Dissection of the Arabidopsis HUA‐PEP gene activity reveals that ovule fate specification requires restriction of the floral A‐function

Abstract

Ovules are essential for sexual plant reproduction and seed formation, and are fundamental for agriculture. However, our understanding of the molecular mechanisms governing ovule development is far from complete. In Arabidopsis, ovule identity is determined by homeotic MADS-domain proteins that define the floral C-(AG) and D-(SHP1/SHP2, STK) functions. Pre-mRNA processing of these genes is critical and mediated by HUA-PEP activity, composed of genes encoding RNA-binding proteins. In strong hua-pep mutants, functional transcripts for C-and D-function genes are reduced, resulting in homeotic transformation of ovules. Thus, hua-pep mutants provide an unique sensitized background to study ovule morphogenesis when C-and D-functions are simultaneously compromised. We found that hua-pep ovules are morphologically sepaloid and show ectopic expression of the homeotic class-A gene AP1. Inactivation of AP1 or AP2 (A-function genes) in hua-pep mutants reduced homeotic conversions, rescuing ovule identity while promoting carpelloid traits in transformed ovules. Interestingly, increased AG dosage led to similar results. Our findings strongly suggest that HUA-PEP activity is required for correct C and D floral functions, which in turn prevents ectopic expression of class-A genes in ovules for their proper morphogenesis, evoking the classic A-C antagonism of the ABC model for floral organ development.

Arabidopsis TCP4 transcription factor inhibits high temperature-induced homeotic conversion of ovules

“…The ovules are specified by AG-SEPs-STK/SHPs, while the carpels are specified by AG-SEPs. The balance between the two kinds of complexes is important for the specification of ovule identity [13][14][15] . Under normal temperature (22 °C), TCPs interacts with BEL1 and AG-SEPs to inhibit the activity of AG-SEPs in wild type.…”

“…The co-existence of SEPs, AG, STK and SHPs suggests that the AG-SEPs-STK/SHPs quartets specifying ovule identity co-exist in the ovules with AG-SEPs quartets specifying carpel identity, suggesting that regulatory mechanisms exert tight control over the activity of AG-SEPs-STK/SHPs quartets and AG-SEPs quartets to promote ovule identity, but not carpel identity, in ovules. Genetic analysis has shown that the activity of AG-SEPs-STK/SHPs quartets and AG-SEPs quartets, and the balance between them, are essential for the specification of ovules [13][14][15] . It has been shown that ag single mutants, sep1 sep2 sep3 triple mutants and sep1 sep2 sep3 sep4 quadruple mutants do not form carpels or ovules because they lack both types of quartets [16][17][18] .…”