The roles of threonine deaminase in Nicotiana attenuata
Threonine deaminase (TD), the first enzyme to convert threonine to α-keto butyrate (α-KB) in the isoleucine (Ile) biosynthetic pathway, is up-regulated after herbivore attack and methyl jasmonate (MeJA) elicitation, and constitutively expressed in floral buds in Nicotiana attenuata. To understand these unusual patterns of expression, we silenced TD by virus-induced gene silencing (VIGS) as well as by Agrobacteriummediated transformation. Plants whose TD had been silenced by VIGS had shortened pistils and anthers that reverted to petals. Antisense-oriented TD transgenic plants (asTDS) were stunted in their growth and delayed in their cotyledon opening and leaf senescence, and produced flowers with shortened pistils and anthers that reverted to petals; that is, all tissues in which TD promotor:GUS transformants revealed high TD expression. The seedling phenotypes of asTDS plants were partially recovered by supplementing the germination media with α-KB, Ile, or jasmonic acid-isoleucine (JA-Ile). The shortened pistils, poor pollen production, petaloid anthers, and reduced trichome numbers are defects shared with other JA-signaling mutants. Floral buds had higher-than-usual jasmonic acid (JA) levels but unaltered salicylic acid levels, and reduced α-KB, Ile, JA-Ile, and trypsin proteinase inhibitor levels. Clearly, TD plays various roles in plant development, some of which are likely attributable to TD’s function in supplying Ile for JA-Ile signaling.