Altered Expression of a Malate-Permeable Anion Channel, OsALMT4, Disrupts Mineral Nutrition

Abstract

ORCID IDs: 0000-0003-1555-4754 (J.L.); 0000-0003-3009-7854 (M.Z.); 0000-0002-1376-9543 (P.R.R.).Aluminum-activated malate transporters (ALMTs) form a family of anion channels in plants, but little is known about most of its members. This study examined the function of OsALMT4 from rice (Oryza sativa). We show that OsALMT4 is expressed in roots and shoots and that the OsALMT4 protein localizes to the plasma membrane. Transgenic rice lines overexpressing (OX) OsALMT4 released malate from the roots constitutively and had 2-fold higher malate concentrations in the xylem sap than nulls, indicating greater concentrations of malate in the apoplast. OX lines developed brown necrotic spots on the leaves that did not appear on nulls. These symptoms were not associated with altered concentrations of any mineral element in the leaves, although the OX lines had higher concentrations of Mn and B in their grain compared with nulls. While total leaf Mn concentrations were not different between the OX and null lines, Mn concentrations in the apoplast were greater in the OX plants. The OX lines also displayed increased expression of Mn transporters and were more sensitive to Mn toxicity than null plants. We showed that the growth of wild-type rice was unaffected by 100 mM Mn in hydroponics but, when combined with 1 mM malate, this concentration inhibited growth. We conclude that increasing OsALMT4 expression affected malate efflux and compartmentation within the tissues, which increased Mn concentrations in the apoplast of leaves and induced the toxicity symptoms. This study reveals new links between malate transport and mineral nutrition.

Genome-Wide Identification of Aluminum-Activated Malate Transporter (ALMT) Gene Family in Rubber Trees (Hevea brasiliensis) Highlights Their Involvement in Aluminum Detoxification

“…Phylogenetic evolutionary analysis plays a vital role in realizing the function of gene families. In earlier studies, 14 ALMT genes in Arabidopsis were classified into three clusters [41]; 34 soybean ALMT genes that belonged to two main groups were further divided into six subgroups [44]; nine ALMT genes in rice were clustered into five clades [45]; 25 ALMT genes in Malus domestica were classified into seven subgroups [38]; 13 grape ALMT genes were clustered into three clades [46]. These results indicated a large phylogenetic evolutionary classification difference among various plant species.…”

GABA signalling in guard cells acts as a ‘stress memory’ to optimise plant water loss

“…GABA concentration oscillates over diel cycles and increases in response to multiple abiotic and biotic stresses including drought, heat, cold, anoxia, wounding pathogen infection and salinity 23 . ALMT have been implicated in modulating multiple developmental and physiological processes in plants 14, 15, 16, 34, 35, 36 including those underpinning nutrient uptake and fertilization that are affected by GABA 13, 37 . Therefore, our discovery that GABA regulates ALMT to form a novel and physiologically relevant signalling mechanism in guard cells is likely to have broad significance beyond stomata, particularly during plant responses to environmental transitions and stress.…”

The ALMT Gene Family Performs Multiple Functions in Plants

“…Growth studies found that the OX lines were more sensitive to Mn toxicity but more tolerant of B toxicity than null plants. The authors conclude that enhancing OsALMT4 expression affected the compartmentation of malate within the tissues which increased Mn concentrations in the apoplast and caused the toxicity symptoms [39].…”

“…OsALMT4 is widely expressed in rice and encodes a malate-permeable anion channel on the plasma membrane. Enhancing OsALMT4 expression affected the compartmentation of malate within the tissues which increased Mn concentrations in the apoplast and caused the toxicity symptoms of dark brown spots and stripes and occasionally necrotic margins [39].…”

“…The characterised ALMT members show a range of functions including Al resistance [1][2][3][4], stomatal function [5][6][7][8][9][10][11], mineral nutrition [12,13,39], microbe interactions [16,17], fruit acidity [14,15], and seed development [18]. Members which have been characterised are widely distributed in higher plant families of Poaceae, Brassicaceae, Fabaceae, Rutaceae, Rosaceae, Vitaceae and Solanaceas (Table 1).…”