Specialized 16SrX phytoplasmas induce diverse morphological and physiological changes in their respective fruit crops

Abstract

The host-pathogen combinations—Malus domestica (apple)/`Candidatus Phytoplasma mali´, Prunus persica (peach)/`Ca. P. prunorum´ and Pyrus communis (pear)/`Ca. P. pyri´ show different courses of diseases although the phytoplasma strains belong to the same 16SrX group. While infected apple trees can survive for decades, peach and pear trees die within weeks to few years. To this date, neither morphological nor physiological differences caused by phytoplasmas have been studied in these host plants. In this study, phytoplasma-induced morphological changes of the vascular system as well as physiological changes of the phloem sap and leaf phytohormones were analysed and compared with non-infected plants. Unlike peach and pear, infected apple trees showed substantial reductions in leaf and vascular area, affecting phloem mass flow. In contrast, in infected pear mass flow and physicochemical characteristics of phloem sap increased. Additionally, an increased callose deposition was detected in pear and peach leaves but not in apple trees in response to phytoplasma infection. The phytohormone levels in pear were not affected by an infection, while in apple and peach trees concentrations of defence- and stress-related phytohormones were increased. Compared with peach and pear trees, data from apple suggest that the long-lasting morphological adaptations in the vascular system, which likely cause reduced sap flow, triggers the ability of apple trees to survive phytoplasma infection. Some phytohormone-mediated defences might support the tolerance.

Application of Plant Defense Elicitors Fails to Enhance Herbivore Resistance or Mitigate Phytoplasma Infection in Cranberries

“…We can also not discard the possibility that other phytohormones besides SA and JA, such as ethylene and abscisic acid (which were not measured here), might play a role in interactions among cranberries, phytoplasma, and insect herbivores ( Dermastia, 2019 ). It is worth noting that phytoplasma infection not only exclusively affects the plant nutritional and biochemical composition, but also induces morphological and physiological changes in their respective host plants ( Gallinger et al, 2021 ); thus, other factors not measured here could have also contributed to the observed improved herbivore performance.…”

Increased susceptibility to Chrysanthemum Yellows phytoplasma infection in Atcals7ko plants is accompanied by enhanced expression of carbohydrate transporters

“…It is puzzling, how infected wild-type plants appear to succeed a better maintenance of supply of the terminal sinks than Atcals7ko plants. A reason may be that callose does not constrict all sieve pores (Gallinger et al 2021 ; Buoso et al 2022 ), but stem stunting (Pagliari et al 2016 ) is indicative of a serious disturbance of phloem function (Maust et al 2003 ). Perhaps, constriction of sieve pores—including those of PPUs—due to phytoplasma infection prevents lateral symplasmic loss from SEs and loss is solely controlled by membrane-located release/retrieval transporters (van Bel 2021 ).…”

Impact of the “Flavescence Dorée” Phytoplasma on Xylem Growth and Anatomical Characteristics in Trunks of ‘Chardonnay’ Grapevines (Vitis vinifera)

“…Phytoplasmas are usually confined in the sieve elements from where they release defence and stress-related effectors that may spread through the vascular system and affect the distal plant organs and tissues, altering their physiology and structures [ 7 , 8 , 9 ]. As a consequence, the phytoplasma-induced impairment of the host plants is usually associated with non-specific symptoms, such as chlorosis, premature reddening, leaf curl, abnormal growth, or reduced vigour, and may highly vary according to the concerned host-pathogen combination [ 10 ]. On grapevines, external visible FDp-induced symptoms mainly consist of colour alterations of the leaf surface and veins, with a yellowing (white-fruited cvs) or reddening (red-fruited cvs) of the lamina.…”