Abstract
We examined the function of OsALMT4 in rice (Oryza sativa L.) which is a member of the aluminum-activated malate transporter family. Previous studies showed that OsALMT4 localizes to the plasma membrane and that expression in transgenic rice lines results in a constitutive release of malate from the roots. Here, we show that OsALMT4 is expressed widely in roots, shoots, flowers, and grain but not guard cells. Expression was also affected by ionic and osmotic stress, light and to the hormones ABA, IAA, and salicylic acid. Malate efflux from the transgenic plants over-expressing OsALMT4 was inhibited by niflumate and salicylic acid. Growth of transgenic lines with either increased OsALMT4 expression or reduced expression was measured in different environments. Light intensity caused significant differences in growth between the transgenic lines and controls. When day-time light was reduced from 700 to 300 μmol m-2s-1 independent transgenic lines with either increased or decreased OsALMT4 expression accumulated less biomass compared to their null controls. This response was not associated with differences in photosynthetic capacity, stomatal conductance or sugar concentrations in tissues. We propose that by disrupting malate fluxes across the plasma membrane carbon partitioning and perhaps signaling are affected which compromises growth under low light. We conclude that OsALMT4 is expressed widely in rice and facilitates malate efflux from different cell types. Altering OsALMT4 expression compromises growth in low-light environments.
Highlights
Aluminum-activated malate transporters (ALMT) form a family of anion channels in plants (Sasaki et al, 2004; Punta et al, 2012; Sharma et al, 2016)
The present study examined the biology of OsALMT4 in more detail by examining where it is expressed in rice and how expression is affected by a range of treatments
We showed previously that OsALMT4 is expressed in roots and leaves of rice seedlings but details of the tissue-specific expression is unknown
Summary
Aluminum-activated malate transporters (ALMT) form a family of anion channels in plants (Sasaki et al, 2004; Punta et al, 2012; Sharma et al, 2016). Anion channels are membrane-bound proteins that facilitate the energetically passive transport of anions across membranes. Plant cells generate a negative electrical potential difference across the plasma membrane and the tonoplast such that anion channels generally mediate anion movement from the cytosol to the apoplast or into the vacuole. Anion channels contribute in many basic cellular functions including signaling, charge balancing and controlling membrane potential, substrate compartmentation, osmotic control, turgor regulation and responses to nutrient deficiency and abiotic stress (Barbier-Brygoo et al, 2011). ALMT proteins typically have five to seven transmembrane regions in the N-terminal half and a long hydrophilic C-terminal tail but predictions of secondary structure vary (Motoda et al, 2007; Dreyer et al, 2012)
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