Abstract
Land plants must balance CO2 assimilation with transpiration in order to minimize drought stress and maximize their reproductive success. The ratio of assimilation to transpiration is called transpiration efficiency (TE). TE is under genetic control, although only one specific gene, ERECTA, has been shown to regulate TE. We have found that the alpha-subunit of the heterotrimeric G protein in Arabidopsis (Arabidopsis thaliana), GPA1, is a regulator of TE. gpa1 mutants, despite having guard cells that are hyposensitive to abscisic acid-induced inhibition of stomatal opening, have increased TE under ample water and drought stress conditions and when treated with exogenous abscisic acid. Leaf-level gas-exchange analysis shows that gpa1 mutants have wild-type assimilation versus internal CO2 concentration responses but exhibit reduced stomatal conductance compared with ecotype Columbia at ambient and below-ambient internal CO2 concentrations. The increased TE and reduced whole leaf stomatal conductance of gpa1 can be primarily attributed to stomatal density, which is reduced in gpa1 mutants. GPA1 regulates stomatal density via the control of epidermal cell size and stomata formation. GPA1 promoter::beta-glucuronidase lines indicate that the GPA1 promoter is active in the stomatal cell lineage, further supporting a function for GPA1 in stomatal development in true leaves.
Highlights
Land plants must balance CO2 assimilation with transpiration in order to minimize drought stress and maximize their reproductive success
In Arabidopsis (Arabidopsis thaliana), multiple quantitative trait loci associated with transpiration efficiency (TE) have been identified, indicating that TE is under genetic control (Juenger et al, 2005; Masle et al, 2005; McKay et al, 2008)
Quantitative trait loci that affect TE have been identified in Arabidopsis (Juenger et al, 2005; Masle et al, 2005), but only one specific gene, ERECTA, has been shown to regulate TE (Masle et al, 2005)
Summary
Given the involvement of GPA1 in the regulation of stomatal movements and ABA signaling (Wang et al, 2001; Pandey et al, 2006), TE was measured on gpa, gpa, and ecotype Columbia (Col) under soil water conditions equivalent to 90% (ample water) or 30% (drought stress) of the soil water-carrying capacity. ABA inhibition of stomatal opening (albeit wild type for ABA promotion of stomatal closure), gpa mutants had reduced discrimination against 13C in rosette tissue compared with Col, even in this experiment where 25 mM ABA was applied directly to the leaves (P , 0.001 for gpa and P = 0.004 for gpa). Gpa mutants have increased TE and increased d13C (reduced discrimination) compared with Col. Mean TE (A) and mean d13C values of rosette tissue (B) of gpa and Col under ample water (white bars) and drought stress (black bars) conditions are shown. Analysis of dry biomass partitioning between the rosette and the inflorescence showed that gpa mutants allocate less biomass to the inflorescence compared with wild-type plants (Fig. 7C; P = 0.0049 for gpa, P = 0.0004 for gpa14) These data suggest that despite the increased TE of gpa, reduced biomass partitioning to the inflorescence has negative fitness consequences for gpa mutants
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