Two Members of the Aluminum-Activated Malate Transporter Family, SlALMT4 and SlALMT5, are Expressed during Fruit Development, and the Overexpression of SlALMT5 Alters Organic Acid Contents in Seeds in Tomato (Solanum lycopersicum).
The aluminum-activated malate transporter (ALMT) family of proteins transports malate and/or inorganic anions across plant membranes. To demonstrate the possible role of ALMT genes in tomato fruit development, we focused on SlALMT4 and SlALMT5, the two major genes expressed during fruit development. Predicted proteins were classified into clade 2 of the family, many members of which localize to endomembranes. Tissue-specific gene expression was determined using transgenic tomato expressing the β-glucuronidase reporter gene controlled by their own promoters. Both the genes were expressed in vascular bundles connecting to developing seeds in fruit and in the embryo of mature seeds. Further, SlALMT5 was expressed in embryo in developing seeds in fruit. Subcellular localization of both proteins to the endoplasmic reticulum (ER) was established by transiently expressing the green fluorescent protein fusions in plant protoplasts. SlALMT5 probably localized to other endomembranes as well. Localization of SlALMT5 to the ER was also confirmed by immunoblot analysis. The transport function of both SlALMT proteins was investigated electrophysiologically in Xenopus oocytes. SlALMT5 transported malate and inorganic anions such as nitrate and chloride, but not citrate. SlALMT4 also transported malate, but the results were less consistent perhaps because it did not localize strongly to the plasma membrane. To elucidate the physiological role of SlALMT5 further, we overexpressed SlALMT5 in tomato. Compared with the wild type, overexpressors exhibited higher malate and citrate contents in mature seeds, but not in fruit. We conclude that the malate transport function of SlALMT5 expressed in developing fruit influences the organic acid contents in mature seeds.
Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, Okayama, 710-0046 Japan tsasaki@okayama-u.ac.jp.
Takayuki
T
Sasaki
Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, Okayama, 710-0046 Japan.
Yoshiyuki
Y
Tsuchiya
Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, Okayama, 710-0046 Japan.
Michiyo
M
Ariyoshi
Graduate School of Environmental and Life Science, Okayama University, Tsushima-Naka, Kita-ku, Okayama, 700-8530 Japan.
Ryohei
R
Nakano
Graduate School of Environmental and Life Science, Okayama University, Tsushima-Naka, Kita-ku, Okayama, 700-8530 Japan.
Koichiro
K
Ushijima
Graduate School of Environmental and Life Science, Okayama University, Tsushima-Naka, Kita-ku, Okayama, 700-8530 Japan.
Yasutaka
Y
Kubo
Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, Okayama, 710-0046 Japan.
Izumi C
IC
Mori
Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, Okayama, 710-0046 Japan.
Emi
E
Higashiizumi
Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, Okayama, 710-0046 Japan.
Ivan
I
Galis
Institute of Plant Science and Resources, Okayama University, Chuo 2-20-1, Kurashiki, Okayama, 710-0046 Japan.
Yoko
Y
Yamamoto
D000535:Aluminum / Q000494:pharmacology*; D019343:Citric Acid / Q000378:metabolism; D055724:Electrophysiological Phenomena / Q000187:drug effects; D005638:Fruit / Q000187:drug effects / Q000235:genetics / Q000254:growth & development* / Q000378:metabolism; D018506:Gene Expression Regulation, Plant / Q000187:drug effects; D017343:Genes, Plant; D018551:Solanum lycopersicum / Q000187:drug effects / Q000235:genetics / Q000502:physiology; D008293:Malates / Q000378:metabolism*; D026901:Membrane Transport Proteins; D009928:Organ Specificity / Q000187:drug effects / Q000235:genetics; D027361:Organic Anion Transporters / Q000235:genetics / Q000378:metabolism*; D010802:Phylogeny; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism*; D030821:Plants, Genetically Modified; D012639:Seeds / Q000378:metabolism*; D013347:Subcellular Fractions / Q000187:drug effects / Q000378:metabolism
D008293:Malates; D026901:Membrane Transport Proteins; D027361:Organic Anion Transporters; D010940:Plant Proteins; D019343:Citric Acid; D000535:Aluminum
Aluminum-activated malate transporter; Endomembrane; Fruit development; Organic acid; Seed; Tomato