In most eukaryotes, the prospective chromosomal positions of meiotic crossovers are marked during meiotic prophase by protein complexes called late recombination nodules (LNs). In tomato (Solanum lycopersicum), a cytological recombination map has been constructed based on LN positions. We demonstrate that the mismatch repair protein MLH1 occurs in LNs. We determined the positions of MLH1 foci along the 12 tomato chromosome pairs (bivalents) during meiotic prophase and compared the map of MLH1 focus positions with that of LN positions. On all 12 bivalents, the number of MLH1 foci was ;70% of the number of LNs. Bivalents with zero MLH1 foci were rare, which argues against random failure of detecting MLH1 in the LNs. We inferred that there are two types of LNs, MLH1-positive and MLH1-negative LNs, and that each bivalent gets an obligate MLH1-positive LN. The two LN types are differently distributed along the bivalents. Furthermore, cytological interference among MLH1 foci was much stronger than interference among LNs, implying that MLH1 marks the positions of a subset of strongly interfering crossovers. Based on the distances between MLH1 foci or LNs, we propose that MLH1-positive and MLH1-negative LNs stem from the same population of weakly interfering precursors.

This review summarizes the available data related to the effects of air pollution on pollen grains from different plant species. Several studies carried out either on in situ harvested pollen or on pollen exposed in different places more or less polluted are presented and discussed. The different experimental procedures used to monitor the impact of pollution on pollen grains and on various produced external or internal subparticles are listed. Physicochemical and biological effects of artificial pollution (gaseous and particulate) on pollen from different plants, in different laboratory conditions, are considered. The effects of polluted pollen grains, subparticles, and derived aeroallergens in animal models, in in vitro cell culture, on healthy human and allergic patients are described. Combined effects of atmospheric pollutants and pollen grains-derived biological material on allergic population are specifically discussed. Within the notion of “polluen,” some methodological biases are underlined and research tracks in this field are proposed.

To investigate the configuration and function of microtubules (MTs) in tip-growing Medicago truncatularoot hairs, we used immunocytochemistry or in vivo decoration by a GFP linked to a MT-binding domain. The two approaches gave similar results and allowed the study of MTs during hair development. Cortical MTs (CMTs) are present in all developmental stages. During the transition from bulge to a tip-growing root hair, endoplasmic MTs (EMTs) appear at the tip of the young hair and remain there until growth arrest. EMTs are a specific feature of tip-growing hairs, forming a three-dimensional array throughout the subapical cytoplasmic dense region. During growth arrest, EMTs, together with the subapical cytoplasmic dense region, progressively disappear, whereas CMTs extend further toward the tip. In full-grown root hairs, CMTs, the only remaining population of MTs, converge at the tip and their density decreases over time. Upon treatment of growing hairs with 1 μm oryzalin, EMTs disappear, but CMTs remain present. The subapical cytoplasmic dense region becomes very short, the distance nucleus tip increases, growth slows down, and the nucleus still follows the advancing tip, though at a much larger distance. Taxol has no effect on the cytoarchitecture of growing hairs; the subapical cytoplasmic dense region remains intact, the nucleus keeps its distance from the tip, but growth rate drops to the same extent as in hairs treated with 1 μm oryzalin. The role of EMTs in growing root hairs is discussed.

A critical step in establishing a successful nitrogen-fixing symbiosis between rhizobia and legume plants is the entrapment of the bacteria between root hair cell walls, usually in characteristic 180°to 360°curls, shepherd's crooks, which are formed by the host's root hairs. Purified bacterial signal molecules, the nodulation factors (NFs), which are lipochitooligosaccharides, induce root hair deformation in the appropriate host legume and have been proposed to be a key player in eliciting root hair curling. However, for curling to occur, the presence of intact bacteria is thought to be essential. Here, we show that, when spot applied to one side of the growing Medicago truncatula root hair tip, purified NF alone is sufficient to induce reorientation of the root hair growth direction, or a full curl. Using wild-type M. truncatula containing the pMtENOD11::GUS construct, we demonstrate that MtENOD11::GUS is expressed after spot application. The data have been incorporated into a cell biological model, which explains the formation of shepherd's crook curls around NF-secreting rhizobia by continuous tip growth reorientation.Bacteria of the genera Rhizobium spp., Bradyrhizobium sp., Azorhizobium sp., Mesorhizobium sp., and Sinorhizobium sp., collectively referred to as rhizobia, can establish a symbiosis with legume plants. The bacteria induce the development of a new plant organ, the root nodule, in which nitrogen fixation takes place. Located in this nodule, the bacteria are provided with photosynthate. In return, the symbiont converts atmospheric nitrogen into ammonia, a form that can be readily assimilated by the host plant. A requirement for successful infection in many legumes is the entrapment of bacteria between root hair cell walls. Usually, this is accomplished by the formation of a tight curl, a shepherd's crook, of the host plant root hairs to which bacteria have become attached (Kijne, 1992;Hadri and Bisseling, 1998). The subsequent formation of the host-produced infection thread (Nutman, 1956) is initiated within this curl, where bacteria have been enclosed between root hair cell walls. In experimental conditions, infection threads can also originate from bacteria that are entrapped between the tips of two uncurled root hairs (Haack, 1964).Nodulation factors (NFs) are molecules synthesized and excreted by rhizobia in response to plant flavonoids (Fisher and Long, 1992). Application of purified NF to legume roots induces the formation of polarized cytoplasmic bridges (pre-infection threads) in the outer cortical cells (van Brussel et al., 1992) and cell divisions in the inner cortex (van Brussel et al., 1992; for review, see Kijne, 1992). Moreover, various ENOD (early nodulin) genes are expressed in response to NF (Scheres et al., 1990;Pichon et al., 1992;Yang et al., 1993;Pingret et al., 1998; Compaan et al., 2001;Journet et al., 2001).One of the best characterized biological activities of purified NF is to induce root hair deformation in the appropriate host. Therefore, root hair deformation assays a...

The Medicago truncatula Does not Make Infections (DMI2) mutant is mutated in the nodulation receptor-like kinase, NORK. Here, we report that NORK-mutated legumes of three species show an enhanced touch response to experimental handling, which results in a nonsymbiotic root hair phenotype. When care is taken not to induce this response, DMI2 root hairs respond morphologically like the wild type to nodulation factor (NF). Global NF application results in root hair deformation, and NF spot application induces root hair reorientation or branching, depending on the position of application. In the presence of Sinorhizobium meliloti, DMI2 root hairs make two-dimensional 1808 curls but do not entrap bacteria in a three-dimensional pocket because curling stops when the root hair tip touches its own shank. Because DMI2 does not express the promoter of M. truncatula Early Nodulin11 (ENOD11) coupled to b-glucuronidase upon NF application, we propose a split in NF-induced signaling, with one branch to root hair curling and the other to ENOD11 expression.