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Plant Physiology 138:1175-1176 (2005) © 2005 American Society of Plant Biologists On The InsidePlant Hormones and Haustorium DevelopmentParasitic plants are not rare oddities: over 4,000 species of angiosperms are able to directly invade and parasitize other plants. A feature common to all parasitic plants is the ability to develop invasive structures called haustoria. The development of haustoria, root structures that allow hemiparasitic plants to change from autotrophic to heterotrophic growth, is rapid, highly synchronous, and readily observed in vitro. After invasion, parasitic plant haustoria function as physiological bridges through which the parasite robs host plants of water and nutrients. In the case of the facultative parasite Triphysaria versicolor, haustorium development can be initiated in aseptic roots by exposing them to phenolic haustorium-inducing factors exuded by the hosts' roots. The morphological changes associated with early haustorium ontogeny include rapid cessation of root elongation, expansion and differentiation of epidermal cells into haustorial hairs, and cortical cell expansion. Tomilov et al. (pp. 1469–1480) show that surgically dissected root tips from aseptic Triphysaria versicolor (Fig. 1) roots formed haustoria if the root was exposed to haustorial-inducing factors prior to dissection. In contrast, root tips that were dissected prior to inducing factor treatment were unable to form haustoria unless supplemented with IAA. Moreover, auxin and ethylene responsive promoters are upregulated when T. versicolor is exposed to either exogenous hormones or purified haustoria-inducing factors. These experiments demonstrate that localized auxin and ethylene accumulation are early events in haustorium development and that parasitic plants recruit established plant developmental mechanisms to realize parasite specific functions. The genetic determinants that distinguish parasitic from nonparasitic plants have yet to be identified but appear to function at a stage prior to hormone action.
Thiamine Induces Disease Resistance in Plants Prominent on the list of essential vitamins required by humans is vitamin B1 (thiamine), a deficiency of which causes beriberi, a potentially lethal disturbance of the central nervous and circulatory systems. In countries where rice (Oryza sativa) is a staple food, thiamine deficiency is prevalent because the polishing of rice removes most of the thiamine in the grain. Although rice bran and legumes are good sources of thiamine for the human diet, little is known about the role of thiamine in plant function. In this issue, Ahn et al. (1505–1515) provide intriguing evidence that thiamine induces systemic acquired resistance (SAR) in plants. Arabidopsis (Arabidopsis thaliana), rice, and other crops showed resistance to fungal, bacterial, and viral infections following treatment with thiamine. The effects of thiamine on disease resistance and defense-related gene expression are systemic and last for more than 15 d after treatment. Thiamine treatment induces the transient expression of pathogenesis-related (PR) genes in rice and other plants, as well as the up-regulation of protein kinase C activity. Infiltration of Arabidopsis plants with the Ca2+ channel blocker LaCl3 prevented the accumulation of PR-1 gene transcript in response to thiamine, suggesting a possible role for Ca2+ influx in thiamine signal transduction. Thiamine prevented bacterial infection in Arabidopsis mutants insensitive to jasmonic acid or ethylene but not in mutants impaired in the SAR transduction pathway. Thus, it appears that thiamine induces SAR in plants through salicylic acid and Ca2+-related signaling pathways. These findings may lead to novel strategies for combating plant diseases. Plant Acetylcholinesterases Are Novel Enzymes Acetylcholine (ACh) is a well-known neurotransmitter in animals that serves to propagate electrical stimuli across synaptic junctions. At the presynaptic neuron end, an electrical impulse triggers the exocytotic release of ACh-filled vesicles into the synaptic cleft. ACh then binds to an ACh-receptor (AChR) on the postsynaptic neuron surface, and the ACh-AChR binding induces subsequent impulses to the postsynaptic neuron. The ACh, which is released again by the receptor into the synaptic cleft, is rapidly degraded by acetylcholinesterase (AChE). Although plants lack a nervous system, both ACh and ACh-hydrolyzing activity have been reported in plants. Sagane et al. (pp. 1359–1371) report on their purification and cloning of AChE from maize (Zea mays). Although the pharmacological attributes of maize AChE were similar to its animal counterparts, kinetic analyses indicated that maize AChE showed a lower affinity for substrates and inhibitors than did animal AChE, and the deduced amino acid sequence exhibited no apparent similarity with that of the animal enzyme. The maize AChE possesses a consensus sequence from the lipase GDSL family, which is distinct from the carboxylesterase family consensus sequence that characterizes animal AChEs. In silico screening indicated that maize AChE homologs are widely distributed in the plant kingdom, but similar genes are not found in any animal, fungal, or bacterial databases. These findings suggest that the plant AChE family comprises a novel family of AChE enzymes that is specifically distributed in the plant kingdom. The authors also provide an interesting discussion of a possible role for ACh in root gravitropism. Auxin's True Role in Apical Dominance One of the first and most enduring roles identified for the plant hormone auxin in plant growth and development is its role in mediating apical dominance. According to long held views, decapitation reduces the auxin stream to lateral buds, which then begin to elongate. Classical decapitation and auxin replacement experiments, and studies using transgenes to manipulate endogenous auxin levels, have lacked the temporal and/or spatial resolution to determine whether auxin is primarily involved in bud growth induction or in the subsequent autoregulation of shoot branching. Morris et al. (pp. 1665–1672) provide an in-depth analysis of the dynamics of IAA transport and auxin levels in relation to axillary bud outgrowth. They present evidence that depleted IAA levels are not the trigger for the initial stages of bud growth in decapitated plants and suggest that auxin is involved in controlling a later stage of bud outgrowth. Moreover, they demonstrate that auxin transport inhibitors cause a similar auxin depletion as decapitation, but do not stimulate bud outgrowth. In pea (Pisum sativum), they conclude, there is evidence for two mechanisms by which decapitation can stimulate axillary bud outgrowth. The first involves a rapidly transmitted and elusive signal that acts independently of auxin. In the second mechanism, which first comes into play around 24 h after decapitation, a lack of auxin allows long-term, sustained bud outgrowth. The possible involvement of two mechanisms is consistent with evidence that outgrowing buds can be restored to a dormant state by exogenous auxin. Previous researchers have defined four stages of bud development including dormancy and sustained growth, as well as two transitional stages (dormancy to growth and growth to dormancy). Morris et al. hypothesize that in intact plants, auxin acts as an autoregulation signal by preventing stimulated buds from completing the transition to sustained growth. Abscisic Acid's Role in Water Uptake by Seeds
Water uptake is a fundamental requirement for the initiation and completion of seed germination. Uptake of water by a dry seed is triphasic with a rapid initial uptake (phase I, i.e. imbibition) followed by a plateau phase (phase II). A further increase in water uptake occurs only after germination is completed, as the embryo axes elongate. Because dormant seeds do not complete germination, they do not enter this postgermination phase of water uptake (phase III). Manz et al. (pp. 1538–1551) have taken advantage of two nondestructive and state-of-the-art techniques, namely in vivo 1H-NMR microimaging and 1H-MAS NMR spectroscopy, to examine the spatial and temporal regulation of water uptake by germinating tobacco (Nicotiana tabacum) seeds. The observations reveal that germination of tobacco seed follows a distinct pattern of events: rupture of the testa is followed by rupture of the endosperm. Abscisic acid (ABA) specifically inhibits endosperm rupture and phase III water uptake but does not alter the spatial and temporal pattern of phase I and II water uptake. NMR microimaging showed that the micropylar region of the endosperm is a major entry point for water uptake during tobacco seed germination. Their results support the proposal that different seed tissues and organs hydrate at different extents and that the endosperm of tobacco acts as a water reservoir for the embryo. ABA and the overexpression of class I The Maize Root Transcriptome SAGE provides an accurate view of expressed genes in tissues or cells, aids in the identification of transcripts, and also permits analyses of changes in the transcript populations of organisms exposed to different conditions, or between distantly related organisms. Poroyko et al. (pp. 1700–1710) have used SAGE to determine the transcriptome of well-watered maize roots. The complexity of the maize root transcriptome is comparable with estimates of the complexity for the Arabidopsis root transcript population. Their analyses indicate that the number could exceed 22,000 transcripts. Comparing the maize root transcriptome with that in other plants indicated that highly expressed transcripts differed substantially; less than 5% of the most abundant transcripts were shared between maize and Arabidopsis.
Department of Natural Sciences Mercy College Dobbs Ferry, New York 10522 FOOTNOTES www.plantphysiol.org/cgi/doi/10.1104/pp.104.900165 Related articles in Plant Physiol.:
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-1,3-glucanase in transgenic tobacco seeds do not alter the spatial pattern of water uptake but affect the temporal pattern and the transition to phase III water uptake. Finally, MAS-13C NMR spectroscopy revealed that ABA did not inhibit tobacco seed oil mobilization.