Global View on the Cytokinin Regulatory System in Potato

Abstract

Cytokinins (CKs) were earlier shown to promote potato tuberization. Our study aimed to identify and characterize CK-related genes which constitute CK regulatory system in the core potato (<i>Solanum tuberosum</i>) genome. For that, CK-related genes were retrieved from the sequenced genome of the <i>S. tuberosum</i> doubled monoploid (DM) Phureja group, classified and compared with Arabidopsis orthologs. Analysis of selected gene expression was performed with a transcriptome database for the <i>S. tuberosum</i> heterozygous diploid line RH89-039-16. Genes responsible for CK signaling, biosynthesis, transport, and metabolism were categorized in an organ-specific fashion. According to this database, CK receptors StHK2/3 predominate in leaves and flowers, StHK4 in roots. Among phosphotransmitters, StHP1a expression largely predominates. Surprisingly, two pseudo-phosphotransmitters intended to suppress CK effects are hardly expressed in studied organs. Among B-type <i>RR</i> genes, <i>StRR1b</i>, <i>StRR11</i>, and <i>StRR18a</i> are actively expressed, with <i>StRR1b</i> expressing most uniformly in all organs and <i>StRR11</i> exhibiting the highest expression in roots. By cluster analysis four types of prevailing CK-signaling chains were identified in (1) leaves and flowers, StHK2/3→<i>S</i> <i>t</i> <i>H</i> <i>P</i>1<i>a</i>→StRR1b/+; (2) shoot apical meristems, stolons, and mature tubers, StHK2/4→<i>S</i> <i>t</i> <i>H</i> <i>P</i>1<i>a</i>→StRR1b/+; (3) stems and young tubers, StHK2/4→<i>S</i> <i>t</i> <i>H</i> <i>P</i>1<i>a</i>→StRR1b/11/18a; and (4) roots and tuber sprouts, StHK4→<i>S</i> <i>t</i> <i>H</i> <i>P</i>1<i>a</i>→StRR11/18a. CK synthesis genes <i>StIPT3</i>/<i>5</i> and <i>StCYP735A</i> are expressed mainly in roots followed by tuber sprouts, but rather weakly in stolons and tubers. By contrast, CK-activation genes <i>StLOGs</i> are active in stolons, and <i>StLOG3b</i> expression is even stolon-confined. Apparently, the main CK effects on tuber initiation are realized via activity of <i>StLOG1/3a/3b/7c/8a</i> genes in stolons. Current advances and future directions in potato research are discussed.