Crosses among single-, double- and multi-petal jasmine cultivars (Jasminum sambac Aiton) are unable to easily generate hybrids. To identify the reproductive barriers restricting hybrid set, dynamic changes in jasmine pollen viability and pistil receptivity were compared at different flowering stages. Pollen-pistil interactions in six reciprocal crosses were also investigated to characterize pollen-stigma compatibility. Additionally, paraffin sections of pollinated embryo sacs were prepared for subsequent analyses of developmental status. Furthermore, pistil cell ultrastructural characteristics were observed to reveal cytological mechanism regulating pistil receptivity and the pollen-pistil interactions. We observed that pollen viability and stigma receptivity varied depending on petal phenotype and flowering stage and were easily lost during flowering. Different reciprocal crosses exhibited varied pollen-stigma compatibilities according to the pollen germination rates. Although some pollen grains germinated normally on maternal stigmas, the pollen tubes were arrested in the pistils and were unable to reach the ovaries. Additionally, the embryo sacs remained unfertilized until degenerating. Therefore, jasmine crosses are affected by pre-fertilization reproductive barriers. Low pollen fertility and poor stigma receptivity are detrimental to pollen germination and pollen-pistil compatibility, indicating they are two factors affecting hybrid set. Ultrastructural observation of the pistil cells revealed that cell death occurred during flowering. Thus, the early and rapid senescence of pistils is likely responsible for the decreased pistil receptivity and inhibited pollen tube growth. These findings may be relevant for future jasmine hybridizations. They provide new insights for the development of methods to overcome reproductive barriers and may also be useful for clarifying the phylogenetic relationships among jasmine cultivars with differing petal phenotypes.
Aquaporin (AQP) proteins can transport water and other small molecules through cellular membranes and are one of the first targets of stress-induced signaling in plants. A number of AQP genes have been identified from glycophytes, and their functions have been studied. However, the reports on AQPs from halophytes and their precise role in abiotic stress response are still rare. In this study, we have identified a PIP1 subgroup AQP gene, designated SbPIP1, from the euhalophyte Salicornia bigelovii and characterized it by overexpressing in tobacco plants. SbPIP1 transcript was induced by cold, but suppressed by NaCl and polyethylene glycol (PEG). Transient expression of GFP (green fluorescent protein)-SbPIP fusion protein indicated its localization in the plasma membrane. Overexpression of SbPIP1 in tobacco (Nicotiana tabacum) plants increased their drought tolerance. Leaf protoplasts from transgenic tobacco plants absorbed water more quickly than those from wild type (WT) plants when they were put into hypotonic solution. In addition, the transgenic tobacco plants possessed higher relative water content (RWC) and proline content, but lower levels of malondialdehyde (MDA) and less ion leakage (IL) when compared with WT under the treatment of the different concentrations of PEG. Taken together, our results demonstrate that heterologous expression of SbPIP1 in tobacco plants confers them drought stress tolerance by reducing membrane injury and increasing the ability to retain water.
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