Catalyzes the isomerization between 2-isopropylmalate and 3-isopropylmalate, via the formation of 2-isopropylmaleate (PubMed:19597944). Plays an essential role in leucine biosynthesis (PubMed:19597944, PubMed:20663849, PubMed:24608865, PubMed:32612621). Functions in both the biosynthesis of leucine, and in the methionine chain elongation pathway of aliphatic glucosinolate formation (PubMed:24608865). Plays an essential role in female gametophyte development (PubMed:19597944, PubMed:20663849, PubMed:24608865). (2R,3S)-3-isopropylmalate = (2S)-2-isopropylmalate Amino-acid biosynthesis; L-leucine biosynthesis; L-leucine from 3-methyl-2-oxobutanoate: step 2/4. Heterodimer of the large LEUC/IIL1 subunit and the small LEUD (SSU1, SSU2 or SSU3) subunits. A number of isoforms are produced. According to EST sequences. Expressed at low levels in roots, root tips, at the basis of the hypocotyls, and in emerging leaves (PubMed:24608865). In young seedlings, expressed in cotyledon epidermal cells (PubMed:32612621). In hypocotyls, expressed in peripheral cells (PubMed:32612621). In seedling roots, expressed in the epidermis, including root hairs, and throughout the cortex (PubMed:32612621). In rosette leaves, expressed in the upper and lower epidermis (PubMed:32612621). In roots of adult plants, expressed in the root tips and cortex of the mature root enclosing the stele (PubMed:32612621). In flowering stalks, expressed in the epidermis (PubMed:32612621). Expressed in the carpel epidermis (PubMed:32612621). Embryonic lethality when homozygous. Plants silencing SSU1 exhibit stunted growth, narrow pale leaf blades with green vasculature, abnormal leaf adaxial-abaxial patterning, and abnormal flower morphology. Belongs to the LeuD family.