Differential Responses of Vanilla Accessions to Root Rot and Colonization by Fusarium oxysporum f. sp. radicis-vanillae

Abstract

Root and stem rot (RSR) disease caused by Fusarium oxysporum f. sp. radicis-vanillae (Forv) is the most damaging disease of vanilla (Vanilla planifolia and V. × tahitensis, Orchidaceae). Breeding programs aimed at developing resistant vanilla varieties are hampered by the scarcity of sources of resistance to RSR and insufficient knowledge about the histopathology of Forv. In this work we have (i) identified new genetic resources resistant to RSR including V. planifolia inbreds and vanilla relatives, (ii) thoroughly described the colonization pattern of Forv into selected vanilla accessions, confirming its necrotic non-vascular behavior in roots, and (iii) evidenced the key role played by hypodermis, and particularly lignin deposition onto hypodermal cell walls, for resistance to Forv in two highly resistant vanilla accessions. Two hundred and fifty-four vanilla accessions were evaluated in the field under natural conditions of infection and in controlled conditions using in vitro plants root-dip inoculated by the highly pathogenic isolate Fo072. For the 26 accessions evaluated in both conditions, a high correlation was observed between field evaluation and in vitro assay. The root infection process and plant response of one susceptible and two resistant accessions challenged with Fo072 were studied using wide field and multiphoton microscopy. In susceptible V. planifolia, hyphae penetrated directly into the rhizodermis in the hairy root region then invaded the cortex through the passage cells where it induced plasmolysis, but never reached the vascular region. In the case of the resistant accessions, the penetration was stopped at the hypodermal layer. Anatomical and histochemical observations coupled with spectral analysis of the hypodermis suggested the role of lignin deposition in the resistance to Forv. The thickness of lignin constitutively deposited onto outer cell walls of hypodermis was highly correlated with the level of resistance for 21 accessions tested. The accumulation of p-coumaric and sinapic acids, two phenolic precursors of lignin, was observed in the resistant plants inoculated with Fo072, but not in the susceptible one. Altogether, our analyses enlightened the mechanisms at work in RSR resistant genotypes and should enhance the development of novel breeding strategies aimed at improving the genetic control of RSR of vanilla.

A new vanilla species from Costa Rica closely related to V. planifolia (Orchidaceae)

“…nov., a species that is genetically, morphologically, and biochemically distinct from V. planifolia. Finally, given its original aromatic content combining vanilyl and anisic notes, its high level of resistance (at least for CR0068) to the Fusarium root rot of vanilla (Koyyappurath et al 2015), and its early and abundant flowering in culture, V. sotoarenasii sp. nov. could be a promising genitor for breeding programs aiming to produce new vanilla varieties for the agroindustry.…”

Relación de la temperatura y humedad relativa con el brote del hongo Fusarium oxysporum f. sp. vanillae

“…La vainilla es la única orquídea con fruto cultivada comercialmente para lo obtención de la vainillina (Ramírez et al, 2015;Koyyappurath et al, 2015). La vainilla necesita de ciertas condiciones para tener un buen desarrollo, como lo son, una temperatura entre 23 y 26°C (Childers, 1948), e incluso entre 20 y 30°C (Hernández, 2011), precipitaciones anuales entre 2 000 y 3 000 mm, una humedad relativa alta, alrededor de 70% y sombra entre 50 y 70% según la época del año (Hernández, 2011).…”

“…La producción se ha visto afectada por la aparición del hongo FOV, causando grandes pérdidas a los productores (Pinaria et al, 2015;Adame et al, 2016). El hongo se encuentra alrededor del mundo e infecta al huésped a través de la raíz (Koyyappurath et al, 2015;Pinaria et al, 2015).…”

“…Además, los métodos actuales de bio-control y uso de fungicidas no han mostrado buenos resultados (Gangadara et al, 2010;Koyyappurath et al, 2015;Adame et al, 2015;Jiménez et al, 2015).…”

“…Vanilla is the only fruit orchid grown commercially to obtain vanillin (Ramírez et al, 2015;Koyyappurath et al, 2015). Vanilla needs certain conditions to have a good development, such as a temperature between 23 and 26 °C (Childers, 1948), and even between 20 and 30 °C (Hernández, 2011), annual precipitations between 2 000 mm and 3 000 mm, a high relative humidity around 70% and shade between 50 and 70% depending on the time of year (Hernández, 2011).…”