Cytokinin-induced parthenocarpy of San Pedro type fig (Ficus carica L.) main crop: explained by phytohormone assay and transcriptomic network comparison

CPPU-induced San Pedro type fig main crop parthenocarpy exhibited constantly increasing IAA content and more significantly enriched KEGG pathways in the receptacle than in female flowers. N-(2-chloro-4-pyridyl)-N-phenylurea (CPPU) was applied to San Pedro fig (Ficus carica L.) main crop to induce parthenocarpy; the optimal effect was obtained with 25 mg L^-1 application to syconia when female flowers were at anthesis. To elucidate the key expression changes in parthenocarpy conversion, significant changes in phytohormone level and transcriptome of fig female flowers and receptacles were monitored. HPLC-MS revealed increased IAA content in female flowers and receptacle 2, 4 and 10 days after treatment (DAT), decreased zeatin level in the receptacle 2, 4 and 10 DAT, decreased GA₃ content 2 and 4 DAT, and increased GA₃ content 10 DAT. ABA level increased 2 and 4 DAT, and decreased 10 DAT. CPPU-treated syconia released more ethylene than the control except 2 DAT. RNA-Seq and bioinformatics analysis revealed notably more differentially expressed KEGG pathways in the receptacle than in female flowers. In the phytohormone gene network, GA-biosynthesis genes GA20ox and GA3ox were upregulated, along with GA signal-transduction genes GID1 and GID2, and IAA-signaling genes AUX/IAA and GH3. ABA-biosynthesis gene NCED and signaling genes PP2C and ABF were downregulated 10 DAT. One ACO gene showed consistent upregulation in both female flowers and receptacle after CPPU treatment, and more than a dozen of ERFs demonstrated opposing changes in expression. Our results revealed early-stage spatiotemporal phytohormone and transcriptomic responses in CPPU-induced San Pedro fig main crop parthenocarpy, which could be valuable for further understanding the nature of the parthenocarpy of different fig types.

Biochemical and molecular characterisation of exogenous cytokinin application on grain filling in rice

BACKGROUND

Poor filling of grains in the basal spikelets of large size panicles bearing numerous spikelets has been a major limitation in attempts to increase the rice production to feed the world's increasing population. Considering that biotechnological intervention could play important role in overcoming this limitation, the role of cytokinin in grain filling was investigated based on the information on cell proliferating potential of the hormone and reports of its high accumulation in immature seeds.

RESULTS

A comparative study considering two rice varieties differing in panicle compactness, lax-panicle Upahar and compact-panicle OR-1918, revealed significant difference in grain filling, cytokinin oxidase (CKX) activity and expression, and expression of cell cycle regulators and cytokinin signaling components between the basal and apical spikelets of OR-1918, but not of Upahar. Exogenous application of cytokinin (6-Benzylaminopurine, BAP) to OR-1918 improved grain filling significantly, and this was accompanied by a significant decrease in expression and activity of CKX, particularly in the basal spikelets where the activity of CKX was significantly higher than that in the apical spikelets. Cytokinin application also resulted in significant increase in expression of cell cycle regulators like cyclin dependent kinases and cyclins in the basal spikelets that might be facilitating cell division in the endosperm cells by promoting G1/S phase and G2/M phase transition leading to improvement in grain filling. Expression studies of type-A response regulator (RR) component of cytokinin signaling indicated possible role of OsRR3, OsRR4 and OsRR6 as repressors of CKX expression, much needed for an increased accumulation of CK in cells. Furthermore, the observed effect of BAP might not be solely because of it, but also because of induced synthesis of trans-zeatin (tZ) and N⁶-(Δ²-isopentenyl)adenine (iP), as reflected from accumulation of tZR (tZ riboside) and iPR (iP riboside), and significantly enhanced expression of an isopentenyl transferase (IPT) isoform.

CONCLUSION

The results suggested that seed-specific overexpression of OsRR4 and OsRR6, and more importantly of IPT9 could be an effective biotechnological intervention towards improving the CK level of the developing caryopses leading to enhanced grain filling in rice cultivars bearing large panicles with numerous spikelets, and thereby increasing their yield potential.