Closing Plant Stomata Requires a Homolog of an Aluminum-Activated Malate Transporter

Abstract

Plant stomata limit both carbon dioxide uptake and water loss; hence, stomatal aperture is carefully set as the environment fluctuates. Aperture area is known to be regulated in part by ion transport, but few of the transporters have been characterized. Here we report that AtALMT12 (At4g17970), a homolog of the aluminum-activated malate transporter (ALMT) of wheat, is expressed in guard cells of Arabidopsis thaliana. Loss-of-function mutations in AtALMT12 impair stomatal closure induced by ABA, calcium and darkness, but do not abolish either the rapidly activated or the slowly activated anion currents previously identified as being important for stomatal closure. Expressed in Xenopus oocytes, AtALMT12 facilitates chloride and nitrate currents, but not those of organic solutes. Therefore, we conclude that AtALMT12 is a novel class of anion transporter involved in stomatal closure.

Cytosolic Nucleotides Block and Regulate the Arabidopsis Vacuolar Anion Channel AtALMT9

“…Because the inhibition of AtALMT9 by ATP in malate-based solutions transform the I-V curve of this channel from monotonic to bell shaped, similar to the one of R-type channels, our data support the idea that AtALMT12/QUAC1 is gated by cytosolic nucleotides and that the typical I-V characteristic of this channel comes from this voltage-dependent block. It has to be noted that in all the situations in which AtALMT12/QUAC1 has been measured ATP was present in significant amounts in the cytosolic solution (5,8). Moreover, the only attempt to remove ATP from the cytosolic solution revealed a change of the bell-shaped I-V characteristic to monotonic one (13).…”

“…The bell shape of the I-V curve results from an abrupt change in channel conductance from positive to negative at very negative membrane potentials. This indicates that the channels start to close and the resulting ion current decrease after a critical membrane potential (5,8). In contrast, the other ALMTs exhibit a monotonic voltage-dependent I-V curve.…”

“…The current-voltage relationship is a fundamental biophysical characteristic of ion transporters providing information on their physiological properties. AtALMT12, which constitutes part of the R-type/QUAC currents in the plasma membrane of guard cells, exhibits a characteristic "bell-shaped" I-V curve (5,8). The bell shape of the I-V curve results from an abrupt change in channel conductance from positive to negative at very negative membrane potentials.…”

Sucrose transporter NtSUT1 confers aluminum tolerance on cultured cells of tobacco (Nicotiana tabacumL.)

“…Protein concentration was determined with a protein assay kit (Bio-Rad Laboratories, Hercules, CA, USA). The resultant crude membrane fractions were used for sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) and immunoblot analysis was performed as described previously (Sasaki et al 2004(Sasaki et al , 2010. Briefly, denatured crude membrane fractions (15 µg for the detection of NtSUT1 and 7.5 µg for the plasma membrane H + -ATPase) were loaded into 10% polyacrylamide gel for SDS-PAGE and electrophoresed to separate proteins, which were blotted to polyvinylidene difluoride membrane by western blotting, following standard protocols.…”

A molecular pathway for CO2 response in Arabidopsis guard cells

“…In addition to SLAC1, Arabidopsis guard cells also express SLAH3, a second S-type anion channel 25 , which is especially important for the extrusion of NO 3 À . In contrast to S-type channels, R-/QUAC-type anion channels may be essential for the efflux of organic acids such as malate, which can accumulate during stomatal opening and be released during closure [26][27][28] .…”