Converts stearoyl-ACP to oleoyl-ACP by introduction of a cis double bond between carbons 9 and 10 of the acyl chain. Required for the activation of certain jasmonic acid (JA)-mediated responses and the repression of the salicylic acid (SA) signaling pathway. 2 H(+) + O2 + octadecanoyl-[ACP] + 2 reduced [2Fe-2S]-[ferredoxin] = (9Z)-octadecenoyl-[ACP] + 2 H2O + 2 oxidized [2Fe-2S]-[ferredoxin] Binds 2 Fe(2+) ions per subunit. Lipid metabolism; fatty acid metabolism. Homodimer. A number of isoforms are produced. According to EST sequences. Ubiquitously expressed. Positively regulated by LEC1. Up-regulated by jasmomic acid (JA). Increased level of stearate (18:0) and reduced level of oleic acid (18:1) in leaves. Spontaneous cell death, constitutive PR genes expression, accumulation of high levels of salicylic acid (SA), enhanced resistance to Pseudomonas syringae, Peronospora parasitica and Cucumber mosaic virus, and enhanced susceptibility to Botrytis cinerea. Belongs to the fatty acid desaturase type 2 family. Sequencing errors.