# Chloroplasts at the Crossroad of Photosynthesis, Pathogen Infection and Plant Defense
## 2 The Chloroplast is a Major Synthesis Site for Many Plant Hormones
### Methionine, the Precursor for ET Biosynthesis, is Made in the Chloroplast  
ET is a two-carbon gaseous plant hormone. ET biosynthesis is a three-step process: (1) the conversion of methionine (Met) to S-adenosyl Met (SAM) by SAM synthetase; (2) the conversion of SAM to 1-aminocyclopropane-1-carboxylic acid (ACC) by ACC synthase; and (3) the conversion of ACC to ET by ACC oxidase [6]. Although ET itself is not produced in the chloroplast, Met, the precursor of ET biosynthesis, is made in the chloroplast. There are three cobalamin-independent Met synthases (MSs) in _Arabidopsis_[102]. MS3 is located in the chloroplast and is required for Met generation from homocysteine synthesized _de novo_ in chloroplasts [102]. MS1 and MS2 are present in the cytosol and  
Figure 1: Interactions among defense-related signals. Red arrow heads represent positive (promoting) effects; blue bars represent negative (inhibitory) effects; grey lines with red arrow heads and blue bars represent both positive and negative effects. This is not an exhaustive presentation of all defense-related signals, but it shows the major ones discussed in this review. are most likely involved in Met regeneration from homocysteine made during the activated methyl cycle [102]. ET is another important regulator of plant growth and development, abiotic stresses, and biotic stresses [6; 103]. ET may influence photosynthesis by regulating stomatal aperture, although the effects of ET on stomatal movements varied in the literature. ET was reported to modulate stomatal opening in different species when epidermal peels were used [104; 105; 106]. However, ET was found to mediate stomatal closure when intact leaves were used [107; 108; 109]. ET may impact photosynthesis by regulating chlorophyll contents, _chlorophyll a/b-binding protein_ (_CAB_) gene expression, PSI and PSII efficiency, and Rubisco activity, on an age-dependent manner [110]. ET-insensitive _Arabidopsis_ and tobacco mutants displayed reduced chlorophyll contents, lower Rubisco activity, and decreased \(CAB\) expression in juvenile non-senescing leaves [111; 112; 113; 114], suggesting that the basal-level ET perception is required for normal photosynthetic capacity in juvenile non-senescing _Arabidopsis_ and tobacco leaves [110]. However, the opposite is true in mature senescing leaves of ET-insensitive _Arabidopsis_, tomato, and tobacco mutants [111; 115; 116; 117]. These age-dependent responses suggest that ET is needed for normal chlorophyll accumulation in young non-senescing leaves but may promote chlorophyll degradation in mature senescing leaves [110]. It should be noted that ET application on juvenile non-senescing _Arabidopsis_ leaves resulted in reduced chlorophyll levels and reduced \(CAB\) transcript contents [111; 118], indicating that excess amounts of ET inhibits photosynthesis [110]. The influence of ET on photosynthesis is also species-specific [110]. The ACS-deficient maize mutant displayed increased levels of chlorophyll and Rubisco and improved leaf performance [119]. Furthermore, exogenous ET treatment did not affect photosynthesis in maize plants [120; 121; 122]. The roles of ET in plant defense against pathogens are also multifaceted. As described above, ET modulates stomatal closure in intact leaves; therefore, ET is important in blocking the entry of bacterial pathogens via stomata. ET-mediated stomatal closure involves other signaling molecules. For example, ET-induced stomatal closure was found to be dependent on apoplastic H\({}_{2}\)O\({}_{2}\) production by RoboF NADPH oxidase located on the guard cell plasma membrane [109], peroxidase on the guard cell wall [123], and polyamine oxidase in the guard cell nucleus, cytoplasm, and cell wall [124]. Intriguingly, ET was found to inhibit ABA-induced stomatal closure by reducing the \(\cdot\)NO content in _Vicia faba_ guard cells [79]. ET plays different roles in plant defense, depending on pathogen types [6].