Brassinosteroid signaling regulates female germline specification in Arabidopsis

Abstract

In most flowering plants, a single megaspore mother cell (MMC) is differentiated from a pool of sporophytic cells in the distal ovule. Cai et al. show that a BR hormone gradient is generated in the distal nucellus of ovule primordia with a concave point in the MMC that is essential to restrict female germline fate to a single sub-epidermal cell.

Ovule initiation: the essential step controlling offspring number in Arabidopsis

“…As long as subsequent developmental processes are successful, promoting ovule initiation will lead to increased number of effective offspring. Most positive regulators of ovule initiation, including developmental signals ( ANT , HLL , LUG , SEU , SIN2 , STK ) and hormone signals (auxin, CK, and BR), also positive regulate subsequent ovule development processes and increase offspring number (Schneitz et al, 1998; Broadhvest et al, 2000; Liu et al, 2000; Azhakanandam et al, 2008; Gomez et al, 2016; Hu et al, 2018; Jia et al, 2020; Terceros et al, 2020; Cucinotta et al, 2021; Cai et al, 2022). However, the function of GA in ovule initiation and seed development is complicated.…”

Misexpression of the Homologues of Bryophyte Gametophyte-to-Sporophyte Control Genes in Arabidopsis Results in Germline Reprogramming and Phenotypes that Mirror Apomictic Development

“…It is notable that the multi-MMC frequencies observed here are among the highest reported in the literature (Cai et al, 2022) and there are often more than two MMCs, each of which appears to be able to undergo meiosis and start development as a gametophyte. Interestingly, in many apomictic angiosperm species, including Hieracium and Hypericum , where ectopic BLH1 expression is known to be important for apomixis (Okada et al, 2013; Galla et al, 2019), atypical development begins from the aposporous MMC with subsequent atypical callose deposition (Galla et al, 2011).…”

“…A common defect in Arabidopsis early female development is the specification of multiple Megaspore Mother Cells (MMCs). This phenotype is present at a low level in WT but occurs frequently in epigenetic mutants (Rodriguez-leal et al ., 2015; Mendes et al ., 2020), and mutants with cell-cell communication defects (Su et al ., 2020; Cai et al ., 2022). This phenotype occurred extremely frequently in several of our experimental lines (fig2), particularly in Quad KNAT and KNAT5 ,7, often with 4-5 MMC-like cells within a single ovule in up to 100% of ovules in some gynoecia.…”

“…In both promSPL:BLH1 and SPL:KNAT ovules with multi-MMCs, often more than one MMC appeared meiotically functional, resulting in dual/multiple gametophytes that arrest by the FG2 stage. Again, callose staining confirmed the presence of multi-meiotic nascent gametophytes (Zhao et al ., 2014; Cai et al ., 2022) with excessive callose deposition in an atypical pattern, often present in multiple cells within the nucellus. This phenotype resembles certain types of apomictic development (Tucker et.…”

“…al , 2001; Galla et. al , 2011) and has previously been used as an indicator of a reported multiple fully functional MMC phenotype (Cai et al ., 2022).…”