Genome-Wide Identification of Tomato (Solanum lycopersicum L.) CKX Gene Family and Expression Analysis in the Callus Tissue under Zeatin Treatment

The cytokinin oxidase/dehydrogenase(CKX)enzyme is essential for controlling thefluctuating levels of endogen-ous cytokinin(CK)and has a significant impact on different aspects of plant growth and development.Nonethe-less,there is limited knowledge about CKX genes in tomato(Solanum lycopersicum L.).Here we performed genome-wide identification and analysis of nine SlCKX family members in tomatoes using bioinformatics tools.The results revealed that nine SlCKX genes were unevenly distributed onfive chromosomes(Chr.1,Chr.4,Chr.8,Chr.10,and Chr.12).The amino acid length,isoelectric points,and molecular weight of the nine SlCKX proteins ranged from 453 to 553,5.77 to 8.59,and 51.661 to 62.494 kD,respectively.Subcellular localization analysis indi-cated that SlCKX2 proteins were located in both the vacuole and cytoplasmic matrix;SlCKX3 and SlCKX5 pro-teins were located in the vacuole;and SlCKX1,4,6,7,8,and 9 proteins were located in the cytoplasmic matrix.Furthermore,we observed differences in the gene structures and phylogenetic relationships of SlCKX proteins among different members.SlCKX1-9 were positioned on two out of the three branches of the CKX phylogenetic tree in the multispecies phylogenetic tree construction,revealing their strong conservation within phylogenetic subgroups.Unique patterns of expression of CKX genes were noticed in callus cultures exposed to varying con-centrations of exogenous ZT,suggesting their roles in specific developmental and physiological functions in the regeneration system.These results may facilitate subsequent functional analysis of SlCKX genes and provide valu-able insights for establishing an efficient regeneration system for tomatoes.

Review: Plant Growth Hormone Cytokinins Control the Crop Seed Yield

This review chronicles the development of the cytokinin research during the last 30 years. Cytokinin and auxin are the two major plant growth hormones that control virtually all aspects of growth and development in higher plants. The pathways for cytokinin biosynthesis and metabolism have been characterized by the identification of isopentenyl pyrophosphate transferase, cytokinin oxidases, cytokinin hydroxylase, zeatin cis-/trans-isomerase, cytokinin phosphoribosyl hydrolases, cytokinin-specific riboside phosphorylase, and others enzymes. Loss-of function mutant phenotypes of cytokinin degradation/activating enzymes indicate the regulation of concentration and spatial distribution of bio-active cytokinin plays a pivotal role in the increase in panicle size, in the numbers of floral organs, and eventually in seed yield. One of the most fundamental questions in the cytokinin field is one concerning the prevalence of cis-zeatin in monocotyledonous crops (rice and maize) and in dicotyledonous legumes (pea, chickpea) and potato/sweet potato. A hypothesis is that cis-zeatin is synthesized by the cis-specific hydroxylation of the terminal methyl group of N6-isopentenyl side chain of N6-isopentenyl adenosine (i6Ado) or of their mono-, di-, or tri-phosphates catalyzed by the cis-specific hydroxylase. A second potential pathway is the isomerization of trans-zeatin to cis-zeatin by zeatin cis-/trans-isomerase. A second fundamental question to be addressed is the physiological role of cis-zeatin. Some have argued for a special function of cis-zeatin to account for the prevalence of the cis-zeatin in the plant kingdom from algae to higher plants.