Two promoters conferring active gene expression in vegetative nuclei of tobacco immature pollen undergoing embryogenic dedifferentiation

Genotype-independent plant transformation

Plant transformation and regeneration remain highly species- and genotype-dependent. Conventional hormone-based plant regeneration via somatic embryogenesis or organogenesis is tedious, time-consuming, and requires specialized skills and experience. Over the last 40 years, significant advances have been made to elucidate the molecular mechanisms underlying embryogenesis and organogenesis. These pioneering studies have led to a better understanding of the key steps and factors involved in plant regeneration, resulting in the identification of crucial growth and developmental regulatory genes that can dramatically improve regeneration efficiency, shorten transformation time, and make transformation of recalcitrant genotypes possible. Co-opting these regulatory genes offers great potential to develop innovative genotype-independent genetic transformation methods for various plant species, including specialty crops. Further developing these approaches has the potential to result in plant transformation without the use of hormones, antibiotics, selectable marker genes, or tissue culture. As an enabling technology, the use of these regulatory genes has great potential to enable the application of advanced breeding technologies such as genetic engineering and gene editing for crop improvement in transformation-recalcitrant crops and cultivars. This review will discuss the recent advances in the use of regulatory genes in plant transformation and regeneration, and their potential to facilitate genotype-independent plant transformation and regeneration.

Species with Haploid or Doubled Haploid Protocols

In this chapter, we present a list of species (and few interspecific hybrids) where haploids and/or doubled haploids have been published, including the method by which they were obtained and the corresponding references. This list is an update of the compilation work of Maluszynski et al. published in 2003, including new species for which protocols were not available at that time, and also novel methodologies developed during these years. The list includes 383 different backgrounds. In this book, we present full protocols to produce DHs in 43 of the species included in this list. In addition, this book includes a chapter for one species not included in the list. This makes a total of 384 species where haploids and/or DHs have been reported up to date.

Timing of the G1/S transition in tobacco pollen vegetative cells as a primary step towards androgenesis in vitro

KEY MESSAGE

Mid-bicellular pollen vegetative cells in tobacco escape from G1 arrest and proceed to the G1/S transition towards androgenesis within 1 day under glutamine starvation conditions in vitro. In the Nicotiana tabacum pollen culture system, immature pollen grains at the mid-bicellular stage can mature in the presence of glutamine; however, if glutamine is absent, they deviate from their native cell fate in a few days. The glutamine-starved pollen grains cannot undergo maturation, even when supplied with glutamine later. Instead, they undergo cell division towards androgenesis slowly within 10 days in a medium containing appropriate nutrients. During the culture period, they ought to escape from G1 arrest to proceed into S phase as the primary step towards androgenesis. However, this event has not been experimentally confirmed. Here, we demonstrated that the pollen vegetative cells proceeded to the G1/S transition within approximately 15-36 h after the start of culture. These results were obtained by analyzing transgenic pollen possessing a fusion gene encoding nuclear-localizing GFP under the control of an E2F motif-containing promoter isolated from a gene encoding one of DNA replication licensing factors. Observations using a 5-ethynyl-2'-deoxyuridine DNA labeling and detection technique uncovered that the G1/S transition was soon followed by S phase. These hallmarks of vegetative cells undergoing dedifferentiation give us new insights into upstream events causing the G1/S transition and also provide a novel strategy to increase the frequency of the androgenic response in tobacco and other species, including recalcitrants.

Promoters of two anther-specific genes confer organ-specific gene expression in a stage-specific manner in transgenic systems

Differential screening of a stage-specific cDNA library of Indica rice has been used to identify two genes expressed in pre-pollination stage panicles, namely OSIPA and OSIPK coding for proteins similar to expansins/pollen allergens and calcium-dependent protein kinases (CDPK), respectively. Northern analysis and in situ hybridizations indicate that OSIPA expresses exclusively in pollen while OSIPK expresses in pollen as well as anther wall. Promoters of these two anther-specific genes show the presence of various cis-acting elements (GTGA and AGAAA) known to confer anther/pollen-specific gene expression. Organ/tissue-specific activity and strength of their regulatory regions have been determined in transgenic systems, i.e., tobacco and Arabidopsis. A unique temporal activity of these two promoters was observed during various developmental stages of anther/pollen. Promoter of OSIPA is active during the late stages of pollen development and remains active till the anthesis, whereas, OSIPK promoter is active to a low level in developing anther till the pollen matures. OSIPK promoter activity diminishes before anthesis. Both promoters show a potential to target expression of the gene of interest in developmental stage-specific manner and can help engineer pollen-specific traits like male-sterility in plants.

Shoot formation from root tip region: a developmental alteration by WUS in transgenic tobacco

We examined the effect of ectopic expression of WUS on the morphology of tobacco seedlings and the segments in vitro. WUS was amplified from Arabidopsis cDNA and introduced into the tobacco genome under the transcriptional control of the beta-estradiol-inducible expression system. When 1-week-old transgenic seedlings were cultured in the presence of beta-estradiol, only the root tip region developed bulbous tissues followed by shoot formation and plant regeneration, suggesting its applicability for improving the strategy of micropropagation in recalcitrant species. Evident abnormality was not observed in the cotyledons, hypocotyl nor root except for the tip. However, ectopic WUS seemed to be functional in those parts through the observation of gene expression and the behavior of cultured segments. Small root segments with a root tip treated with beta-estradiol also showed bulbing but no shoots unless exogenous cytokinin was supplied. These findings suggest the existence of unknown factors regulating ectopic WUS function in the seedling.

An aluminum-activated citrate transporter in barley

Soluble ionic aluminum (Al) inhibits root growth and reduces crop production on acid soils. Al-resistant cultivars of barley (Hordeum vulgare L.) detoxify Al by secreting citrate from the roots, but the responsible gene has not been identified yet. Here, we identified a gene (HvAACT1) responsible for the Al-activated citrate secretion by fine mapping combined with microarray analysis, using an Al-resistant cultivar, Murasakimochi, and an Al-sensitive cultivar, Morex. This gene belongs to the multidrug and toxic compound extrusion (MATE) family and was constitutively expressed mainly in the roots of the Al-resistant barley cultivar. Heterologous expression of HvAACT1 in Xenopus oocytes showed efflux activity for (14)C-labeled citrate, but not for malate. Two-electrode voltage clamp analysis also showed transport activity of citrate in the HvAACT1-expressing oocytes in the presence of Al. Overexpression of this gene in tobacco enhanced citrate secretion and Al resistance compared with the wild-type plants. Transiently expressed green fluorescent protein-tagged HvAACT1 was localized at the plasma membrane of the onion epidermal cells, and immunostaining showed that HvAACT1 was localized in the epidermal cells of the barley root tips. A good correlation was found between the expression of HvAACT1 and citrate secretion in 10 barley cultivars differing in Al resistance. Taken together, our results demonstrate that HvAACT1 is an Al-activated citrate transporter responsible for Al resistance in barley.