Brazil’s Suzana Garcia recently defended her PhD thesis in Bioscience Engineering at KU Leuven University on the study of ABA-responsive mechanisms in banana plants. Rony Swennen, an RTB scientist, was the first promoter of her work, with other Belgian professor S. Carpentier, Dr. Bart Panis and Brazil’s Dr. M.T. Souza Junior.
“We reported for the first time that Cachaco and Mbwazirume, two banana varieties with varying degrees of drought tolerance, presented different responsiveness towards abscisic acid (ABA)”, Suzana explains. The drought-tolerant variety, Cachaco, exhibited greater responsiveness towards ABA at the morphological, physiological and cellular levels. These results help explain why Cachaco is more drought-tolerant than Mbwazirume and support drought research in banana in support of breeding bananas for climatic change.”
While the thesis is the outcome of Suzana’s work, it owes a lot to the partnership between the Brazilian Agricultural Research Corporation (Embrapa) and two key European universities partnering with RTB: Wageningen University in the Netherlands and KU Leuven in Belgium. During her master studies, Suzana did a 9-month internship at the Plant Research International (PRI) associated with Wageningen University. She was under the supervision of Dr. Manoel Souza, a researcher working in advanced biology at Embrapa LABEX Europe. Dr. Souza established the collaboration between Embrapa and KU Leuven on banana research via Prof. Rony Swennen. Suzana was introduced to Prof. Swennen during this time, which resulted in the idea of a PhD project to investigate mechanisms of drought tolerance in banana plants as this is so much relevant for banana growers worldwide but especially in Brazil.
The PhD research was made possible thanks to Brazil’s National Council for Scientific and Technological Development (CNPq), which granted a 4-year scholarship to Suzana, and also thanks to the Belgian Development Cooperation and RTB center Bioversity International, which funded this research.
Banana is an important food crop for millions of people from developing countries in Latin America, Asia and Africa. More than 400 million people depend on it as a staple food. In addition, bananas are extremely important for export with a value of nearly 8 billion USD. Banana cultivation suffers from drought in the dry season and this is expected to increase with climate change. This should foster research on water stress demand and on the characterization of the banana biodiversity towards drought finally enabling the development of more drought tolerant banana varieties. Drought tolerance in banana has been linked to the presence of the B genome. This seems to be supported by our findings that the ‘Cachaco’ variety (ABB) is more drought tolerant than the ‘Mbwazirume’ variety (AAA). However, the understandings of the drought tolerance mechanisms in banana are still limited. A crucial mechanism developed by the plants to cope with water stress is the closure of stomata to avoid water loss. Stomatal closure is induced by abscisic acid (ABA), a phytohormone whose production is increased under drought stress. Different plant tissues can produce ABA and its production increases depending on where the actual stress takes place. When roots are stressed by water shortage, they produce ABA, which can then be transported from the roots to the leaves to anticipate and control transpiration. Additionally, ABA triggers other mechanisms of drought tolerance in other tissues/cell types, such as root conductance to increase the water uptake from the soil. It also slows down plant growth and metabolism, resulting in saved energy during the period of water scarcity. To study the role of ABA in banana plants, we first optimized an existing autotrophic system to grow banana plantlets under controlled light, humidity and temperature conditions. The root system of the plantlets was submersed into a hydroponic solution, and both pseudostem and leaves were exposed to ambient conditions within an incubator. This system allowed for reproducible plant responses measurements in a relatively short time period.
To evaluate how banana plants react to ABA and how ABA might contribute to drought tolerance, we selected two banana varieties: ‘Cachaco’ and ‘Mbwazirume’. We applied ABA into the hydroponic solution so that the submersed roots could mimic a root-derived stress signal. Subsequently, we also sprayed ABA on the leaves, to mimic a leaf-derived signal. We showed that exogenous ABA application to roots decreased plant growth and total water loss, and increased guttation. The ABA concentration in the leaves was also shown to increase. We also showed that ABA sprayed on the leaves decreased total plant water loss and subsequently increased leaf temperature. Additionally, we concluded that ‘Cachaco’ presented greater responsiveness towards ABA in terms of growth reduction and stomata sensitivity. We subsequently focused on the role of ABA at the cellular level and studied plasma membrane proteins extracted from root cells because they are involved in the transmission of external signals to the interior of the cell, and in the selective transport of water, nutrients and ions across the plasma membrane. The study of plasma membrane proteins is challenging because of their poor solubility in aqueous media and low relative abundance. We showed that a microsomal-based strategy proved to be a valuable tool to isolate plasma membrane proteins. Then we extracted microsomal fractions from ‘Cachaco’ and ‘Mbwazirume’ roots treated or not with ABA and applied quantitative high throughput proteomics for data analysis. We showed that ABA significantly changed the root proteome of both varieties and most of the ABA responsive proteins identified were predicted to be membrane-associated. The number of ABA responsive proteins identified in ‘Cachaco’ was higher than in ‘Mbwazirume’ suggesting that the ABA effect was more pronounced in ‘Cachaco’ proteins. We concluded – based on the ABA response at the morphological, physiological and cellular levels – that ‘Cachaco’ was more responsive to ABA and this may contribute to its better drought tolerance.
By Véronique Durroux-Malpartida