Abstract
Tendrils are contact-sensitive, filamentous organs that permit climbing plants to tether to their taller neighbors. Tendrilled legume species are grown as field crops, where the tendrils contribute to the physical support of the crop prior to harvest. The homeotic tendril-less (tl) mutation in garden pea (Pisum sativum), identified almost a century ago, transforms tendrils into leaflets. In this study, we used a systematic marker screen of fast neutron-generated tl deletion mutants to identify Tl as a Class I homeodomain leucine zipper (HDZIP) transcription factor. We confirmed the tendril-less phenotype as loss of function by targeting induced local lesions in genomes (TILLING) in garden pea and by analysis of the tendril-less phenotype of the t mutant in sweet pea (Lathyrus odoratus). The conversion of tendrils into leaflets in both mutants demonstrates that the pea tendril is a modified leaflet, inhibited from completing laminar development by Tl. We provide evidence to show that lamina inhibition requires Unifoliata/LEAFY-mediated Tl expression in organs emerging in the distal region of the leaf primordium. Phylogenetic analyses show that Tl is an unusual Class I HDZIP protein and that tendrils evolved either once or twice in Papilionoid legumes. We suggest that tendrils arose in the Fabeae clade of Papilionoid legumes through acquisition of the Tl gene.
Highlights
Many climbing plants use specialized organs called tendrils for support
Compared with the wild-type leaf (Figure 1A), narrow, subterminal leaflets were found in place of tendrils in heterozygous fast neutron (FN) mutants (Figure 1B), as expected for this semidominant mutation, while the homozygous FN mutants displayed a classic homeotic transformation of tendrils into leaflets (Figure 1C)
Tendril-less F1 progeny were obtained from tendril-less FN mutants crossed to lines carrying the tl-w type allele, confirming that the new FN mutants all carried allelic mutations
Summary
Many climbing plants use specialized organs called tendrils for support. Some tendrils explore the physical environment with characteristic circling movements (Darwin, 1875) followed by contact-induced coiling (Jaffe and Galston, 1968), permitting the plant to obtain support by grasping onto and entwining its neighbors. Plant tendrils may be derived from a variety of structures, such as leaf parts, whole leaves, or stems (Bell, 1991); for example, the grapevine tendril is a gibberellin-inhibited inflorescence (Boss and Thomas, 2002) Such diverse derivations, and the fact that tendrilled taxa are widespread in flowering plants (Darwin, 1875), suggest that tendrils are an example of convergent evolution. The fact that tendrilled taxa are widespread in flowering plants (Darwin, 1875), suggest that tendrils are an example of convergent evolution These novel organs enable plants to reach the canopy, where they can spread and maximize opportunities for pollination, photosynthesis, and seed dispersal with minimal energy investment in expensive supporting structures. A better understanding of tendril formation has the potential to aid agronomic performance and to provide insight on convergent morphological evolution
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