Neurobiological View of Plants and Their Body Plan - Communication in Plants: Neuronal Aspects of Plant Life - page 23

auxin transport (Blilou et al. 2005; Kepinsky and Leyser 2005). On the other
hand, shoots, bearing leaves and flowers, are more specialized to perform
photosynthesis and sexual reproduction. Of course, flowers do entertain
interactions with insects and even small birds (Raguso 2004), to allow
effective spread of pollen, but flower cells do not interact directly with
insect cells as is the case of root cells invaded by symbiotic bacteria and
fungi.Thelatteractonlyaspathogensiftheyinteractwithshootsandleaves.
Parasitic plants provide very convincing evidence that roots represent
the essential part of the plant, whereas shoots can be dispensable. If the
plant nutrition is achieved by heterotrophic mechanisms then the plant is
highlyreducedtoahaustorialsystem,derivedfromroots,specializedfor
organic nutrition. For instance, in holoparasitic plants, such as Rafflesia ,
the aboveground green part of the plant is completely missing (Brown
1822; Barkman et al. 2004). Nevertheless, haustoria of Rafflesia generate
the largest flowers in the plant kingdom, which reveals that this unique
organism really belongs to plants. Moreover, the primary role of roots in
determining the nature of shoots is obvious also from grafting experiments
which show that the rootstocks determine several shoot characteristics such
as photosynthesis performance, shoot branching, leaf development, vein
patterning, pathogen sensitivity, and stress tolerance (Jensen et al. 2003;
Booker et al. 2004; Van Norman et al. 2004; Nelson 2004; Estan et al. 2005).
Interestingly in this respect, non-pathogenic rhizobacteria interacting with
roots can elicit induced systemic resistance in diverse plants against fungi,
bacteria, and viruses (van Loon et al. 1998).
2.3
Shoot Apex as the Posterior Pole of the Plant Body
If the root apex is the anterior pole of the plant body then the shoot
apex must represent the posterior pole. In all multicellular organisms, the
posterior pole is specialized for excretion of metabolites and for sexual
reproduction. Plants conform very well with this expectation. Their shoots
harbour organs of excretion - the trichomes and hydathodes. Moreover,
stomata perform gas exchange. Trichomes are unicellular or multicellular
protuberances of shoot and leaf epidermis which allow removal of excess
ions from the plant and can excrete toxic compounds via pores (striae) at
their tips (Wagner et al. 2004; Kolb and Müller 2004). Trichomes also protect
plants from herbivores, heat, and sunlight, and control leaf temperature and
water loss, as well as regulating apoplasmic calcium (Fahn 2000; DeSilva
et al. 2001; Jensen et al. 2003; Wagner et al. 2004; Kolb and Müller 2004).
Interestingly, hydathodes seem to function analogously to the kidney (Pilot
et al. 2004).