Yuki Fujiki

An Arabidopsis Homolog of Yeast ATG6/VPS30 Is Essential for Pollen Germination

Yeast (Saccharomyces cerevisiae) Atg6/Vps30 is required for autophagy and the sorting of vacuolar hydrolases, such as carboxypeptidase Y. In higher eukaryotes, however, roles for ATG6/VPS30 homologs in vesicle sorting have remained obscure. Here, we show that AtATG6, an Arabidopsis (Arabidopsis thaliana) homolog of yeast ATG6/VPS30, restored both autophagy and vacuolar sorting of carboxypeptidase Y in a yeast atg6/vps30 mutant. In Arabidopsis cells, green fluorescent protein-AtAtg6 protein localized to punctate structures and colocalized with AtAtg8, a marker protein of the preautophagosomal structure. Disruption of AtATG6 by T-DNA insertion resulted in male sterility that was confirmed by reciprocal crossing experiments. Microscopic analyses of AtATG6 heterozygous plants (AtATG6/atatg6) crossed with the quartet mutant revealed that AtATG6-deficient pollen developed normally, but did not germinate. Because other atatg mutants are fertile, AtAtg6 likely mediates pollen germination in a manner independent of autophagy. We propose that Arabidopsis Atg6/Vps30 functions not only in autophagy, but also plays a pivotal role in pollen germination.

Novel families of vacuolar amino acid transporters

Amino acids are compartmentalized in the vacuoles of microorganisms and plants. In Saccharomyces cerevisiae, basic amino acids accumulate preferentially into vacuoles but acidic amino acids are almost excluded from them. This indicates that selective machineries operate at the vacuolar membrane. The members of the amino acid/auxin permease family and the major facilitator superfamily involved in the vacuolar compartmentalization of amino acids have been recently identified in studies using S. cerevisiae. Homologous genes for these transporters are also found in plant and mammalian genomes. The physiological significance in response to nitrogen starvation can now be discussed.

Leucine and its keto acid enhance the coordinated expression of genes for branched-chain amino acid catabolism in Arabidopsis under sugar starvation.

Branched‐chain α‐keto acid dehydrogenase (BCKDH), a multienzyme complex, plays a key role in branched‐chain amino acid catabolism. However, it remains unclear whether expression of each subunit is coordinately regulated in plants, which should be important for the efficient assembly of subunits into a functional multienzyme complex. We show that the transcripts from the Arabidopsis E1α subunit gene accumulated in dark‐adapted leaves and in sugar‐starved suspension cells. These results are complementary to our previous report that the transcripts for the E1β and E2 subunit genes accumulated in sugar‐starved cells. Expression of the E1α gene is likely to be regulated by hexokinase‐mediated sugar signaling, indicating that sugar plays a regulatory role in the coordinated expression of BCKDH subunit genes. Furthermore, Leu and its metabolite α‐ketoisocaproate have synergistic effects on the enhanced expression of BCKDH subunit genes under sugar starvation. We hence suggest that branched‐chain amino acids activate their own degradation pathway in sugar‐starved cells through co‐induction of each subunit gene of BCKDH.