The redox homeostasis of plant cells depends upon a balance between generation of reactive oxygen species (ROS) and quenching of ROS by antioxidants, and this balance influences susceptibility to pathogens and other stresses (Trchounian et al., 2016). Plants commonly produce ROS such as superoxide (O −2 ) and hydrogen peroxide (H 2 O 2 ) in response to biotic and abiotic stresses, resulting in an oxidative response (Suzuki et al., 2012;Trchounian et al., 2016). Depending upon the timing, magnitude, and persistence of ROS accumulation, stress-responsive oxidative responses may be adaptive to the plant or may contribute to symptom development. Prolonged and uncontrolled ROS production can damage cell membranes, nucleic acids, and proteins in plant cells; this results in oxidative stress and can in some cases facilitate pathogen infection in compatible interactions (Demidchik, 2015;Petrov et al., 2015;Rossi et al., 2017). Moreover, rapid and transient ROS accumulation can function to trigger plant immune responses, and is commonly observed in the plant apoplast at the site of infection during incompatible interactions with pathogens (Apel & Hirt, 2004;Lamb & Dixon, 1997). This so-called oxidative burst in the apoplast typically relies on generation of O − 2 by NADPH oxidases at the plasma membrane and conversion of O − 2 to H 2 O 2 by superoxide dismutase; H 2 O 2 may also be generated by cell wall peroxidases (

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 CO 2 levels; in addition, spr2 had significantly thinner leaf blades, which may help maintain normal levels of CO 2 diffusion despite the lower number of stomata. Surprisingly, spr2 also had significantly higher carbon assimilation (A) and maximum quantum efficiency of PSII (F v /F m) 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, CO 2 response curves indicated higher in vivo Rubisco activity (V cmax) 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 (J max). 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.

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