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

Abstract

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 Knottedl (Knf)-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 Knl-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 nove1 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 t o coding position 64 of the TKn2 transcript, leaving the TKn2 homeobox intact. We suggest that class I Knl-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.

Clausa, a Tomato Mutant with a Wide Range of Phenotypic Perturbations, Displays a Cell Type-Dependent Expression of the Homeobox Gene LeT6/TKn2

“…To determine the expression pattern of LeT6 in various tissues, we performed in situ RNA hybridization in shoot apices, mature leaves, and carpels derived from wild-type and clau:shl mutant plants. Consistent with previous reports (Chen et al, 1997;Parnis et al, 1997;Janssen et al, 1998aJanssen et al, , 1998b, LeT6 was found to be expressed in wild-type meristems, leaf primordia (not shown), as well as vascular tissues (Fig. 5B), but not in mature wild-type leaflets (Fig.…”

“…This is different from the adherent1 (ad1) mutant of maize in which tissue identity is preserved at the attached region (Sinha and Lynch, 1998). The control over organ fusion in the clau mutant cannot be solely attributed to LeT6, because tomato plants misexpressing this gene do not exhibit fusion of organs (Chen et al, 1997;Janssen et al, 1998a;Parnis et al, 1997). Taken together, the phenotypic alterations exerted by the clau mutation suggest that CLAUSA is involved not only in controlling the expression pattern of LeT6, but also in the control of other developmental genes in tomato plants.…”

“…The mechanism for the abnormal expression of TKn2 in Cu leaves is yet unknown. Parnis et al (1997) proposed that overexpression of TKn2 in Cu leaves may result from a mutation within a putative silencer in the long intron 2, or within 0.1 cM (50-100 kb) of the Cu locus. This suggests that the spatiotemporal regulation of knox gene expression requires cis regulatory elements that lie outside the transcribed region of the gene, but within its regulatory region (50-100 kb).…”

KNOX genes: versatile regulators of plant development and diversity

“…For example, dominant Mouse ear (Me) and Curl (Cu) mutations cause aberrant transcription of the tomato Tkn2 gene, yet Me leaves vary between highly ramified vegetative leaves and bladeless reproductive leaves, while the growth of Cu leaves is severely arrested (Parnis et al, 1997). Variation in the expression domain of ARP repressors during development may influence these phenotypes; however, ARP expression has been reported to be both absent and present in the SAM of tomato so this issue requires further investigation (Kim et al, 2003;Pien et al, 2001).…”

Trifoliate encodes an MYB transcription factor that modulates leaf and shoot architecture in tomato

“…The encoded homeodomain transcription factors delay maturation and thereby facilitate the formation of leaflets (8). Overexpression of KNOXI genes during tomato leaf development leads to a strong increase in leaf complexity (5,28,29,36). We tested the interrelation between LeT6/TKn2 gene activity and Tf function by combining KNOXI gain of function mutants showing an increased leaf complexity with the tf mutation.…”