NEW INSIGHTS REVEALED THE EVOLUTION OF THE AFL SUBFAMILY OF B3 TRANSCRIPTION FACTORS FROM CHLOROPHYTA AND ITS REQUISITE IN LAND PLANTS

ABI3, FUS3 and LEC2 are the master regulators of seed development. The functions of these transcription factors have been well characterized in some plants, but there is some limited understanding of their evolutionary and structural features. In this study, AFLs were investigated in 64 genome-sequenced plant species. Identification and multiple sequence alignment of key domains analysis revealed that the primary type of ABI3 (pri-ABI3) is present in two of the green algae and ABI3 is present in all the tested plants. However, FUS3 exists in angiosperms and LEC2 only appears in dicots. Phylogenetic tree analysis of AFLs revealed that pri-ABI3 genes are closest to an AtHSI2 outgroup and monocotyledonous FUS3 genes are closer to ABI3 genes from non-vascular terrestrial plants. Collectively, ABI3 might have evolved first in Chlorophyta, followed by FUS3 in angiosperms and LEC2 in dicots. FUS3 and LEC2 underwent tandem duplication, while ABI3 in plants was the result of both tandem and segmental duplication events. AFL genes in plants are mainly evolved from basicity to acidity. Gene structure analysis suggests that most AFLs in plants trend towards a fixed number of exons; the intron phase of AFLs is mainly 0, 0, 2, 1 and 0. This finding indicates that most of the vascular plants shared a conserved gene structure during biological evolution. Expression analysis revealed that pri-ABI3 genes are induced under light and anaerobic conditions. AFL genes in oil seed crops are mainly expressed in seed and siliques, data that indicate their roles in seed development. Moreover, ABI3 in Amaranthus hypochondriacus and FUS3 in Himulus guttatus are also highly expressed in green cotyledons, evidence that implicates their unknown roles in other tissues.