Vegetative phase change and shoot maturation in plants

Abstract

As a plant shoot develops, it produces different types of leaves, buds, and internodes, and eventually acquires the capacity to produce structures involved in sexual reproduction. Morphological and anatomical traits that change in coordinated fashion at a predictable time in vegetative development allow this process to be divided into several more-or-less discrete phases; the transition between these phases is termed "vegetative phase change." Vegetative phase change is regulated by a decrease in the expression of the related microRNAs, miR156, and miR157, which act by repressing the expression of squamosa promoter binding protein/SBP-like (SBP/SPL) transcription factors. SBP/SPL proteins regulate a wide variety of processes in shoot development, including flowering time and inflorescence development. Answers to long-standing questions about the relationship between vegetative and reproductive maturation have come from genetic analyses of the transcriptional and posttranscriptional regulatory networks in which these proteins are involved. Studies conducted over several decades indicate that carbohydrates have a significant effect on phase-specific leaf traits, and recent research suggests that sugar may be the leaf signal that promotes vegetative phase change.

Keywords: Flowering time; Heteroblasty; Heterochrony; Phase change; SBP; SPL; Shoot morphogenesis; miR156; miR172.

Figure 1

The abundance of miR156, miR157, and miR172 is correlated with the vegetative phase of woody species. A–E, Juvenile and adult leaves of A) Acacia confusa, B) Acacia colei, C) Eucalyptus globulus, D) Hedera helix (English ivy), and E) Quercus acutissima (sawtooth oak). The leaves shown in A and B are successive leaves on the same shoot. F) Northern blots of small RNA from the leaves shown in A–E, hybridized with probes to the indicated transcripts. Scale bars correspond to 2 cm. Modified from (Wang et al, 2011).

Figure 2

miR156 controls reproductive competence by repressing the expression of genes required for flower production. The pathways illustrated here are based on validated miRNA-target interactions and transcription factor binding studies described in the text. Black indicates genes that are expressed; gray represents repressed genes.