Integration of the SMXL/D53 strigolactone signalling repressors in the model of shoot branching regulation in Pisum sativum

Abstract

DWARF53 (D53) in rice (Oryza sativa) and its homologs in Arabidopsis (Arabidopsis thaliana), SUPPRESSOR OF MAX2-LIKE 6 (SMXL6), SMXL7 and SMXL8, are well established negative regulators of strigolactone (SL) signalling in shoot branching regulation. Little is known of pea (Pisum sativum) homologs and whether D53 and related SMXLs are specific to SL signalling pathways. Here, we identify two allelic pea mutants, dor-mant3 (dor3), and demonstrate through gene mapping and sequencing that DOR3 corresponds to a homolog of D53 and SMXL6/SMXL7, designated PsSMXL7. Phenotype analysis, gene expression, protein and hormone quantification assays were performed to determine the role of PsSMXL7 in regulation of bud outgrowth and the role of PsSMXL7 and D53 in integrating SL and cytokinin (CK) responses. Like D53 and related SMXLs, we show that PsSMXL7 can be degraded by SL and induces feedback upregulation of PsSMXL7 transcript. Here we reveal a system conserved in pea and rice, whereby CK also upregulates PsSMXL7/D53 transcripts, providing a clear mechanism for SL and CK cross-talk in the regulation of branching. To further deepen our understanding of the branching network in pea, we provide evidence that SL acts via PsSMXL7 to modulate auxin content via PsAFB5, which itself regulates expression of SL biosynthesis genes. We therefore show that PsSMXL7 is key to a triple hormone network involving an auxin-SL feedback mechanism and SL-CK cross-talk.

Sucrose promotes D53 accumulation and tillering in rice

“…Recent findings in pea suggested that CK promotes PsSMXL7 protein accumulation, the homologue of the rice D53, through increasing the expression of the PsSMXL7 gene (Kerr et al ., 2021). Since sucrose was reported to promote CK synthesis in different species (Barbier et al ., 2015b; Kiba et al ., 2019; Salam et al ., 2021), it would be tempting to hypothesize that sucrose induces D53 protein levels through a CK‐mediated increase of D53 expression.…”

Masks Start to Drop: Suppressor of MAX2 1-Like Proteins Reveal Their Many Faces

“…Since the discovery of D53/SMXL proteins in rice ( Jiang et al, 2013 ; Zhou et al, 2013 ; Zheng et al, 2020 ) and Arabidopsis ( Stanga et al, 2013 ; Soundappan et al, 2015 ), SMXL family members have gradually been characterized in additional plant species, including wheat ( Liu et al, 2017 ), apple ( Li et al, 2018 ), woodland strawberry ( Wu et al, 2019 ), lotus ( Carbonnel et al, 2020 ), and pea ( Kerr et al, 2021 ). Recent efforts to unravel the evolutionary history of this gene family have shown that SMXLs are both unique to and ubiquitously present in all land plants ( Figure 2A ; Walker et al, 2019 ).…”

“…The involvement of the D2 domain in the MAX2-dependent degradation of SMXLs can be attributed to the presence of the Walker A motif. D2-Walker A is required in several species for the degradation of SMAX1 ( Khosla et al, 2020 ; Wang et al, 2020b ; Zheng et al, 2020 ; Mizuno et al, 2021 ) and SMXL6/7/8 ( Jiang et al, 2013 ; Zhou et al, 2013 ; Soundappan et al, 2015 ; Wang et al, 2015 ; Liang et al, 2016 ; Struk et al, 2018 ; Kerr et al, 2021 ). Several publications term this motif P-loop or (F)RGKT, according to its structure or its amino-acid sequence, respectively ( Zhou et al, 2013 ; Soundappan et al, 2015 ; Wang et al, 2015 ; Liang et al, 2016 ; Struk et al, 2018 ).…”

“…This pathway regulates several physiological processes, including inhibition of shoot branching ( Soundappan et al, 2015 ; Wang et al, 2015 ), cotyledon expansion ( Soundappan et al, 2015 ) and lateral root outgrowth ( Soundappan et al, 2015 ; Villaécija-Aguilar et al, 2019 ); increase of branch angle ( Liang et al, 2016 ) and leaf and petiole length ( Soundappan et al, 2015 ); promotion of stem elongation ( Soundappan et al, 2015 ; Liang et al, 2016 ), leaf senescence ( Bennett et al, 2016 ) and secondary growth ( Liang et al, 2016 ); and protection against drought stress through stomatal closure ( Yang et al, 2020a ), thickening of the cuticle and production of anthocyanins ( Figure 1 ; Table 1 ; Li et al, 2020 ). The role for SMXL6/7/8 in shoot branching or tillering in monocotylednous plants, has been studied in several additional species, including rice, wheat ( Triticum aestivum ), and pea ( Pisum sativum ), suggesting that this role for SL signaling is conserved at least across angiosperms ( Jiang et al, 2013 ; Zhou et al, 2013 ; Liu et al, 2017 ; Kerr et al, 2021 ). In apple ( Malus domestica ) and woodland strawberry ( Fragaria vesca ), SMXL6/7/8 has been inferred to play a role in abiotic stress and flower development respectively, although not yet confirmed by functional characterization ( Li et al, 2018 ; Wu et al, 2019 ).…”

Cytokinins: Wide-Spread Signaling Hormones from Plants to Humans with High Medical Potential

“…In a recent study, it was found that CK signaling is a potential target for enhancing future shoot regeneration efficiency, as it activates the dedication of the shoot progenitor at later stages and allows chromatin to maintain shoot identity genes at the priming stage [ 67 ]. In the context of the plant growth phases, it is frequently reported that CKs have an important regulatory role, for instance, vegetative phase change in Arabidopsis thaliana through the miR172/TOE1-TOE2 module [ 68 ], development and environmental responses of plants through CKRs [ 69 ], and shoot branching regulation in Pisum sativum through the SMXL/D53 strigolactone signaling repressors and up-regulation of PsSMXL7/D53 transcripts [ 70 ].…”