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The inability of plants to ‘run away’ has implications for the ways in which they interact with their environment. One feature that is encountered by all plants is mechanical perturbation; in plants growing on land this may include wind and rain action, herbivore activity, encounters with other objects (such as stones, rocks and even other plants) and touch. It is this latter factor that has been investigated by Porter et al., in Hawai'i (pp. 847–858). They have focused especially on changes in plant growth and morphology (thigmomorphogenesis) in response to a touch treatment in the tropical fruit tree, Carica papaya. This species was chosen because its small genome has been sequenced, it is readily transformed by GM techniques and it grows rapidly. Touch treatment consisted of gently bending seedling stems to 45° forward, back, left and right. This was done three times during the day. After 25 days of this treatment, seedlings were stunted (46% decrease in height) but the hypocotyls were 36% broader. A very striking feature was the loss of pigmentation. Chlorophyll content was reduced by more than 50% while anthocyanins (which normally colour the hypocotyl dark purple) were almost completely absent. By contrast, lignin content increased by 12% and the authors suggest that the dramatic reduction in anthocyanins may be caused by diversion of precursors from the anthocyanin to the lignin pathway. Touch treatment also led to the development, on the abaxial side of the petioles, of novel periderm (cork) outgrowths that contained suberin. Genetic analysis showed that Carica has fewer touch-inducible (TCH) and mechano-sensitive channel of small conductance-like (MSL-like) genes than Arabidopsis, possibly reflecting the relative sizes of their genomes. Study of TCH gene expression in Carica revealed that two homologues of the Arabidopsis TCH1 genes were only slightly up-regulated by the touch treatment but were significantly up-regulated by a different mechanical perturbation, namely spraying with water.
Rice is arguably the most important crop in the world with half the world's population relying on it. Increased understanding of how rice plants withstand environmental stress may well contribute to world food security. Thus, Zhang et al. (Nanjing, China and Tokyo, Japan, pp. 923–930) have investigated the effects of Cu on germinating rice embryos. Like many micronutrients, Cu is toxic in higher concentrations. The authors have focused especially on metallothioneins (MTs), low-molecular-weight, cysteine-rich, metal-binding proteins. Studies of plant MTs at the protein level have been limited, but more is known about MT genes and gene expression. For example, expression of type-2 MTs is associated with Cu tolerance in Arabidopsis. Rice seeds were germinated in the presence of 0–200 µm CuSO4; radicle lengths were measured at 4, 6 and 8 days. At 4 days, only 200 µm Cu had any inhibitory effect, but by day 6 there was some inhibition at Cu concentrations of 50 µm and greater. Proteins were extracted from 6-day germinating embryos and fractionated by 1-D and 2-D gel electrophoresis. The latter revealed that 13 protein spots in the 5–25 kDa molecular-weight range were up-regulated (by up to 5·3-fold) by Cu treatment, while three were down-regulated. These 16 protein spots were digested with trypsin and subjected to peptide-mass fingerprinting using a MALDI-TOF mass spectrometer. Data were then used to search several protein databases. This showed clearly that the protein exhibiting the greatest extent of up-regulation was an MT-like protein. Other up-regulated proteins included a putative cell-wall-associated protein kinase, two pathogenesis-related proteins and a small GTP-binding protein. One of the down-regulated proteins was a small cytochrome P450 (CYP90D2). The authors state that this is the first time that proteomic evidence has been obtained that MT and CYP90D2 are Cu-responsive proteins in plants. From a wider perspective, the results also increase our knowledge of Cu tolerance mechanisms in this important crop.
Long before computer-based resources such as biological databases were even thought of, a different type of botanical data bank, the herbarium, was in existence. Herbaria, collections of preserved dried plants, often dating back several centuries, contain a wealth of data waiting to be ‘mined’. Fernando et al. (Melbourne and Toowong, Australia, pp. 931–939) have thus used herbarium specimens to search for Mn hyperaccumulators. Hyperaccumulators, which comprise less than 0·2% of angiosperm species, in general have the ability to take up and sequester amounts of particular metals at levels many times greater than would normally be toxic, even to tolerant species. They have been suggested as agents for bio-remediation of contaminated soils and even as sources of metals for commercial extraction. Mn hyperaccumulators are relatively scarce with only about 11 species known, most of which are native to New Caledonia. The authors rationale was that eastern Australia is geographically close to New Caledonia and is home to ecologically equivalent species to the Mn hyperaccumulators. Leaf fragments were taken from herbarium specimens representing 47 species in seven genera spread across three families; fragments were assayed for Mn, Ni, Fe, Al and Ti. In the family Myrtaceae, five Gossia species were definitely identified as Mn hyperaccumulators (including G. bidwillii, which was already known to accumulate Mn to high levels); one of these, G. fragrantissima, also accumulated Ni. It was confirmed that Macadamia species (family Proteaceae) accumulate either Mn or Al. In the family Celestaceae, Maytenus cunninghamii was shown for the first time to be a hyperaccumulator. Interestingly, this species exhibits considerable variation in its ability to accumulate Mn, and at the two ends of the range the plants are morphologically distinct from each other, suggesting the existence of subspecies. Overall, the authors' rationale has been supported by the results. What is now needed is assay of plants growing in the field.
The evolutionary advantages of sexual reproduction have been discussed widely since Darwin's time. Despite these advantages, sex is expensive and if suitable mates are in short supply, may fail completely. Reliance or partial reliance on asexual reproduction, such as happens in many plants, may therefore be regarded as a form of reproductive ‘safety net’. However, there is a half-way situation between asexual reproduction and full-scale sexual reproduction, namely self-fertilization. This process, which has evolved from outbreeding, is widespread in plants and enables seed set when suitable mates and/or pollination agents are scarce or absent. At the molecular and cellular level it involves loss of self-incompatibility, whilst at the morphological level changes in floral architecture occur. Vallejo-Marín and Barrett at Toronto (pp. 951–962) have studied a species, Eichornia paniculata, an aquatic annual, in which there is a wide variety in the extent of inbreeding, associated with variation in floral architecture. It therefore looks as if evolution of selfing is actually in progress in this species. The authors are interested to know whether changes in floral architecture occur together or arise independently during this evolutionary process. Plants from 31 Brazilian populations were grown under uniform conditions. The main focus was on herkogamy (stigma–anther separation) and flower size in relation to stylar morphology (long, medium or short styles). Reduction in herkogamy (i.e. stigma and anther being closer together), a feature of inbreeding plants, occurred only in the mid-styled morph and was independent of changes in flower size. Herkogamy was clearly under genetic control but was also subject to environmental influences, leading to phenotypic plasticity; moreover, there was genetic variation in the degree of sensitivity to environmental factors. A further interesting feature of the results was the existence of individual plants with both normal and reduced herkogamy – a clear indication of the status of this species as an intermediate form.