Novel natural genetic variation controlling the competence to form adventitious roots and shoots from the tomato wild relative Solanum pennellii

Approaching the genetic dissection of indirect adventitious organogenesis process in tomato explants

The screening of 862 T-DNA lines was carried out to approach the genetic dissection of indirect adventitious organogenesis in tomato. Several mutants defective in different phases of adventitious organogenesis, namely callus growth (tdc-1), bud differentiation (tdb-1, -2, -3) and shoot-bud development (tds-1) were identified and characterized. The alteration of the TDC-1 gene blocked callus proliferation depending on the composition of growth regulators in the culture medium. Calli from tds-1 explants differentiated buds but did not develop normal shoots. Histological analysis showed that their abnormal development is due to failure in the organization of normal adventitious shoot meristems. Interestingly, tdc-1 and tds-1 mutant plants were indistinguishable from WT ones, indicating that the respective altered genes play specific roles in cell proliferation from explant cut zones (TDC-1 gene) or in the organization of adventitious shoot meristems (TDS-1 gene). Unlike the previous, plants of the three mutants defective in the differentiation of adventitious shoot-buds (tdb-1, -2, -3) showed multiple changes in vegetative and reproductive traits. Cosegregation analyses revealed the existence of an association between the phenotype of the tdb-3 mutant and a T-DNA insert, which led to the discovery that the SlMAPKKK17 gene is involved in the shoot-bud differentiation process.

Comparative Analysis of In Vitro Responses and Regeneration between Diverse Bioenergy Sorghum Genotypes

Sorghum has been considered a recalcitrant plant in vitro and suffers from a lack of regeneration protocols that function broadly and efficiently across a range of genotypes. This study was initiated to identify differential genotype-in vitro protocol responses across a range of bioenergy sorghum parental lines and the common grain sorghum genotype Tx430 in order to characterize response profiles for use in future genetic studies. Two different in vitro protocols, LG and WU, were used for comparisons. Distinct genotype-protocol responses were observed, and the WU protocol performed significantly better for plantlet regeneration. Most bioenergy genotypes performed as well, if not better than Tx430, with Rio and PI329311 as the top regenerating lines. Genotypes displayed protocol-dependent, differential phenolic exudation responses, as indicated by medium browning. During the callus induction phase, genotypes prone to medium browning exhibited a response on WU medium which was either equal or greater than on LG medium. Genotype- and protocol-dependent albino plantlet regeneration was also noted, with three of the bioenergy genotypes showing albino plantlet regeneration. Grassl, Rio and Pink Kafir were susceptible to albino plantlet regeneration, with the response strongly associated with the WU protocol. These bioenergy parental genotypes, and their differential responses under two in vitro protocols, provide tools to further explore and assess the role of genetic loci, candidate genes, and allelic variants in the regulation of in vitro responsiveness in sorghum.

The major-effect quantitative trait locus CsARN6.1 encodes an AAA ATPase domain-containing protein that is associated with waterlogging stress tolerance by promoting adventitious root formation

In plants, the formation of hypocotyl-derived adventitious roots (ARs) is an important morphological acclimation to waterlogging stress; however, its genetic basis remains fragmentary. Here, through combined use of bulked segregant analysis-based whole-genome sequencing, SNP haplotyping and fine genetic mapping, we identified a candidate gene for a major-effect QTL, ARN6.1, that was responsible for waterlogging tolerance due to increased AR formation in the cucumber line Zaoer-N. Through multiple lines of evidence, we show that CsARN6.1 is the most possible candidate for ARN6.1 which encodes an AAA ATPase. The increased formation of ARs under waterlogging in Zaoer-N could be attributed to a non-synonymous SNP in the coiled-coil domain region of this gene. CsARN6.1 increases the number of ARs via its ATPase activity. Ectopic expression of CsARN6.1 in Arabidopsis resulted in better rooting ability and lateral root development in transgenic plants. Transgenic cucumber expressing the CsARN6.1^Asp allele from Zaoer-N exhibited a significant increase in number of ARs compared with the wild type expressing the allele from Pepino under waterlogging conditions. Taken together, these data support that the AAA ATPase gene CsARN6.1 has an important role in increasing cucumber AR formation and waterlogging tolerance.

Genome editing as a tool to achieve the crop ideotype and de novo domestication of wild relatives: Case study in tomato

The ideotype is a theoretical model of an archetypal cultivated plant. Recent progress in genome editing is aiding the pursuit of this ideal in crop breeding. Breeding is relatively straightforward when the traits in question are monogenic in nature and show Mendelian inheritance. Conversely, traits with a diffuse, polygenic basis such as abiotic stress resistance are more difficult to harness. In recent years, many genes have been identified that are important for plant domestication and act by increasing yield, grain or fruit size or altering plant architecture. Here, we propose that (a) key monogenic traits whose physiology has been unveiled can be molecularly tailored to achieve the ideotype; and (b) wild relatives of crops harboring polygenic stress resistance genes or other traits of interest could be de novo domesticated by manipulating monogenic yield-related traits through state-of-the-art gene editing techniques. An overview of the genomic and physiological challenges in the world's main staple crops is provided. We focus on tomato and its wild Solanum (section Lycopersicon) relatives as a suitable model for molecular design in the pursuit of the ideotype for elite cultivars and to test de novo domestication of wild relatives.

Chrysanthemum transcription factor CmLBD1 direct lateral root formation in Arabidopsis thaliana

The plant-specific LATERAL ORGAN BOUNDARIES DOMAIN (LBD) genes are important regulators of growth and development. Here, a chrysanthemum class I LBD transcription factor gene, designated CmLBD1, was isolated and its function verified. CmLBD1 was transcribed in both the root and stem, but not in the leaf. The gene responded to auxin and was shown to participate in the process of adventitious root primordium formation. Its heterologous expression in Arabidopsis thaliana increased the number of lateral roots formed. When provided with exogenous auxin, lateral root emergence was promoted. CmLBD1 expression also favored callus formation from A. thaliana root explants in the absence of exogenously supplied phytohormones. In planta, CmLBD1 probably acts as a positive regulator of the response to auxin fluctuations and connects auxin signaling with lateral root formation.

Adventitious roots and lateral roots: similarities and differences

In addition to its role in water and nutrient uptake, the root system is fundamentally important because it anchors a plant to its substrate. Although a wide variety of root systems exist across different species, all plants have a primary root (derived from an embryonic radicle) and different types of lateral roots. Adventitious roots, by comparison, display the same functions as lateral roots but develop from aerial tissues. In addition, they not only develop as an adaptive response to various stresses, such as wounding or flooding, but also are a key limiting component of vegetative propagation. Lateral and adventitious roots share key elements of the genetic and hormonal regulatory networks but are subject to different regulatory mechanisms. In this review, we discuss the developmental processes that give rise to lateral and adventitious roots and highlight knowledge acquired over the past few years about the mechanisms that regulate adventitious root formation.