Photoperiod and Vernalization Control of Flowering-Related Genes: A Case Study of the Narrow-Leafed Lupin (Lupinus angustifolius L.)

Abstract

Narrow-leafed lupin (<i>Lupinus angustifolius</i> L.) is a moderate-yielding legume crop known for its high grain protein content and contribution to soil improvement. It is cultivated under photoperiods ranging from 9 to 17 h, as a spring-sown (in colder locations) or as an autumn-sown crop (in warmer regions). Wild populations require a prolonged cold period, called vernalization, to induce flowering. The key achievement of <i>L. angustifolius</i> domestication was the discovery of two natural mutations (named <i>Ku</i> and <i>Jul</i>) conferring vernalization independence. These mutations are overlapping deletion variants in the promoter of <i>LanFTc1</i>, a homolog of the <i>Arabidopsis thaliana FLOWERING LOCUS T</i> (<i>FT</i>) gene. The third deletion, named here as <i>Pal</i>, was recently found in primitive germplasm. In this study, we genotyped <i>L. angustifolius</i> germplasm that differs in domestication status and geographical origin for <i>LanFTc1</i> alleles, which we then phenotyped to establish flowering time and vernalization responsiveness. The <i>Ku</i> and <i>Jul</i> lines were vernalization-independent and early flowering, wild (<i>ku</i>) lines were vernalization-dependent and late flowering, whereas <i>the Pal</i> line conferred intermediate phenotype. Three lines representing <i>ku</i>, <i>Pal</i>, and <i>Ku</i> alleles were subjected to gene expression surveys under 8- and 16-h photoperiods. <i>FT</i> homologs (<i>LanFTa1</i>, <i>LanFTa2</i>, <i>LanFTc1</i>, and <i>LanFTc2</i>) and some genes selected by recent expression quantitative trait loci mapping were analyzed. Expression profiles of <i>LanFTc1</i> and <i>LanAGL8</i> (<i>AGAMOUS-like 8</i>) matched observed differences in flowering time between genotypes, highlighted by high induction after vernalization in the <i>ku</i> line. Moreover, these genes revealed altered circadian clock control in <i>Pal</i> line under short days. <i>LanFD</i> (<i>FD</i>) and <i>LanCRLK1</i> (<i>CALCIUM/CALMODULIN-REGULATED RECEPTOR-LIKE KINASE 1</i>) were negatively responsive to vernalization in <i>Ku</i> and <i>Pal</i> lines but positively responsive or variable in <i>ku</i>, whereas <i>LanUGT85A2</i> (<i>UDP-GLUCOSYL TRANSFERASE 85A2</i>) was significantly suppressed by vernalization in all lines. Such a pattern suggests the opposite regulation of these gene pairs in the vernalization pathway. <i>LanCRLK1</i> and <i>LanUGT85A2</i> are homologs of <i>A. thaliana</i> genes involved in the <i>FLOWERING LOCUS C</i> (<i>FLC</i>) vernalization pathway. Lupins, like many other legumes, do not have any <i>FLC</i> homologs. Therefore, candidate genes surveyed in this study, namely <i>LanFTc1</i>, <i>LanAGL8, LanCRLK1</i>, and <i>LanUGT85A2</i>, may constitute anchors for further elucidation of molecular components contributing to vernalization response in legumes.