A Stimulatory Role for Cytokinin in the Arbuscular Mycorrhizal Symbiosis of Pea

Abstract

The arbuscular mycorrhizal (AM) symbiosis between terrestrial plants and AM fungi is regulated by plant hormones. For most of these, a role has been clearly assigned in this mutualistic interaction; however, there are still contradictory reports for cytokinin (CK). Here, pea plants, the wild type (WT) cv. Sparkle and its mutant E151 (Pssym15), were inoculated with the AM fungus Rhizophagus irregularis. E151 has previously been characterized as possessing high CK levels in non-mycorrhizal (myc-) roots and exhibiting high number of fungal structures in mycorrhizal (myc+) roots. Myc- and myc+ plants were treated 7, 9, and 11 days after inoculation (DAI) with synthetic compounds known to alter CK status. WT plants were treated with a synthetic CK [6-benzylaminopurine (BAP)] or the CK degradation inhibitor INCYDE, whereas E151 plants were treated with the CK receptor antagonist PI-55. At 13 DAI, plant CK content was analyzed by mass spectrometry. The effects of the synthetic compounds on AM colonization were assessed at 28 (WT) or 35 (E151) DAI via a modified magnified intersections method. The only noticeable difference seen between myc- and myc+ plants in terms of CK content was in the levels of nucleotides (NTs). Whereas WT plants responded to fungi by lowering their NT levels, E151 plants did not. Since NTs are thought to be converted into active CK forms, this result suggests that active CKs were synthesized more effectively in WT than in E151. In general, myc+ and myc- WT plants responded similarly to INCYDE by lowering significantly their NT levels and increasing slightly their active CK levels; these responses were less obvious in BAP-treated WT plants. In contrast, the response of E151 plants to PI-55 depended on the plant mycorrhizal status. Whereas treated myc- plants exhibited high NT and low active CK levels, treated myc+ plants displayed low levels of both NTs and active CKs. Moreover, treated WT plants were more colonized than treated E151 plants. We concluded that CKs have a stimulatory role in AM colonization because increased active CK levels were paralleled with increased AM colonization while decreased CK levels corresponded to reduced AM colonization.

Targeting Cytokinin Homeostasis in Rapid Cycling Brassica rapa with Plant Growth Regulators INCYDE and TD-K

“…We noted that, recently, there are contrasting reports on the efficacy of INCYDE in response to biotic and abiotic stressors [ 34 , 51 , 59 ], as well as reports showing both enhanced [ 54 , 55 ] and decreased content of cytokinins [ 58 ].…”

“…However, recent research has also shown contradictory effects of INCYDE. INCYDE decreased the content of cytokinins in non-mycorrhizal control pea plants, particularly the nucleotide forms [ 58 ], while in Arabidopsis subjected to heat stress, F-INCYDE (a fluorinated analog of INCYDE, 2-fluoro-6-(3-methoxyphenyl)aminopurine [ 19 ]) exacerbated negative stress effects and delayed the stress response of non-acclimated plants [ 59 ]. A decrease in cytokinin nucleotide forms was also detected.…”

Hormones as go‐betweens in plant microbiome assembly

“…The enhancement of pathogen resistance promoted by CK has been shown in several plant–pathogen systems, which seems to be synergistic with, and dependent on, SA‐responsive pathways (Choi et al ., 2010 ; Jiang et al ., 2013 ; Reusche et al ., 2013 ). Contrastingly, CK appears to support the progression of fungal biotrophic pathogens and stimulate beneficial AMF interactions in pea ( Pisum sativum ) (Walters and McRoberts, 2006 ; Chanclud et al ., 2016 ; Morrison et al ., 2017 ; Goh et al ., 2019 ).…”

Isopentenyltransferases as master regulators of crop performance: their function, manipulation, and genetic potential for stress adaptation and yield improvement

“…Plants co‐exist with microorganisms in nature, and plant growth‐promoting microbiomes help plants resist stress via CTKs (Egamberdieva et al, 2017; Goh et al, 2019; Jorge et al, 2019). As CTKs are interkingdom signalling molecules, investigations into the possible function of IPTs , and how bacteria and fungi enhance plant stress resistance would increase our understanding of these beneficial interactions and could also provide novel technologies for crop stress management.…”