Genomic factors responsible for abnormal morphology of pollen tubes in the interspecific cross Nicotiana tabacum × N. rustica

Let's shape again: the concerted molecular action that builds the pollen tube

The pollen tube is being subjected to control by a complex network of communication that regulates its shape and the misfunction of a single component causes specific deformations. In flowering plants, the pollen tube is a tubular extension of the pollen grain required for successful sexual reproduction. Indeed, maintaining the unique shape of the pollen tube is essential for the pollen tube to approach the embryo sac. Many processes and molecules (such as GTPase activity, phosphoinositides, Ca²⁺ gradient, distribution of reactive oxygen species and nitric oxide, nonuniform pH values, organization of the cytoskeleton, balance between exocytosis and endocytosis, and cell wall structure) play key and coordinated roles in maintaining the cylindrical shape of pollen tubes. In addition, the above factors must also interact with each other so that the cell shape is maintained while the pollen tube follows chemical signals in the pistil that guide it to the embryo sac. Any intrinsic changes (such as erroneous signals) or extrinsic changes (such as environmental stresses) can affect the above factors and thus fertilization by altering the tube morphology. In this review, the processes and molecules that enable the development and maintenance of the unique shape of pollen tubes in angiosperms are presented emphasizing their interaction with specific tube shape. Thus, the purpose of the review is to investigate whether specific deformations in pollen tubes can help us to better understand the mechanism underlying pollen tube shape.

The Role of Calcium/Calcium-Dependent Protein Kinases Signal Pathway in Pollen Tube Growth

Pollen tube (PT) growth as a key step for successful fertilization is essential for angiosperm survival and especially vital for grain yield in cereals. The process of PT growth is regulated by many complex and delicate signaling pathways. Among them, the calcium/calcium-dependent protein kinases (Ca2+/CPKs) signal pathway has become one research focus, as Ca2+ ion is a well-known essential signal molecule for PT growth, which can be instantly sensed and transduced by CPKs to control myriad biological processes. In this review, we summarize the recent progress in understanding the Ca2+/CPKs signal pathway governing PT growth. We also discuss how this pathway regulates PT growth and how reactive oxygen species (ROS) and cyclic nucleotide are integrated by Ca2+ signaling networks.

Genes in S and T subgenomes are responsible for hybrid lethality in interspecific hybrids between Nicotiana tabacum and Nicotiana occidentalis

BACKGROUND

Many species of Nicotiana section Suaveolentes produce inviable F(1) hybrids after crossing with Nicotiana tabacum (genome constitution SSTT), a phenomenon that is often called hybrid lethality. Through crosses with monosomic lines of N. tabacum lacking a Q chromosome, we previously determined that hybrid lethality is caused by interaction between gene(s) on the Q chromosome belonging to the S subgenome of N. tabacum and gene(s) in Suaveolentes species. Here, we examined if hybrid seedlings from the cross N. occidentalis (section Suaveolentes)×N. tabacum are inviable despite a lack of the Q chromosome.

METHODOLOGY/PRINCIPAL FINDINGS

Hybrid lethality in the cross of N. occidentalis×N. tabacum was characterized by shoots with fading color. This symptom differed from what has been previously observed in lethal crosses between many species in section Suaveolentes and N. tabacum. In crosses of monosomic N. tabacum plants lacking the Q chromosome with N. occidentalis, hybrid lethality was observed in hybrid seedlings either lacking or possessing the Q chromosome. N. occidentalis was then crossed with two progenitors of N. tabacum, N. sylvestris (SS) and N. tomentosiformis (TT), to reveal which subgenome of N. tabacum contains gene(s) responsible for hybrid lethality. Hybrid seedlings from the crosses N. occidentalis×N. tomentosiformis and N. occidentalis×N. sylvestris were inviable.

CONCLUSIONS/SIGNIFICANCE

Although the specific symptoms of hybrid lethality in the cross N. occidentalis×N. tabacum were similar to those appearing in hybrids from the cross N. occidentalis×N. tomentosiformis, genes in both the S and T subgenomes of N. tabacum appear responsible for hybrid lethality in crosses with N. occidentalis.

Transgene escape: what potential for crop-wild hybridization?

To date, regional surveys assessing the risk of transgene escape from GM crops have focused on records of spontaneous hybridization to infer the likelihood of crop transgene escape. However, reliable observations of spontaneous hybridization are lacking for most floras, particularly outside Europe. Here, we argue that evidence of interspecific reproductive compatibility derived from experimental crosses is an important component of risk assessment, and a useful first step especially where data from field observations are unavailable. We used this approach to assess the potential for transgene escape via hybridization for 123 widely grown temperate crops and their indigenous and naturalized relatives present in the New Zealand flora. We found that 66 crops (54%) are reproductively compatible with at least one other indigenous or naturalized species in the flora. Limited reproductive compatibility with wild relatives was evident for a further 12 crops (10%). Twenty-five crops (20%) were found to be reproductively isolated from all their wild relatives in New Zealand. For the remaining 20 crops (16%), insufficient information was available to determine levels of reproductive compatibility with wild relatives. Our approach may be useful in other regions where spontaneous crop-wild hybridization has yet to be well documented.