Arabidopsis class A S-acyl transferases modify the pollen receptors LIP1 and PRK1 to regulate pollen tube guidance.

Protein S-acylation catalyzed by protein S-acyl transferases (PATs) is a reversible lipid modification regulating protein targeting, stability, and interaction profiles. PATs are encoded by large gene families in plants, and many proteins including receptor-like cytoplasmic kinases (RLCKs) and receptor-like kinases (RLKs) are subject to S-acylation. However, few PATs have been assigned substrates, and few S-acylated proteins have known upstream enzymes. We report that Arabidopsis (Arabidopsis thaliana) class A PATs redundantly mediate pollen tube guidance and participate in the S-acylation of POLLEN RECEPTOR KINASE1 (PRK1) and LOST IN POLLEN TUBE GUIDANCE1 (LIP1), a critical RLK or RLCK for pollen tube guidance, respectively. PAT1, PAT2, PAT3, PAT4, and PAT8, collectively named PENTAPAT for simplicity, are enriched in pollen and show similar subcellular distribution. Functional loss of PENTAPAT reduces seed set due to male gametophytic defects. Specifically, pentapat pollen tubes are compromised in directional growth. We determine that PRK1 and LIP1 interact with PENTAPAT, and their S-acylation is reduced in pentapat pollen. The plasma membrane (PM) association of LIP1 is reduced in pentapat pollen, whereas point mutations reducing PRK1 S-acylation affect its affinity with its interacting proteins. Our results suggest a key role of S-acylation in pollen tube guidance through modulating PM receptor complexes.

CBP60b clade proteins are prototypical transcription factors mediating immunity.

Transcriptional reprogramming is critical for plant immunity. Several calmodulin (CaM)-binding protein 60 (CBP60) family transcription factors (TFs) in Arabidopsis (Arabidopsis thaliana), including CBP60g, systemic acquired resistance deficient 1 (SARD1), CBP60a, and CBP60b, are critical for and show distinct roles in immunity. However, there are additional CBP60 members whose function is unclear. We report here that Arabidopsis CBP60c-f, 4 uncharacterized CBP60 members, play redundant roles with CBP60b in the transcriptional regulation of immunity responses, whose pCBP60b-driven expression compensates the loss of CBP60b. By contrast, neither CBP60g nor SARD1 is interchangeable with CBP60b, suggesting clade-specific functionalization. We further show that the function of CBP60b clade TFs relies on DNA-binding domains (DBDs) and CaM-binding domains, suggesting that they are downstream components of calcium signaling. Importantly, we demonstrate that CBP60s encoded in earliest land plant lineage Physcomitrium patens and Selaginella moellendorffii are functionally homologous to Arabidopsis CBP60b, suggesting that the CBP60b clade contains the prototype TFs of the CBP60 family. Furthermore, tomato and cucumber CBP60b-like genes rescue the defects of Arabidopsis cbp60b and activate the expression of tomato and cucumber SALICYLIC ACID INDUCTION DEFICIIENT2 (SID2) and ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) genes, suggesting that immune response pathways centered on CBP60b are also evolutionarily conserved. Together, these findings suggest that CBP60b clade TFs are functionally conserved in evolution and positively mediate immunity.

RHO OF PLANT proteins are essential for pollen germination in Arabidopsis.

Pollen germination is a process of polarity establishment, through which a single and unique growth axis is established. Although most of the intracellular activities associated with pollen germination are controlled by RHO OF PLANTs (ROPs) and increased ROP activation accompanies pollen germination, a critical role of ROPs in this process has not yet been demonstrated. Here, by genomic editing of all 4 Arabidopsis (Arabidopsis thaliana) ROPs that are preferentially expressed in pollen, we showed that ROPs are essential for polarity establishment during pollen germination. We further identified and characterized 2 ROP effectors in pollen germination (REGs) through genome-wide interactor screening, boundary of ROP domain (BDR) members BDR8 and BDR9, whose functional loss also resulted in no pollen germination. BDR8 and BDR9 were distributed in the cytosol and the vegetative nucleus of mature pollen grains but redistributed to the plasma membrane (PM) of the germination site and to the apical PM of growing pollen tubes. We demonstrated that the PM redistribution of BDR8 and BDR9 during pollen germination relies on ROPs but not vice versa. Furthermore, enhanced expression of BDR8 partially restored germination of rop1 pollen but had no effects on that of the quadruple rop pollen, supporting their genetic epistasis. Results presented here demonstrate an ROP signaling route essential for pollen germination, which supports evolutionarily conserved roles of Rho GTPases in polarity establishment.

Arabidopsis HAPLESS13/AP-1µ is critical for pollen sac formation and tapetal function.

The production of excess and viable pollen grains is critical for reproductive success of flowering plants. Pollen grains are produced within anthers, the male reproductive organ whose development involves precisely controlled cell differentiation, division, and intercellular communication. In Arabidopsis thaliana, specification of an archesporial cell (AC) at four corners of a developing anther, followed by programmed cell divisions, generates four pollen sacs, walled by four cell layers among which the tapetum is in close contact with developing microspores. Tapetum secretes callose-dissolving enzymes to release microspores at early stages and undergoes programmed cell death (PCD) to deliver nutrients and signals for microspore development at later stages. Except for transcription factors, plasma membrane (PM)-associated and secretory peptides have also been demonstrated to mediate anther development. Adaptor protein complexes (AP) recruit both cargos and coat proteins during vesicle trafficking. Arabidopsis AP-1µ/HAPLESS13 (HAP13) is a core component of AP-1 for protein sorting at the trans-Golgi network/early endosomes (TGN/EE). We report here that Arabidopsis HAP13 is critical for pollen sac formation and for sporophytic control of pollen production. Functional loss of HAP13 causes a reduction in pollen sac number. It also results in the dysfunction of tapetum such that secretory function of tapetum at early stages and PCD of tapetum at later stages are both compromised. We further show that the expression of SPL, the polar distribution of auxin maximum, as well as the asymmetric distribution of PIN1 are interfered in hap13 anthers, which in combination may lead to male sterility in hap13.

Arabidopsis COP1 suppresses root hair development by targeting type I ACS proteins for ubiquitination and degradation.

Root hairs (RHs) are an innovation of vascular plants whose development is coordinated by endogenous and environmental cues, such as ethylene and light conditions. However, the potential crosstalk between ethylene and light conditions in RH development is unclear. We report that Arabidopsis constitutive photomorphogenic 1 (COP1) integrates ethylene and light signaling to mediate RH development. Darkness suppresses RH development largely through COP1. COP1 inhibits both cell fate determination of trichoblast and tip growth of RHs based on pharmacological, genetic, and physiological analyses. Indeed, COP1 interacts with and catalyzes the ubiquitination of ACS2 and ACS6. COP1- or darkness-promoted proteasome-dependent degradation of ACS2/6 leads to a low ethylene level in underground tissues. The negative role of COP1 in RH development by downregulating ethylene signaling may be coordinated with the positive role of COP1 in hypocotyl elongation by upregulating ethylene signaling, providing an evolutionary advantage for seedling fitness.