Eliezer Lifschitz

The Making of a Compound Leaf: Genetic Manipulation of Leaf Architecture in Tomato

The most distinctive morphogenetic feature of leaves is their being either simple or compound. To study the basis for this dichotomy, we have exploited the maize homeobox-containing Knotted-1 (Kn1) gene in conjunction with mutations that alter the tomato compound leaf. We show that misexpression of Kn1 confers different phenotypes on simple and compound leaves. Up to 2000 leaflets, organized in compound reiterated units, are formed in tomato leaves expressing Kn1. In contrast, Kn1 induces leaf malformations but fails to elicit leaf ramification in plants with inherent simple leaves such as Arabidopsis or in tomato mutant plants with simple leaves. Moreover, the tomato Kn1 ortholog, unlike that of Arabidopsis, is expressed in the leaf primordia. Presumably, the two alternative leaf forms are conditioned by different developmental programs in the primary appendage that is common to all types of leaves.

Isolation of the tomato AGAMOUS gene TAG1 and analysis of its homeotic role in transgenic plants

To understand the details of the homeotic systems that govern flower development in tomato and to establish the ground rules for the judicious manipulation of this floral system, we have isolated the tomato AGAMOUS gene, designated TAG1, and examined its developmental role in antisense and sense transgenic plants. The AGAMOUS gene of Arabidopsis is necessary for the proper development of stamens and carpels and the prevention of indeterminate growth of the floral meristem. Early in flower development, TAG1 RNA accumulates uniformly in the cells fated to differentiate into stamens and carpels and later becomes restricted to specific cell types within these organs. Transgenic plants that express TAG1 antisense RNA display homeotic conversion of third whorl stamens into petaloid organs and the replacement of fourth whorl carpels with pseudocarpels bearing indeterminate floral meristems with nested perianth flowers. A complementary phenotype was observed in transgenic plants expressing the TAG1 sense RNA in that first whorl sepals were converted into mature pericarpic leaves and sterile stamens replaced the second whorl petals.

The flowering hormone florigen functions as a general systemic regulator of growth and termination

The florigen paradigm implies a universal flowering-inducing hormone that is common to all flowering plants. Recent work identified FT orthologues as originators of florigen and their polypeptides as the likely systemic agent. However, the developmental processes targeted by florigen remained unknown. Here we identify local balances between SINGLE FLOWER TRUSS (SFT), the tomato precursor of florigen, and SELF-PRUNING (SP), a potent SFT-dependent SFT inhibitor as prime targets of mobile florigen. The graft-transmissible impacts of florigen on organ-specific traits in perennial tomato show that in addition to import by shoot apical meristems, florigen is imported by organs in which SFT is already expressed. By modulating local SFT/SP balances, florigen confers differential flowering responses of primary and secondary apical meristems, regulates the reiterative growth and termination cycles typical of perennial plants, accelerates leaf maturation, and influences the complexity of compound leaves, the growth of stems and the formation of abscission zones. Florigen is thus established as a plant protein functioning as a general growth hormone. Developmental interactions and a phylogenetic analysis suggest that the SFT/SP regulatory hierarchy is a recent evolutionary innovation unique to flowering plants.

The TM5 MADS Box Gene Mediates Organ Differentiation in the Three Inner Whorls of Tomato Flowers.

The tomato MADS box gene no. 5 (TM5) is shown here to be expressed in meristematic domains fated to form the three inner whorls-petals, stamens, and gynoecia-of the tomato flower. TM5 is also expressed during organogenesis and in the respective mature organs of these three whorls. This is unlike the major organ identity genes of the MADS box family from Antirrhinum and Arabidopsis, which function in overlapping primordial territories consisting of only two floral whorls each. The developmental relevance of the unique expression pattern of this putative homeotic gene was examined in transgenic plants. In agreement with the expression patterns, antisense RNA of the TM5 gene conferred both early and late alterations of morphogenetic markers. Early defects consist of additional whorls or of a wrong number of organs per whorl. Late, organ-specific changes include evergreen, cauline, and unabscised petals; green, dialytic, and sterile anthers; and sterile carpels and defective styles on which glandular trichomes characteristic of sepals and petals are ectopically formed. However, a complete homeotic transformation of either organ was not observed. The early and late floral phenotypes of TM5 antisense plants suggest that TM5 mediates two unrelated secondary regulatory systems. One system is the early function of the floral meristem identity genes, and the other system is the function of the major floral organ identity genes.

Morphogenesis of Simple and Compound Leaves: A Critical Review

The leaves of seed plants evolved from a primitive shoot system and are generated as determinate dorsiventral appendages at the flanks of radial indeterminate shoots. The remarkable variation of leaves has remained a constant source of fascination, and their developmental versatility has provided an advantageous platform to study genetic regulation of subtle, and sometimes transient, morphological changes. Here, we describe how eudicot plants recruited conserved shoot meristematic factors to regulate growth of the basic simple leaf blade and how subsets of these factors are subsequently re-employed to promote and maintain further organogenic potential. By comparing tractable genetic programs of species with different leaf types and evaluating the pros and cons of phylogenetic experimental procedures, we suggest that simple and compound leaves, and, by the same token, leaflets and serrations, are regulated by distinct ontogenetic programs. Finally, florigen, in its capacity as a general growth regulator, is presented as a new upper-tier systemic modulator in the patterning of compound leaves.

The tomato 66.3-kD polyphenoloxidase gene: molecular identification and developmental expression.

A gene coding for a polypeptide abundant in tomato floral meristems was isolated and shown to represent a tomato 66.3-kD polyphenoloxidase. Analysis of cDNA clones and a corresponding intronless genomic clone indicated that the plastid-bound 587-residue-long polypeptide, designated P2, contains two conserved copper-binding domains, similar to those found in fungal and mammalian tyrosinases. P2 transcripts and polypeptides are accumulated in the arrested floral primordia of the anantha mutant inflorescences and are equally abundant in primordia of wild-type flowers; the gene continues to be expressed at high levels in developing floral organs. In young expanding leaves, P2 protein is concentrated in palisade cells and in epidermal trichomes. Expression patterns of P2 in plant meristems permit molecular distinction between floral and vegetative primordia, and, in a companion study, comparison with dUTPase suggests that the two genes mark two alternative complementary developmental programs in the floral and vegetative meristems of the tomato plants.

Biosynthetic threonine deaminase gene of tomato: isolation, structure, and upregulation in floral organs.

The gene encoding the plant biosynthetic threonine deaminase (Td; EC 4.2.1.16) has been cloned as a result of its unusual upregulation in tomato flowers. The Td gene of tomato encodes a polypeptide of 595 residues, the first 80 of which comprise a putative two-domain transit peptide cleaved at position 51. Comparison of the amino acid sequence with the corresponding enzymes from yeast and bacteria reveals a near identity of the important catalytic regions and greater than 40% overall similarity. The Td gene is unique in the tomato genome and its coding region is interrupted by eight introns. Its expression is greater than 50-fold higher in sepals and greater than 500-fold higher in the rest of the flower than in leaves or roots. Its overexpression, however, is strictly confined to the parenchymal cells of the floral organs. In young tomato leaves, the chloroplast-bound enzyme is found almost exclusively in the subepidermal spongy mesophyll cells.

A Cytokinin-Activating Enzyme Promotes Tuber Formation in Tomato

BACKGROUND Dedicated storage organs in the form of tubers are evolutionary novelties that share a common function but originate in diverse species from different organs. Tubers in potato, Solanum tuberosum, are derived from the swollen tips of specialized basal lateral juvenile shoots, called stolons. Lateral buds of tomato, Solanum lycopersicum, a potato sibling species, only form regular shoots. The evo-devo mechanisms restricting tuber formation to basal juvenile axillary meristems of potato while completely inhibiting it in tomato meristems are not currently understood. RESULTS Ectopic expression of tomato LONELY GUY (LOG1), a cytokinin (CK) biosynthesis gene, imparts potential to the outgrowing juvenile tomato buds to generate, de novo, aerial minitubers (TMTs). TMTs are morphologically, developmentally, and metabolically homologous to aerial potato tubers and display a unique transcriptome with altered hormonal signaling networks. The new hormonal balance stimulates ectopic branching of dormant axillary meristems and loss of apical dominance without disruption of polar auxin transport and obviates the need for specific branching genes. miR156, a master regulator of juvenility, extends tuber-forming potential to distal axillary buds in both wild-type potato and tomato primed by LOG1 signaling. CONCLUSIONS Ubiquitous activation of TLOG1 uncovered a developmentally suppressed tuber-forming potential within tomato axillary meristems. Other meristems in other plants may also carry hidden, suppressed organogenesis potentials. The unlocking of this potential by the activity of a single gene represents a prime example of an evolutionary novelty in the making and suggests that CKs may function as universal regulators of storage-organ formation in plants.

A meristem-related gene from tomato encodes a dUTPase: analysis of expression in vegetative and floral meristems.

A meristem-specific gene coding for deoxyuridine triphosphatase (EC 3.6.1.23) (dUTPase) in tomato was isolated, and its developmental expression in vegetative and floral apices was monitored. An 18-kD polypeptide, P18, was isolated as a consequence of its accumulation in arrested floral meristems of anantha mutant plants. The corresponding cDNA isolated from an expression library exhibited a 40 to 60% similarity with the pseudoprotease sequences of poxviruses, genes that have been suggested to encode dUTPases. Enzymatic tests and conservation of peptide motifs common to bacterial and viral genes verified that the P18 cDNA clone indeed represents a eukaryotic dUTPase. Immunogold localization and in situ hybridization experiments showed that polypeptides and transcripts of dUTPase are maintained at high levels in apical meristematic cells of vegetative and floral meristems. dUTPase gene activity is also high in the potentially meristematic cells of the provascular and vascular system. Its expression is lower in the immediate parenchymal derivatives of the apical meristematic cells, and this downregulation marks, perhaps, the transition from totipotency to the first differentiated state.

Florigen and anti-florigen – a systemic mechanism for coordinating growth and termination in flowering plants

Genetic studies in Arabidopsis established FLOWERING LOCUS T (FT) as a key flower-promoting gene in photoperiodic systems. Grafting experiments established unequivocal one-to-one relations between SINGLE FLOWER TRUSS (SFT), a tomato homolog of FT, and the hypothetical florigen, in all flowering plants. Additional studies of SFT and SELF PRUNING (SP, homolog of TFL1), two antagonistic genes regulating the architecture of the sympodial shoot system, have suggested that transition to flowering in the day-neutral and perennial tomato is synonymous with “termination.” Dosage manipulation of its endogenous and mobile, graft-transmissible levels demonstrated that florigen regulates termination and transition to flowering in an SP-dependent manner and, by the same token, that high florigen levels induce growth arrest and termination in meristems across the tomato shoot system. It was thus proposed that growth balances, and consequently the patterning of the shoot systems in all plants, are mediated by endogenous, meristem-specific dynamic SFT/SP ratios and that shifts to termination by changing SFT/SP ratios are triggered by the imported florigen, the mobile form of SFT. Florigen is a universal plant growth hormone inherently checked by a complementary antagonistic systemic system. Thus, an examination of the endogenous functions of FT-like genes, or of the systemic roles of the mobile florigen in any plant species, that fails to pay careful attention to the balancing antagonistic systems, or to consider its functions in day-neutral or perennial plants, would be incomplete.