KNOTTED1 Cofactors, BLH12 and BLH14, Regulate Internode Patterning and Vein Anastomosis in Maize

Abstract

Monocot stems lack the vascular cambium and instead have characteristic structures in which intercalary meristems generate internodes and veins remain separate and scattered. However, developmental processes of these unique structures have been poorly described. BELL1-like homeobox (BLH) transcription factors (TFs) are known to heterodimerize with KNOTTED1-like homeobox TFs to play crucial roles in shoot meristem maintenance, but their functions are elusive in monocots. We found that maize (Zea mays) BLH12 and BLH14 have redundant but important roles in stem development. BLH12/14 interact with KNOTTED1 (KN1) in vivo and accumulate in overlapping domains in shoot meristems, young stems, and provascular bundles. Similar to kn1 loss-of-function mutants, blh12 blh14 (blh12/14) double mutants fail to maintain axillary meristems. Unique to blh12/14 is an abnormal tassel branching and precocious internode differentiation that results in dwarfism and reduced veins in stems. Micro-computed tomography observation of vascular networks revealed that blh12/14 double mutants had reduced vein number due to fewer intermediate veins in leaves and precocious anastomosis in young stems. Based on these results, we propose two functions of BLH12/14 during stem development: (1) maintaining intercalary meristems that accumulate KN1 and prevent precocious internode differentiation and (2) preventing precocious anastomosis of provascular bundles in young stems to ensure the production of sufficient independent veins.

BELL1‐like homeobox genes regulate inflorescence architecture and meristem maintenance in rice

“…OSH15, a BP‐like KNOX‐type homeodomain protein, is also involved in this process through its formation of a heterodimer with qSH1 and SH5 (Yoon et al ., ). Recently, it has been reported in maize that mutations in the two BELL1‐LIKE HOMEOBOX (BLH) genes BLH12 and BLH14 , which are close homologs of PNY and PNF , cause abnormalities in internode patterning and vascular bundles anastomosis, in addition to indeterminate branch formation in the tassel (Tsuda et al ., ).…”

Moving on up – controlling internode growth

“…A similar effect was observed following GA treatment of rice intercalary meristems (Sauter et al , ), suggesting that regulatory pathways target cell division orientation in both rib and intercalary‐mediated elongation. PNY also limits axial vascular development (Smith & Hake, ), and two BLHs in maize maintain intercalary activity and internode length by preventing premature and ectopic internode vascularisation (Tsuda et al , ). Thus, BLHs such as PNY may help foster extensive axial growth by constraining the development of boundary and vascular tissues, potentially linked to controlling cell division orientation.…”

How plants grow up

“…Controlling elongation by specification and/or proliferation of stem vasculature appears conserved in maize internodes, despite its different proliferation and vascularization pattern compared to dicots (Figure B). Exciting recent work by Tsuda et al () showed that loss of function of BLH12 and BLH14 (maize BLH s of the PNY clade), caused truncated internode elongation due to earlier arrest of intercalary meristems, as well as premature vein anastomoses. Given that BLH12, BLH14 and KN1 proteins interact and are all detected in the intercalary meristem, BLH12/14 and KN1 may work together to maintain intercalary meristems and prevent their precocious differentiation (Tsuda et al ).…”

“…Exciting recent work by Tsuda et al () showed that loss of function of BLH12 and BLH14 (maize BLH s of the PNY clade), caused truncated internode elongation due to earlier arrest of intercalary meristems, as well as premature vein anastomoses. Given that BLH12, BLH14 and KN1 proteins interact and are all detected in the intercalary meristem, BLH12/14 and KN1 may work together to maintain intercalary meristems and prevent their precocious differentiation (Tsuda et al ). BLH12 and BLH14 are also important to slow the lignification program in internodes, consistent with their expression in the stem provasculature (Tsuda et al ).…”

“…Given that BLH12, BLH14 and KN1 proteins interact and are all detected in the intercalary meristem, BLH12/14 and KN1 may work together to maintain intercalary meristems and prevent their precocious differentiation (Tsuda et al ). BLH12 and BLH14 are also important to slow the lignification program in internodes, consistent with their expression in the stem provasculature (Tsuda et al ). KN1 is also expressed in early vascular strands (Jackson et al ), which may be relevant to BLH12/14 function.…”