Research into the flowering dynamics and molecular regulation of heterodichogamy in avocado trees holds significant importance for both agricultural practice and scientific understanding. Avocado is a valuable crop worldwide, with increasing demand driven by its nutritional value. However, optimizing fruit yield and quality proves challenging because of the unique reproductive strategy of avocado trees. With Type A and B trees flowering at different times of the day promoting cross-pollination becomes crucial. Understanding the genetic mechanisms underlying the timing of flower maturation is crucial for efficient pollination and fruit set, directly impacting yield and profitability for avocado growers. By elucidating the key genes involved in heterodichogamy and their regulatory pathways, this research can inform targeted breeding efforts to enhance pollination success and fruit production in avocado orchards.

Moreover, unravelling the molecular intricacies of flower development in avocado contributes to broader scientific knowledge in plant biology and reproductive biology. Avocado exhibits a specialized form of heterodichogamy, a reproductive strategy that sets it apart from many other plant species. Insight gained from this research can advance our understanding of how plants coordinate flowering and reproductive timing. Additionally, identifying molecular markers associated with flower type and heterodichogamy facilitates the development of screening tools for growers to accurately classify avocado trees and optimize orchard management strategies.

Therefore, the research aims to identify differentially expressed genes between Type A and B avocado trees and to develop a putative model for heterodichogamy in avocado using differentially expressed genes from an RNA-seq experiment, with the goal of developing a screening tool for the identification of Type A and B avocado trees. This research bridges fundamental and applied science, offering practical solutions for sustainable avocado production while enriching our understanding of plant reproductive biology.

ARP Team Members

Johane Cilliers

Dematophora necatrix (previously Rosellinia necatrix Berl. ex Prill.) is an ascomycete pathogen that targets a multitude of different plant hosts in various tropical and temperate regions. As the causal agent of white root rot (WRR) it has caused significant economic losses within the agricultural and forestry industries, including apple, citrus and avocado. Symptoms and the presence of D. necatrix were confirmed in a commercial avocado orchard in the Limpopo Province (South Africa) in 2016. Since then, it has been detected in Mpumalanga, Western Cape and Kwa-Zulu Natal.

Control options for WRR are imperfect due to the pathogen’s hardy resting structures, deep soil penetration, and resistance to common fungicides. However, various approaches have been used to manage D. necatrix infection, alone or in combination, which includes; (i) cultural practices, such as the removal of diseased plant material; (ii) the use of uninfected and/ or D. necatrix resistant/ tolerant plant material; (iii) physical practices, such as soil solarization; (iv) the use of chemical control agents, such as, fluazinam; (v) the use of biological control agents, including, plant growth-promoting rhizobacteria and Trichoderma.; (vi) and lastly, periodic testing for the presence of the pathogen in soil using baiting and PCR-based detection techniques.

Currently, research in the ARP is aimed at understanding D. necatrix population diversity within South Africa and the rhizosphere microbial communities in avocado trees infected with D. necatrix. As well as, to develop an effective management strategy for D. necatrix in avocado orchards using chemical control and biological control agents. Additionally, the ARP is investigating the potential of CRISPR-Cas9 genome editing technology in D. necatrix targeting putative pathogenicity genes, which will be used for functional analysis.

* Read more about Dematophora necatrix on our Fact sheet here.

ARP Team Members

Phinda Magagula: The detection and management of Dematophora necatrix in avocado orchards.

Tsakane Miyambo: Investigating the genetic diversity, population structure and virulence of Dematophora necatrix in South Africa.

Raven Wienk: Investigating the Persea americana - Dematophora necatrix - Trichoderma interaction.

Dr Molly Malefo: CRISPR/Cas9 ribonucleoprotein (RNP)-based genome editing in D. necatrix.

Images from left to right: 1. D. necatrix on semi-selective media. 2. D. necatrix infected soil material in an orchard. 3. White mycelial fans of D. necatrix at the crown of an infected tree.

Avocado sunblotch is a disease caused by avocado sunblotch viroid (ASBVd) – a circular single-stranded RNA molecule in the family Avsunviroidae which is only about 250 nucleotides in length. Despite its tiny size, and the fact that this viroid is not classified as a living organism, the presence of ASBVd in an avocado host can lead to the appearance of severe symptoms such as the formation of coloured, sunken lesions on avocado fruit, discoloured streaks on young stems, and discolouration and malformation of leaves. Although the appearance of these symptoms will not usually lead to death of a healthy avocado tree, fruit symptoms will impact market value of the fruit, and asymptomatic trees may have significantly reduced yield when compared to their uninfected counterparts.

While previous research on ASBVd has been largely focused on mapping its distribution, refining detection techniques and monitoring the physiological effect of infection on avocado trees and fruit, there is as yet no definitive evidence to explain how the viroid causes disease at the molecular level. Current research within the ARP aims to elucidate molecular mechanisms of avocado sunblotch disease by investigating host responses to viroid infection, as well as the viroid variants associated with chlorotic symptoms in avocado hosts. By expanding our understanding of how ASBVd causes host symptoms at the molecular level we will pave the way for future research which will aid South African avocado growers in combatting this disease.

*Read more about avocado sunblotch viroid on our Fact sheet here.

ARP Team Members

Melissa Joubert: Investigating molecular mechanisms for disease caused by avocado sunblotch viroid (ASBVd).

Images from left to right: 1. Typical fruit symptoms caused by ASBVd infection include the formation of yellow, sunken lesions on avocado. 2. ASBVd infection may cause bleaching symptoms in avocado leaves, characterised by yellowing of leaf tissues near or along the leaf midvein. 3. PhD candidate Melissa Joubert sampling avocado tissues with sunblotch symptoms. Symptomatic tissues may be restricted to only one part of an infected tree, while the rest of the tree remains free of chlorotic symptoms.

The advent of next-generation sequencing (NGS) technologies has revolutionised research capacity and its broad applicability over the past decade. These technologies have directly impacted researchers’ ability to understand host-pathogen interactions, holistically, and on a molecular scale. Recently, the avocado industry has begun to unravel some of the underlying mysteries that pertain to prevalent avocado pests and diseases; the Avocado Research Programme (ARP) has been highly influential in this space.

As a founding member of the Avocado Genome Consortium - an international collaborative effort which was established in 2016 - the ARP has directly contributed to the development of a high quality, chromosome-level reference genome assembly for Persea americana (avocado). The Avocado Genome Consortium has used this new reference genome to re-sequence more than 10 additional cultivars and rootstocks as part of an ongoing project, with plans to increase this number soon. Provided with this data, researchers will be able to accelerate the arduous process of selecting rootstocks and cultivars with desirable traits.

As part of the broader objectives of the Avocado Genome Consortium, transcriptomic data has also been generated. This data will be utilized to answer fundamental questions pertaining to the evolutionary biology, gene expression, physiological processes, and molecular pathways in avocado. Transcriptomic data from a dual RNA-sequencing experiment - involving both susceptible and partially resistant avocado rootstocks challenged with Phytophthora cinnamomi - was also used to identify avocado defence targets and discovery of pathogen effectors involved in disease development. The data from this work has been published across several research articles and will be used to further our understanding of the avocado-P. cinnamomi interaction.

The researchers involved are:

• David Kuhn, USDA, Florida
• Patricia Manosalva, UCR, California
• Noëlani van den Berg, UP, South Africa
• Antonio Javier Matas Arroyo, Departamento de Biología Vegetal, University of Malaga, Spain
• Aureliano Bombarley Gomez, Virginia Tech Horticulture, USA
• Randy Ploetz, University of Florida, USA
• Alan Chambers, University of Florida, USA