Persistent Calyxes in Postbloom Fruit Drop: A Microscopy and Microanalysis Perspective.

João Paulo Rodrigues Marques,Geraldo José Silva Junior,Lilian Amorim,Beatriz Appezzato-Da-Glória,Gabriel Sgarbiero Montanha,Marcel Bellato Spósito,Hudson Wallace Pereira De Carvalho

doi:10.3390/pathogens9040251

João Paulo Rodrigues Marques, Geraldo José Silva Junior + Show 5 more

Open Access

https://doi.org/10.3390/pathogens9040251

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Abstract

Citrus postbloom fruit drop, caused by Colletotrichum spp., is an important disease in the Americas. The pathogen infects citrus flowers, produces orange-brown lesions on petals, and may cause the abscission of young fruit. In diseased flowers, the calyxes remain attached to the peduncle after the young fruit drop. No anatomical and microanalysis studies have been conducted to determine whether calyx tissues can be infected by Colletotrichum spp. and why calyxes remain attached to the peduncle. Based on light microscopy, we demonstrate that the ovary abscission zone exhibits a separation region composed of layers of thickened lignified walled cells, indicating that abscission involves the disruption of cell walls. The first layers of the protective zone (PZ) are composed of densely packed cells with suberized walls produced by the wound meristem. Beneath the PZ, there is a compact mass of small cells that accumulate starch grains. X-ray fluorescence microanalysis (µ-XRF) confirmed the increased accumulation of calcium in the receptacle of the persistent calyxes compared to non-inoculated citrus flowers. Moreover, the peduncle pith and the receptacle exhibit hypertrophied cells with thick walls that may be related to calyx retention. Fungal structures are not observed inside the persistent calyx tissues.

Highlights

Postbloom fruit drop (PFD) is an important citrus disease caused by Colletotrichum spp
Histochemical tests with zinc iodine chloride revealed the absence of starch in the receptacle pith cells (Figure 1D)
Our results showed a new abscission zone produced by citrus plants in response to Colletotrichum acutatum infection

Summary

Introduction

Postbloom fruit drop (PFD) is an important citrus disease caused by Colletotrichum spp. PFD is responsible for causing serious economic losses to citrus growers, and it is considered a limiting factor for citrus production in countries of Central America and Brazil [6]. Colletotrichum spp. are responsible for causing orange lesions on petals, which can coalesce and compromise the entire surface and colonize all its tissues [10,11,12]. The stigma may present lesions when infected, but these lesions are necrotic and localized since the fungus does not colonize the stigma tissues [13]. After the period of the colonization of petal tissues, premature fall of the ovary occurs, as does the production of persistent calyxes or “starlets”. Persistent calyxes are diagnostic for Pathogens 2020, 9, 251; doi:10.3390/pathogens9040251 www.mdpi.com/journal/pathogens

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