Promoters of Forisome Genes MtSEO2 and MtSEO3 Direct Gene Expression to Immature Sieve Elements in Medicago truncatula and Nicotiana tabacum

Forisomes are mechanoproteins that function as valves in the phloem sieve tubes of the Fabaceae. Changes in the Ca²⁺ level that occur after wounding trigger an instantaneous switch between the condensed and dispersed conformations, allowing forisomes to seal the sieve plate pores of injured sieve elements. Recently, three genes encoding forisome components from Medicago truncatula were identified [Medicago truncatula sieve element occlusion 1 (MtSEO1), MtSEO2 and MtSEO3] and MtSEO1 expression was shown to be restricted to immature sieve elements. Here, we present a detailed molecular analysis of the MtSEO2 and MtSEO3 promoters and show, through reverse transcriptase-polymerase chain reaction and reporter-transgenic experiments in M. truncatula roots and tobacco plants, that their expression profiles are comparable to MtSEO1. The impact of these data on the likelihood of MtSEO gene co-regulation by common transcription factors is discussed.