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This study investigates how two genetic signalling pathways, D14L karrikin signalling and the common symbiosis signalling pathway (CSSP) anchored by the kinase CCaMK, interact to regulate arbuscular mycorrhizal (AM) fungal colonisation in rice roots. By expressing a constitutively active form of CCaMK in D14L-deficient mutants, the researchers confirmed that CSSP acts downstream of D14L signalling and demonstrated that CCaMK activation alone can expand fungal access to normally restrictive root tissues such as the meristematic zone and endodermis. They also found that D14L retains CCaMK-independent roles in controlling arbuscule development and intraradical colonisation quality, and transcriptome analysis suggested CCaMK primarily exerts its transcriptional effects through AP2 transcription factors.
Why it matters
AM symbiosis is central to phosphate uptake in most crops, and understanding the genetic architecture that controls colonisation could inform efforts to engineer or enhance mycorrhizal associations in agricultural species, potentially reducing dependence on synthetic phosphate fertilisers.
⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
Arbuscular mycorrhizal (AM) symbiosis is conserved across land plants and is the default nutrient uptake strategy in nature. Within roots, AM colonisation is tightly patterned and dynamically tuned by nutritional cues. Multiple genetic modules contribute to this regulation, including the phosphate starvation response, DWARF14-LIKE (D14L) karrikin signalling, and the common symbiosis signalling pathway (CSSP). Transcriptional overlap among these has led to the hypothesis that phosphate starvation and D14L signalling act upstream of the CSSP. Here, we examined the epistatic relationship between D14L and CSSP in rice. Overexpression of an autoactive gain-of-function CCaMK (gofCCaMKox) restored AM colonisation and symbiosis marker gene expression in d14l mutants to wild-type levels or above, whereas overexpression of wild-type CCaMK did not, confirming that CSSP operates downstream of D14L signalling. However, gofCCaMKox did not rescue the d14l mesocotyl elongation phenotype, supporting a bifurcation of D14L into developmental and symbiotic outputs. Unexpectedly, gofCCaMKox also expanded fungal access to normally restrictive tissue domains (the meristematic zone and endodermis) assigning a role for CCaMK activation in defining root zone and cell-type competence for AM colonisation. Despite restored colonisation, introduction of gofCCaMKox into d14l produced arbuscules, which however were less developed and had increased hyphal septation, revealing a CCaMK-independent role for D14L in intraradical colonisation and arbuscule development. Transcriptome profiling resolved AM-relevant genes into modules controlled by CCaMK activation alone, in combination with D14L, or requiring additional colonisation-associated cues, and further suggested CCaMK primarily acts through AP2 transcription factors. Together, these findings reinforce CCaMK as a master regulator of AM symbiosis at the genetic, transcriptomic and anatomical levels while uncovering CCaMK-independent functions of D14L in arbuscule development.