Matt Pepper
Matt Jackson
April 15th, 2011
Telome Theory
Introduction:
The Megaphyll leaves are the most common form of leaves utilized by euphyllophytes (e.g. ferns, gymnosperms, angiosperms). These leaves are characterized as broad, flat blades with a complex vascular system and are the fundamental light capturing organs of the plant. The formation of these leaves has had a profound impact on the evolution of plant and animal life, as well as the energy exchange between the land surface and atmosphere. Proposed by German palaeobotanist Walter Zimmerman, the telome theory describes the evolutionary sequence of modifications leading up to the appearance of megaphyll leaves on ancestral early axial vascular land plants in three fundamental steps: overtopping, plantation, and webbing [1].
Telome Theory:
In general, the telome theory proposes that the combination and modification of multicellular telomes through one or more developmental processes explains all plant morphologies, extant or extinct [2]. The first step is overtopping, where one branch of a dichotomous pair of stems outgrows the other, leaving the smaller stem to branch out 3-dimensionally. Planation is the flattening of the neighboring branch systems into a single plane. The final step, webbing is the growth of photosynthetic tissue joining the segments of the planated branches to form a leaf blade [2].
Overtopping:
Telome theory explains that overtopped branches were the precursors of megaphyll leaves [2]. Overtopping requires two criteria, the initiation of branches, and control of relative growth rates of the different branches. Two types of overtopping are expressed in extant plants, sympodial branching typical of lycopods and some ferns, and monopdial branching usually expressed in the majority of higher plants [6]. For angiosperms, branching requires formation of axillary meristems. Formation of axillary meristem is linked to the shoot apical meristem (SAM) [6]. The SAM is distinguishable from the axillary meristem in that its growth is indeterminate, that is, unlike the axillary, it never stops growing. This is explained by using the KNOX (knotted-like homebox)/ARP module of transcription factors [6]. The SAM expresses KNOX genes and lack ARP gene expression, while the axillary meristems express the opposite.
Planation:
Planation developed as a means to maximize spore dispersal, light interception, and possibly mechanical stability of the plant [7]. Developmentally, planation requires precise positioning of branch initiation points and the coordination of relative branch growth. In modern plants, branch points are determined largely by leaf position, of which the growth factor auxin plays a huge role. The flux of auxin through plant tissues is controlled by the expression of auxin and influx carriers. The accumulation of auxin at sites within the meristem determines the postion of leaf formation [8].
Webbing:
Once branching systems had become overtopped and planated, thin lateral outgrowths aligning the branches marked the formation of the true, laminate leaf [9]. Developmentally, the webbing of branches requires the formation of lateral outgrowths and either congenital or post-genital fusion of adjacent branches. The formation of the leaf is controlled by a complex transcriptional and signaling network requiring the juxtaposition of fields of cells expressing either adaxial or abaxial identity genes [9]. For modern day angiosperms, fusion events in adjacent primordial are observed in certain genetic contexts. Expression of genes is seen in the prevention of fusion between forming primordial, with fusion occurring in their absence. Interestingly, no obvious fusion events occur in during angiosperm leaf initiation and development. Recent data show auxin influx at certain sites on the SAM is involved with the positioning of primary vascular tissue [9].
High Carbon Dioxide:
Recent evidence indicates that ancient plants were capable of producing and assembling leaves millions of years before evolutionary innovations explained by telome theory [10]. High CO2 concentrations are believed to have played a part in this. Studies identify High carbon dioxide (HIC) as a gene that affects stomatal density in plants. At elevated CO2 levels, plants with reduced HIC expression had increased stomatal density of up to 40%, while normal (wildtype) plants expressed little change [10]. It is hypothesized that HLC uses long-chained fatty acids to regulate stomatal development by blocking diffusion of an unknown signaling compound that represses stomatal development. Elevated CO2 levels disrupt fatty acid synthesis, reducing production of the repressor, allowing stomatal density to increase [10].
References
Beerling, D. J., and Fleming, A. J., 2006, Zimmerman’s telome theory of megaphyllleaf evolution: a molecular and cellular critique, Current Opinion in Plant Biology, v. 10, p. 4-12.