilomycin A1 and concanamycin A, which are inhibitors of the vacuole-type H + -ATPase, and therefore, prevents vacuolar hydrolase activity.…”

Transcription Analysis of Arabidopsis Membrane Transporters and Hormone Pathways during Developmental and Induced Leaf Senescence

“…Knockout or RNAi mutants of AtATG4a/b, AtATG5, AtATG7, AtATG9, and AtATG18a display accelerated senescence and hypersensitivity to nutrient starvation (Doelling et al, 2002;Hanaoka et al, 2002;Yoshimoto et al, 2004;Thompson et al, 2005;Xiong et al, 2005), indicating that autophagy is not essential for the senescence program but plays an important role in nutrient remobilization. Consistently, in Arabidopsis suspension-cultured cells, several ATG genes are up-regulated at the onset of carbon starvation (Contento et al, 2004;Rose et al, 2006).…”

The ARC1 E3 Ligase Promotes Two Different Self-Pollen Avoidance Traits inArabidopsis

“…In contrast, A. thaliana Sha SCRb-SRKb + Al-ARC1 line 2 contained potential autophagic organelles in the vacuole in 90% of the samples studied ( Figures 9E and 9F, Table 4). These autophagic organelles (membrane enclosed structures) are similar in appearance to the Class 2 and Class 3 autophagic vacuoles described by Rose et al (2006) where the contents are in the process of being digested. Interestingly, in one A. thaliana Sha SCRb-SRKb line 14 sample, we observed what appears to be an autophagic vacuole in the cytoplasm ( Figure 9C), while another sample had both an MVB and a potential phagophore engulfing the cytoplasm ( Figure 9D) in the process of forming an autophagosome (reviewed in Yoshimoto, 2012).…”