Size of chloroplasts in Arabidopsis mesophyll cells affects jasmonate biosynthesis

Chloroplasts are highly dynamic organelles whose morphology responds to developmental and environmental cues, yet whether organellar architecture directly influences metabolic capacity remains unclear. Using Arabidopsis arc3 and arc5 mutants harbouring defective division machinery resulting in giant chloroplasts, we investigated how chloroplast morphology is linked to the biosynthesis of jasmonates, such as jasmonic acid (JA) and jasmonoyl-isoleucine (JA-Ile). Quantitative three-dimensional analysis using confocal laser scanning microscopy, jasmonate determination after wounding, galactolipid profiling and ultrastructural analysis were performed to characterize arc mutants relative to wild type, the chloroplast positioning mutant chup1 and the chloroplast movement mutant kac1/2. Upon wounding, arc mutants accumulated higher levels of JA and JA-Ile than the wild type, whereas levels of galactolipids enriched in α-linolenic acid, the primary fatty acid substrate for jasmonate biosynthesis, were reduced. Giant chloroplasts in arc mutants possessed loosely organized thylakoid membranes with expanded stromal regions. Moreover, protein abundance of the JA biosynthetic enzyme allene oxide cyclase was increased in arc mutants. Enhanced jasmonate production resulted in a stronger mechanostimulation response in these mutants: Compared to wild type, arc mutants exhibited a more pronounced flowering delay under repeated mechanical stimulation. The combination of altered membrane architecture and increased enzyme abundance likely underlies enhanced jasmonate production. Collectively, these findings identify chloroplast morphology as a previously unrecognized factor associated with jasmonate biosynthesis and suggest a link between organellar architecture and hormone biosynthesis during plant stress adaptation.