The dominant developmental mutants of tomato, Mouse-ear and Curl, are associated with distinct modes of abnormal transcriptional regulation of a Knotted gene.

The Curl (Cu) and Mouse-ear (Me) mutations of tomato cause two seemingly unrelated developmental syndromes with a wide range of pleiotropic phenotypes. Yet, the distinct morphogenic alterations in shoots, leaves, and inflorescences conferred by the two mutations appear to be caused by unchecked meristematic activity that characterizes dominant mutations in Knotted1 (Kn1)-like genes of monocot plants. We have been unable to separate the two closely linked Cu and Me mutations, and they may lie in the same gene. A homeobox-containing class I Kn1-like gene, TKn2, also maps to the same location. Significantly, the dominant mutations are associated with two aberrant modes of TKn2 transcription. Overexpression of the two in-frame wild-type transcripts of TKn2 is associated with the Cu mutation, whereas misexpression of an abundant and oversized fusion mRNA is associated with the Me mutation. Available molecular evidence strongly suggests that the defective Me-TKn2 transcript is generated via a novel splicing event that merges transcripts of two closely linked genes. The translated fusion product is comprised of most of the 5' end of the adjacent PPi-dependent fructose 6-phosphate phosphotransferase (PFP) transcript spliced in-frame to coding position 64 of the TKn2 transcript, leaving the TKn2 homeobox intact. We suggest that class I Kn1-like genes were selected early during evolution to regulate basic programs of aerial meristems and that subtle alterations in their function may be the basis for the wide diversity in growth parameters of shoot systems, leaves, and inflorescences among plant species.