Abstract
Fatty Acid Desaturase 7 (FAD7) generates polyunsaturated fatty acids, promoting the desaturation of chloroplast membranes; it also provides an essential precursor for the synthesis of jasmonic acid (JA), a phytohormone that can influence plant growth, development, and primary metabolism. This study examined the effects of spr2, a null mutation in SlFAD7, on the growth, morphology, and photosynthetic traits of tomato, Solanum lycopersicum. Although the spr2 mutant had a lower density of stomata than wild type plants, the two genotypes had comparable stomatal conductance, transpiration rates, and intracellular CO2 levels; in addition, spr2 had significantly thinner leaf blades, which may help maintain normal levels of CO2 diffusion despite the lower number of stomata. Surprisingly, spr2 also had significantly higher carbon assimilation (A) and maximum quantum efficiency of PSII (Fv/Fm) than wild type plants at both of the light intensities tested here (220 or 440 µmol m−2 s−1), despite having lower levels of chlorophyll than wild type plants under low light (220 µmol m−2 s−1). Furthermore, CO2 response curves indicated higher in vivo Rubisco activity (Vcmax) in spr2 compared to wild type plants, as well as an enhanced maximum rate of electron transport used in the regeneration of ribulose-1,5-bisphosphate (Jmax). These data indicate that loss of function of FAD7 can enhance the efficiency of both light-dependent and light-independent reactions in photosynthesis. Consistent with this, the spr2 mutant also displayed enhanced growth, with significantly more leaves and a more compact growth habit. In contrast to spr2, another tomato mutant impaired in JA synthesis (acx1) showed no enhancements in growth or photosynthetic efficiency, suggesting that the enhancements observed in spr2 are independent of the effects of this mutation on JA synthesis. These data demonstrate that loss of function of FAD7 can enhance photosynthesis and growth, potentially through its impacts on the chloroplast membranes.
Highlights
Fatty Acid Desaturase 7 (FAD7) is an w-3-desaturase found throughout the plant kingdom that influences the physical properties of chloroplast membranes
In order to investigate the impacts of the spr2 mutation on plant development and photosynthetic efficiency, plant growth measurements in spr2 and wild type plants were taken at four weeks after germination under typical growth chamber conditions (220 μmol m−2 s−1), and chlorophyll content and the maximum quantum efficiency of PSII in dark-adapted leaves (Fv/maximal fluorescence of a dark-adapted leaf (Fm)) were measured with a MultispeQ handheld spectrometer
variable fluorescence of a dark-adapted leaf (Fv)/ Fm was significantly higher in spr2 than in WT (Figure 2B), indicating that the light-dependent reactions of photosynthesis are more efficient in spr2 mutants than in wild type
Summary
Fatty Acid Desaturase 7 (FAD7) is an w-3-desaturase found throughout the plant kingdom that influences the physical properties of chloroplast membranes. Levels of FAD7 activity modulate the relative abundance of trienoic versus dienoic fatty acids that are in membrane lipids This ratio influences the physical characteristics of the lipids, such as their tendency to form a lipid bilayer and the temperature at which they transition from a solid (gel) to a liquid phase within that bilayer (Gounaris et al, 1983; Quinn and Williams, 1983). Through its effects on membrane properties, FAD7 activity could potentially influence photosynthesis, which is dependent upon membrane-associated protein complexes in the chloroplast
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