Are phloem-derived amino acids the origin of the elevated malate concentration in the xylem sap following mineral N starvation in soybean?

Autor(es)

Vitor, Simone C. ^[1] ; do Amarante, Luciano ^{[1, 2]} ; Sodek, Ladaslav ^[1]

Afiliação do(s) autor(es)

^[1] Univ Campinas UNICAMP, Inst Biol, Dept Plant Biol, POB 6109, BR-13083970 Campinas, SP - Brazil

^[2] Univ Fed Pelotas, Dept Bot, CP 354, BR-96160000 Pelotas, RS - Brazil

• Tipo de documento: Artigo Científico
• Fonte: PLANTA; v. 248, n. 2, p. 437-449, AUG 2018.
• Citações Web of Science: 3

Resumo

A substantial increase in malate in the xylem sap of soybean subjected to mineral N starvation originates mainly from aspartate, a prominent amino acid of the phloem. A substantial increase in xylem malate was found when non-nodulated soybean plants were transferred to a N-free medium. Nodulated plants growing in the absence of mineral N and, therefore, dependent on symbiotic N-2 fixation also contained elevated concentrations of malate in the xylem sap. When either nitrate or ammonium was supplied, malate concentrations in the xylem sap were low, both for nodulated and non-nodulated plants. Evidence was obtained that the elevated malate concentration of the xylem was derived from amino acids supplied by the phloem. Aspartate was a prominent component of the phloem sap amino acids and, therefore, a potential source of malate. Supplying the roots of intact plants with C-13-aspartate revealed that malate of the xylem sap was readily labelled under N starvation. A hypothetical scheme is proposed whereby aspartate supplied by the phloem is metabolised in the roots and the products of this metabolism cycled back to the shoot. Under N starvation, aspartate metabolism is diverted from asparagine synthesis to supply N for the synthesis of other amino acids via transaminase activity. The by-product of aspartate transaminase activity, oxaloacetate, is transformed to malate and its export accounts for much of the elevated concentration of malate found in the xylem sap. This mechanism represents a new additional role for malate during mineral N starvation of soybean, beyond that of charge balance. (AU)

• Processo FAPESP: 13/03325-8 - Papel dos ácidos orgânicos no metabolismo e reciclagem de N na raiz da soja
• Beneficiário: Simone Cespedes Vitor
• Modalidade de apoio: Bolsas no Brasil - Doutorado