Abstract
Cytokinins (CKs) are a chemically diverse class of plant growth regulators, exhibiting wide-ranging actions on plant growth and development, hence their exploitation in agriculture for crop improvement and management. Their coordinated regulatory effects and cross-talk interactions with other phytohormones and signaling networks are highly sophisticated, eliciting and controlling varied biological processes at the cellular to organismal levels. In this review, we briefly introduce the mode of action and general molecular biological effects of naturally occurring CKs before highlighting the great variability in the response of fruit crops to CK-based innovations. We present a comprehensive compilation of research linked to the application of CKs in non-model crop species in different phases of fruit production and management. By doing so, it is clear that the effects of CKs on fruit set, development, maturation, and ripening are not necessarily generic, even for cultivars within the same species, illustrating the magnitude of yet unknown intricate biochemical and genetic mechanisms regulating these processes in different fruit crops. Current approaches using genomic-to-metabolomic analysis are providing new insights into the in planta mechanisms of CKs, pinpointing the underlying CK-derived actions that may serve as potential targets for improving crop-specific traits and the development of new solutions for the preharvest and postharvest management of fruit crops. Where information is available, CK molecular biology is discussed in the context of its present and future implications in the applications of CKs to fruits of horticultural significance.
Highlights
Cytokinins (CKs) are a unique class of plant growth regulators (PGRs) with a long and interesting history
Synthetic compounds such as CPPU, a phenylurea synthetic CK that has become one of the widely used chemicals with CK-like effects in fruit production globally, still show inconsistencies in its effects, which may depend on factors such as genotype, phenological stage, concentration, and synergistic interactions arising from combinations with other PGRs [14]
This is largely reflective of a past mindset in applied agricultural research that did not necessarily examine questions related to the mechanistic aspects; the fundamental information of these processes remains unknown
Summary
Cytokinins (CKs) are a unique class of plant growth regulators (PGRs) with a long and interesting history. Fundamental knowledge of the diverse roles of CKs in plants remains fragmented, and there is greater scope to deepen our knowledge of how CKs function and regulate cellular mechanisms that control plant growth and development This knowledge will enable greater exploitation and application of CKs in horticultural fruit production. The systematic application of biostimulants, plant growth-promoting rhizobacteria (PGPR) and mycorrhizal fungi have been demonstrated to hold the potential to mitigate biotic and abiotic stresses as well as boost fruit crop production [18,30] In addition to these approaches, the diverse roles of PGRs, especially CKs, offer a potential avenue that requires more detailed attention [31,32,33,34,35,36]. A prominent model organism for scientific studies, is excluded due to the extensive existing literature focusing on different aspects of the plant [15,27,41,42,43]
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