Generation of composite Persea americana (Mill.) (avocado) plants: A proof-of-concept-study

Agrobacterium rhizogenes: paving the road to research and breeding for woody plants

Woody plants play a vital role in global ecosystems and serve as valuable resources for various industries and human needs. While many woody plant genomes have been fully sequenced, gene function research and biotechnological breeding advances have lagged behind. As a result, only a limited number of genes have been elucidated, making it difficult to use newer tools such as CRISPR-Cas9 for biotechnological breeding purposes. The use of Agrobacterium rhizogenes as a transformative tool in plant biotechnology has received considerable attention in recent years, particularly in the research field on woody plants. Over the past three decades, numerous woody plants have been effectively transformed using A. rhizogenes-mediated techniques. Some of these transformed plants have successfully regenerated. Recent research on A. rhizogenes-mediated transformation of woody plants has demonstrated its potential for various applications, including gene function analysis, gene expression profiling, gene interaction studies, and gene regulation analysis. The introduction of the Ri plasmid has resulted in the emergence of several Ri phenotypes, such as compact plant types, which can be exploited for Ri breeding purposes. This review paper presents recent advances in A. rhizogenes-mediated basic research and Ri breeding in woody plants. This study highlights various aspects of A. rhizogenes-mediated transformation, its multiple applications in gene function analysis, and the potential of Ri lines as valuable breeding materials.

An efficient root transformation system for CRISPR/Cas9-based analyses of shoot-root communication in cucurbit crops

Cucurbit crops are suitable models for studying long-distance signaling in horticultural plants. Although thousands of substances are graft transmissible in cucurbits, functional studies have been hampered by the lack of efficient genetic transformation systems. Here, we report a convenient and efficient root transformation method for several cucurbit crops that will facilitate studies of functional genes and shoot-root crosstalk. We obtained healthy plants with completely transformed roots and non-transgenic shoots within 6 weeks. Furthermore, we combined this root transformation method with grafting, which allowed for gene manipulation in the rootstock. We validated our system by exploring salt tolerance mechanisms using a cucumber (Cucumis sativus)/pumpkin (Cucurbita moschata Duch.) (scion/rootstock) graft in which the sodium transporter gene High-affinity K+ transporter1 (CmoHKT1;1) was edited in the pumpkin rootstock, and by overexpressing the pumpkin tonoplast Na+/H+ antiporter gene Sodium hydrogen exchanger4 (CmoNHX4) in cucumber roots.

Advances in Understanding Defense Mechanisms in Persea americana Against Phytophthora cinnamomi

Avocado (Persea americana) is an economically important fruit crop world-wide, the production of which is challenged by notable root pathogens such as Phytophthora cinnamomi and Rosellinia necatrix. Arguably the most prevalent, P. cinnamomi, is a hemibiotrophic oomycete which causes Phytophthora root rot, leading to reduced yields and eventual tree death. Despite its' importance, the development of molecular tools and resources have been historically limited, prohibiting significant progress toward understanding this important host-pathogen interaction. The development of a nested qPCR assay capable of quantifying P. cinnamomi during avocado infection has enabled us to distinguish avocado rootstocks as either resistant or tolerant - an important distinction when unraveling the defense response. This review will provide an overview of our current knowledge on the molecular defense pathways utilized in resistant avocado rootstock against P. cinnamomi. Notably, avocado demonstrates a biphasic phytohormone profile in response to P. cinnamomi infection which allows for the timely expression of pathogenesis-related genes via the NPR1 defense response pathway. Cell wall modification via callose deposition and lignification have also been implicated in the resistant response. Recent advances such as composite plant transformation, single nucleotide polymorphism (SNP) analyses as well as genomics and transcriptomics will complement existing molecular, histological, and biochemical assay studies and further elucidate avocado defense mechanisms.

An improved method for Agrobacterium rhizogenes-mediated transformation of tomato suitable for the study of arbuscular mycorrhizal symbiosis

BACKGROUND

Solanum lycopersicum, an economically important crop grown worldwide, has been used as a model for the study of arbuscular mycorrhizal (AM) symbiosis in non-legume plants for several years and several cDNA array hybridization studies have revealed specific transcriptomic profiles of mycorrhizal tomato roots. However, a method to easily screen candidate genes which could play an important role during tomato mycorrhization is required.

RESULTS

We have developed an optimized procedure for composite tomato plant obtaining achieved through Agrobacterium rhizogenes-mediated transformation. This protocol involves the unusual in vitro culture of composite plants between two filter papers placed on the culture media. In addition, we show that DsRed is an appropriate molecular marker for the precise selection of cotransformed tomato hairy roots. S. lycopersicum composite plant hairy roots appear to be colonized by the AM fungus Rhizophagus irregularis in a manner similar to that of normal roots, and a modified construct useful for localizing the expression of promoters putatively associated with mycorrhization was developed and tested.

CONCLUSIONS

In this study, we present an easy, fast and low-cost procedure to study AM symbiosis in tomato roots.